Water Quality and Trend Analysis 

of Colorado–Big Thompson System 

Reservoirs and Related Conveyances, 

1969 Through 2000

By Michael R. Stevens

Prepared in cooperation with the

NORTHERN COLORADO WATER CONSERVANCY DISTRICT, 

BUREAU OF RECLAMATION, and the

CITY OF FORT COLLINS

Water-Resources Investigations Report 03–4044

U.S. Department of the Interior

U.S. Geological Survey
U.S. Department of the Interior

Gale A. Norton, Secretary

U.S. Geological Survey

Charles G. Groat, Director

U.S. Geological Survey, Reston, Virginia: 2003

For sale by U.S. Geological Survey, Information Services

Box 25286, Denver Federal Center

Denver, CO 80225

For more information about the USGS and its products:

Telephone: 1-888-ASK-USGS

World Wide Web: http://www.usgs.gov/

Any use of trade, product, or firm names in this publication is for descriptive purposes only and does not imply 

endorsement by the U.S. Government.
iii

Contents

Abstract.......................................................................................................................................................... 1

Introduction ................................................................................................................................................... 2

Purpose and Scope ............................................................................................................................. 2

Previous Studies and Acknowledgments........................................................................................ 3

Description of Study Area .................................................................................................................. 4

Hydrology ....................................................................................................................................................... 6

Water Quality................................................................................................................................................. 8

Chemical Characteristics ................................................................................................................... 9

Field Measurements and Major-Ion Concentrations.......................................................... 13

Nutrients..................................................................................................................................... 13

Trace Elements.......................................................................................................................... 17

Colorado Water-Quality Standards........................................................................................ 22

Limnology ............................................................................................................................................ 22

Reservoir Profile Characteristics........................................................................................... 22

Hypolimnetic Oxygen Dynamics............................................................................................. 23

Trophic Indicators and Nutrient Limitation........................................................................... 23

Trend Analysis ............................................................................................................................................. 26

Time-Series Data ............................................................................................................................... 26

Statistical Testing............................................................................................................................... 28

Summary and Conclusions........................................................................................................................ 31

References Cited ........................................................................................................................................ 33

Hydrologic and Water-Quality Data......................................................................................................... 35

Figures

 1. Map showing location of study area and sampling sites ..................................................... 3

 2. Schematic diagram of CBT Project facilities .......................................................................... 4

 3. Graph showing percentage of Colorado–Big Thompson water supplied for 

municipal use ............................................................................................................................... 5

 4–6. Maps showing:

 4. Land uses in watershed upstream from Lake Granby.................................................... 6

 5. Land uses in watershed upstream from the mouth of Big Thompson Canyon .......... 7

 6. Land uses in watershed draining directly to Horsetooth Reservoir ............................ 8

 7–11. Graphs showing:

 7. Maximum, mean, and minimum daily streamflows for the period of record 

 (1953 to 2000) at Colorado River below Baker Gulch (09010500) .................................. 9

 8. Time-series plots of operation of selected Colorado–Big Thompson 

 conveyances ....................................................................................................................... 10

 9. Time-series plots of operation of selected Colorado–Big Thompson 

 reservoirs............................................................................................................................. 11

 10. Monthly average discharge characteristics of selected Colorado–

 Big Thompson conveyances ............................................................................................ 12

 11. Monthly average reservoir contents of selected Colorado–Big Thompson 

 reservoirs............................................................................................................................. 12
iv

 12. Boxplots of selected water-quality constituents ................................................................. 14

 13. Piper diagram showing major-ion composition of samples at selected sites................. 17

 14–17. Graphs showing:

 14. May and October field-measurement profiles at Lake Granby (dam) ....................... 18

 15. May and October field-measurement profiles at Shadow Mountain Lake .............. 19

 16. May and October field-measurement profiles at Carter Lake .................................... 20

 17. May and October field-measurement profiles at Horsetooth Reservoir 

 (Soldier Canyon Dam) ........................................................................................................ 21

 18. Time-series plots of annual areal hypolimnetic oxygen deficit in reservoirs.................. 24

 19. Annual trophic status index (TSI) computations for Secchi depth, total 

phosphorus, and chlorophyll-a by the Carlson (1977) method........................................... 25

 20. Nonmonotonic distribution and seasonal Kendall test results for pre-1983 

and post-1983 of ammonia plus organic nitrogen (total) at Adams Tunnel 

east portal ................................................................................................................................... 27

21. Monotonic distribution and seasonal Kendall test results for all samples 

of dissolved iron at Adams Tunnel east portal...................................................................... 27

 22. Time-series plots of reservoir reciprocal hydraulic residence time in terms 

of the proportion of water entering or leaving the water body.......................................... 31

 23–37. Graphs showing temporal variations in periodic measurements and 

concentrations at:

 23. Lake Granby near spillway, sampled near the surface....................................................... 38

 24. Lake Granby near spillway, sampled near the bottom ........................................................ 44

 25. Lake Granby near Rainbow Bay, sampled near the surface.............................................. 49

 26. Lake Granby near Rainbow Bay, sampled near the bottom ............................................... 55

 27. Granby Pump Canal................................................................................................................... 60

 28. Shadow Mountain Lake, sampled at the surface................................................................. 65

 29. Shadow Mountain Lake, sampled near the bottom ............................................................. 71

 30. Adams Tunnel, east portal, near Estes Park ......................................................................... 76

 31. Olympus Tunnel.......................................................................................................................... 81

 32. Carter Lake, sampled near the surface.................................................................................. 86

 33. Carter Lake, sampled near the bottom................................................................................... 92

 34. Horsetooth Reservoir, near Soldier Canyon Dam, sampled near the surface................. 97

 35. Horsetooth Reservoir, near Soldier Canyon Dam, sampled near the bottom................ 103

 36. Horsetooth Reservoir, near Spring Canyon Dam, sampled near the surface................ 108

 37. Horsetooth Reservoir, near Spring Canyon Dam, sampled near the bottom................. 114

Tables

 1. Water-quality sampling sites ..................................................................................................... 2

 2. Seasonal periods and period of record tested by seasonal Kendall test ........................ 29

 3. Summary of significant upward and downward temporal trends at Colorado–

Big Thompson sampling sites.................................................................................................. 30

 4. Summary statistics for site 1, Lake Granby, near spillway, near surface ...................... 119

 5. Summary statistics for site 1, Lake Granby, near spillway, near bottom........................ 120

 6. Summary statistics for site 2, Lake Granby (Rainbow Bay), sampled near surface..... 121

 7. Summary statistics for site 2, Lake Granby (Rainbow Bay), sampled near bottom...... 122
v

 8. Summary statistics for site 3, Granby Pump Canal ............................................................ 123

 9. Summary statistics for site 4, Shadow Mountain Lake, sampled at the surface.......... 124

 10. Summary statistics for site 4, Shadow Mountain Lake, sampled near bottom ............. 125

 11. Summary statistics for site 5, Alva B. Adams Tunnel east portal .................................... 126

 12. Summary statistics for site 6, Lake Estes, sampled at surface........................................ 127

 13. Summary statistics for site 6, Lake Estes, sampled near bottom .................................... 128

 14. Summary statistics for site 7, Olympus Tunnel ................................................................... 129

 15. Summary statistics for site 8, Carter Lake, sampled near surface.................................. 130

 16. Summary statistics for site 8, Carter Lake, sampled near bottom................................... 131

 17. Summary statistics for site 9, Horsetooth Reservoir (Soldier Canyon Dam), 

sampled near surface ............................................................................................................. 132

 18. Summary statistics for site 9, Horsetooth Reservoir (Soldier Canyon Dam), 

sampled near bottom .............................................................................................................. 133

 19. Summary statistics for site 10, Horsetooth Reservoir (Spring Canyon Dam), 

sampled at surface.................................................................................................................. 134

 20. Summary statistics for site 10, Horsetooth Reservoir (Spring Canyon Dam), 

sampled near bottom .............................................................................................................. 135

 21. Trend analysis results for site 1, Granby Reservoir, near dam, surface samples......... 136

 22. Trend analysis results for site 1, Granby Reservoir, near dam, bottom samples .......... 137

 23. Trend analysis results for site 2, Granby Reservoir (Rainbow Bay), surface 

samples ..................................................................................................................................... 138

 24. Trend analysis results for site 2, Granby Reservoir (Rainbow Bay), bottom 

samples ..................................................................................................................................... 139

 25. Trend analysis results for site 3, Granby Pump Canal, samples ...................................... 140

 26. Trend analysis results for site 4, Shadow Mountain Reservoir, surface samples........ 141

 27. Trend analysis results for site 4, Shadow Mountain Reservoir, bottom samples ......... 142

 28. Trend analysis results for site 5, Adams Tunnel east portal ............................................. 143

 29. Trend analysis results for site 7, Olympus Tunnel, samples ............................................. 144

 30. Trend analysis results for site 8, Carter Reservoir, surface samples.............................. 145

 31. Trend analysis results for site 8, Carter Reservoir bottom samples................................ 146

 32. Trend analysis results for site 9, Horsetooth Reservoir (Soldier Canyon Dam), 

surface samples....................................................................................................................... 147

 33. Trend analysis results for site 9, Horsetooth Reservoir (Soldier Canyon Dam), 

bottom samples........................................................................................................................ 148

 34. Trend analysis results for site 10, Horsetooth Reservoir (Spring Canyon Dam), 

surface samples....................................................................................................................... 149

 35. Trend analysis results for site 10, Horsetooth Reservoir (Spring Canyon Dam), 

bottom samples........................................................................................................................ 150
vi

Conversion Factors, Vertical Datum, and 

Abbreviations

Multiply By To obtain

acre 0.00156 square mile

acre-foot (acre-ft) 1,233 cubic meter

cubic foot per second (ft3

/s) 0.02832 cubic meter per second

foot (ft) 0.3048 meter

inch 2.54 centimeter

mile 1.609 kilometer

millimeter 0.03937 inch

square centimeter (cm2

) 0.1550 square inch (in2)

square foot (ft2

) 0.0929 square meter

square mile (mi2

) 2.590 square kilometer (km2)

Temperature in degrees Celsius (°C) may be converted to degrees Fahrenheit (°F) as follows:

°F = 1.8(°C) + 32

Vertical coordinate information is referenced to the National Geodectic Vertical Datum of 1929 (NGVD 29); 

horizontal coordinate information is referenced to the North American Datum of 1927 (NAD 27).

Additional Abbreviations

MRL minimum reporting level

mg milligrams

mg/L milligrams per liter

mm millimeters

mL milliliters

NTU nephelometric turbidity units

ROE residue on evaporation

μg/L micrograms per liter

μS/cm microsiemens per centimeter at 25 degrees Celsius

WY water year, begins on October 1 and ends on September 30 of the following year. 

 Identified by year in which it ends (water year 2000 ended on September 30, 2000).
Abstract

The U.S. Geological Survey, in an ongoing cooperative monitoring program with the Northern Colorado 

Water Conservancy District, Bureau of Reclamation, 

and City of Fort Collins, has collected water-quality data 

in north-central Colorado since 1969 in reservoirs and 

conveyances, such as canals and tunnels, related to the 

Colorado–Big Thompson Project, a water-storage, collection, and distribution system. Ongoing changes in water use 

among agricultural and municipal users on the eastern slope 

of the Rocky Mountains in Colorado, changing land use in 

reservoir watersheds, and other water-quality issues among 

Northern Colorado Water Conservancy District customers 

necessitated a reexamination of water-quality trends in 

the Colorado–Big Thompson system reservoirs and related 

conveyances. The sampling sites are on reservoirs, canals, 

and tunnels in the headwaters of the Colorado River (on the 

western side of the transcontinental diversion operations) 

and the headwaters of the Big Thompson River (on the 

eastern side of the transcontinental diversion operations). 

Carter Lake Reservoir and Horsetooth Reservoir are offchannel water-storage facilities, located in the foothills of 

the northern Colorado Front Range, for water supplied from 

the Colorado–Big Thompson Project. The length of waterquality record ranges from approximately 3 to 30 years 

depending on the site and the type of measurement or constituent. Changes in sampling frequency, analytical methods, and minimum reporting limits have occurred repeatedly over the period of record.

The objective of this report was to complete a retrospective water-quality and trend analysis of reservoir 

profiles, nutrients, major ions, selected trace elements, 

chlorophyll-a, and hypolimnetic oxygen data from 1969 

through 2000 in Lake Granby, Shadow Mountain Lake, and 

the Granby Pump Canal in Grand County, Colorado, and 

Horsetooth Reservoir, Carter Lake, Lake Estes, Alva B. 

Adams Tunnel, and Olympus Tunnel in Larimer County, 

Colorado. 

This report summarizes and assesses:

1. Water-quality and field-measurement profile data collected by the U.S. Geological Survey and stored in the 

U.S. Geological Survey National Water Information 

System, 

2. Time-series trends of chemical constituents and physical 

properties, 

3. Trends in oxygen deficits in the hypolimnion of the reservoirs in the late summer season by the seasonal Kendall 

trend test method, 

4. Nutrient limitation and trophic status indicators, and

5. Water-quality data in terms of Colorado water-quality 

standards.

Water quality was generally acceptable for primary uses 

throughout the Colorado–Big Thompson system over the site 

periods of record, which are all within the span of 1969 to 

2000. Dissolved solids and nutrient concentrations were low 

and typical of a forested/mountainous/crystalline bedrock 

hydrologic setting. Most of the more toxic trace elements 

were rarely detected or were found in low concentrations, due 

at least in part to a relative lack of ore-mineral deposits within 

the drainage areas of the Colorado–Big Thompson Project.

Constituent concentrations consistently met waterquality standard thresholds set by the State of Colorado. 

Trophic-State Index Values indicated mesotrophic conditions 

generally prevailed at reservoirs, based on available Secchi 

depth, total phosphorus concentrations, and chlorophyll-a

concentrations. 

Based on plots of time-series values and concentrations 

and seasonal Kendall nonparametric trends testing, dissolved 

solids and most major ions are decreasing at most sites. Many 

of the nutrient data did not meet the minimum criteria for 

time-series testing; but for those that did, nutrient concentrations were generally stable (no statistical trend) or decreasing 

(ammonia plus organic nitrogen and total phosphorus). Iron 

and manganese concentrations were stable or decreasing at 

most sites that met testing criteria. Chlorophyll-a data were 

only collected for 11 years but generally indicated quasi-stable 

or downward temporal trends. 

Water Quality and Trend Analysis of Colorado–

Big Thompson System Reservoirs and Related 

Conveyances, 1969 Through 2000

By Michael R. Stevens
2 Water Quality and Trend Analysis of Colorado–Big Thompson System Reservoirs and Related Conveyances

Introduction

The U.S. Geological Survey (USGS), in an ongoing 

cooperative monitoring program with the Northern 

Colorado Water Conservancy District (NCWCD), Bureau 

of Reclamation (BOR), and City of Fort Collins, has collected 

water-quality data in north-central Colorado since 1969 in 

reservoirs and conveyances such as canals and tunnels related 

to the Colorado–Big Thompson (CBT) Project, a waterstorage, collection, and distribution system. The sampling 

sites are on reservoirs, canals, and tunnels in the headwaters 

of the Colorado River (on the western side of the transcontinental diversion operations) and the headwaters of the Big 

Thompson River (on the eastern side of the transcontinental 

diversion operations). Carter Lake and Horsetooth Reservoir 

are off-channel water-storage facilities, located in the foothills 

of the northern Colorado Front Range, for water supplied from 

the CBT Project (table 1, fig. 1). The length of water-quality 

record ranges from approximately 3 to 30 years depending on 

the site and the type of measurement or constituent. Sampling 

frequency, analytical methods, and minimum reporting limits 

have changed repeatedly over the period of record.

The CBT Project was authorized by Congress in 1937 

and completed in 1956, resulting in the construction of several 

reservoirs, more than 120 miles of tunnels and canals, and 

numerous power-generation facilities. The USGS became 

involved in monitoring in the CBT Project area as part of a 

cooperative water-quality evaluation of the BOR Pick-Sloan 

Missouri Basin Program. When operation of the CBT Project 

was assumed by NCWCD, collection of baseline waterquality information continued. A report by Mueller (1990) 

summarized water-quality data available at the time, analyzed 

time-series trends, and evaluated the sampling network design 

for most of the sampling sites considered in this report and 

other sites on the eastern plains where the water is used for 

agricultural purposes. 

Purpose and Scope

Part of the value of such a long-term monitoring record 

is the ability to test for temporal trends in water quality. Temporal trends in nutrients, major ions, trophic variables, and 

oxygen can indicate changes in water quality related to eutrophication. Ongoing changes in water use among agricultural 

and municipal users on the eastern slope of Colorado, changing land uses in reservoir watersheds, and other water-quality 

issues among NCWCD customers necessitated a reexamination of water-quality trends in CBT system reservoirs and 

related conveyances. 

The overall objective of the project is to complete a 

retrospective analysis of reservoir profiles, nutrients, major 

ions, selected trace elements, chlorophyll-a, and hypolimnetic oxygen data from 1969 through 2000 in Lake Granby, 

Shadow Mountain Lake, and the Granby Pump Canal in Grand 

County, Colo., and Horsetooth Reservoir, Carter Lake, Lake 

Estes, east portal of the Alva B. Adams Tunnel (hereinafter 

“Adams Tunnel”), and Olympus Tunnel in Larimer County, 

Colo. Sampling sites in this report are in the headwater areas 

of the CBT Project where diversion and storage facilities are 

located. Areas downstream from Lake Granby on the western 

slope and water delivery areas downstream from Carter Lake 

and Horsetooth Reservoir are beyond the scope of this report. 

Table 1. Water-quality sampling sites.

[15-digit U.S. Geological Survey (USGS) identification numbers are latitude and longitude of site location with a 2-digit code at the end; 8-digit identification 

numbers are USGS downstream order numbering]

Site

number

(fig. 1)

U.S. Geological Survey 

identification number Site name Number of samples

Period 

of record

1 09018500 Lake Granby near Granby, Colo. (near spillway) 154 1973–75, 

1979–present

2 400844105530800 Lake Granby near Granby, Colo. (near dam in Rainbow Bay) 123 1989–present

3 09018300 Granby Pump Canal near Grand Lake, Colo. 179 1970–present

4 09014500 Shadow Mountain Lake near Grand Lake, Colo. 125 1989–present

5 09013000 Alva B. Adams Tunnel east portal, near Estes Park, Colo. 283 1970–present

6 402231105291900 Lake Estes near dam near Estes Park, Colo. 16 1998–present

7 06734900 Olympus Tunnel at Lake Estes, Colo. 247 1970–present

8 06742500 Carter Lake near Berthoud, Colo. 309 1970–present

9 06737500 Horsetooth Reservoir near Fort Collins, Colo. (Soldier Canyon Dam) 518 1969–present

10 403147105083800 Horsetooth Reservoir near Fort Collins, Colo. (Spring Canyon Dam) 100 1983–present

11 09010500 Colorado River below Baker Gulch near Grand Lake, Colo. -- 1953–present
Introduction 3

Figure 1. Location of study area and sampling sites (modified from Mueller, 1990).

This report summarizes water-quality and field-measurement 

profile data collected by the USGS and stored in the USGS 

National Water Information System; assesses time-series 

trends of selected chemical constituents and physical properties, trends in oxygen deficits in the hypolimnion of the reservoirs in the late summer season by the seasonal Kendall trend 

test method, and nutrient limitation and trophic status indicators; and evaluates water-quality data in terms of Colorado 

water-quality standards.

Previous Studies and Acknowledgments

A report by Mueller (1990) discussed trends determined 

by using the seasonal Kendall method; general water-quality 

analysis and sampling network evaluation were the primary 

focus of that report. Trend analysis at the Adams Tunnel 

east portal that is included in this report also was computed 

for selected chemical constituents in Middelburg (1993, 

p. 204). Jassby and Goldman (1999) studied water-quality 

Grand Junction

SOUTH

PLATTE RIVER

DENVER

Study area

COLORADO 

GUNNISON RIVER

COLORADO R

Pueblo

r 

106°30'

40°

40°30'

41°

106° 105°30' 105° 104°30'

Base from U.S. Geological Survery Digital Data

Transverse Mercator Projection

Central Meridian 

Datum 1983 0 20 40 MILES 

0 20 40 KILOMETERS 

EXPLANATION

Towns

Sites and number

Roads

Streams

Counties

11

5

6

7

8

9

10

4

3

1

2

2

Roaring Fork

North Platt e River

Colorado River

Piney River

Fraser River

Big Thomps on River

Horsetooth

Rerservoir

Little Thompson River

JACKSON

COUNTY

WELD

COUNTY

BOULDER

COUNTY

SUMMIT

COUNTY

GRAND

COUNTY

CLEAR

CREEK

COUNTY

JEFFERSON

COUNTY

ADAMS

COUNTY DENVER

COUNTY

LARIMER

COUNTY

WYOMING

COLORADO

Cache La Poudre River

Saint Vrain Creek

Boulder Creek

South

Platte River

Grand

Lake

Lake

Granby

Shadow

Mountain

Lake

Estes Park

Lyons

Longmont

Fort Collins

CONTINENTAL DIVIDE

Rocky Mountain

National Park

ADAMS

TUNNEL

OLYMPUS

TUNNEL
4 Water Quality and Trend Analysis of Colorado–Big Thompson System Reservoirs and Related Conveyances

and biological conditions in Horsetooth Reservoir and Carter 

Lake in 1998–99. Greve (1999) designed a water-quality 

monitoring network for the Big Thompson watershed in cooperation with the Big Thompson Watershed Forum, a watershed 

group within the study area. Johnson and Goettl (1999) studied 

food web changes for 14 years following the introduction of 

rainbow smelt into Horsetooth Reservoir.

The author thanks Don Carlson at the NCWCD and 

Gene Price and Jeff Lucero at the BOR for their assistance 

in bringing this report to fruition, and Dave Dzurovshin at the 

Colorado State Engineers Office for assistance in retrieving 

discharge data at some sites.

Description of Study Area

The study area involves parts of two mountainous 

watersheds. On the western side of the Continental Divide, 

several major tributaries in the headwaters of the Colorado 

River (North Fork Colorado River, North Inlet, East Inlet, and 

Arapaho Creek) are connected to parts of the western slope 

CBT reservoir system (Lake Granby, Shadow Mountain Lake, 

and Grand Lake) and, thus, are the primary sources of water 

diverted by the CBT Project (fig. 2). Other water sources 

include the Willow Creek Canal, which conveys water stored 

in Willow Creek Reservoir, and water pumped from Windy 

Gap Reservoir (built in 1985 by the Municipal Subdistrict of 

the NCWCD to use the unused capacity in CBT facilities), 

located on the Colorado River downstream from the confluence with the Fraser River. Storage on the western slope is primarily in Lake Granby (539,800 acre-ft total capacity). Water 

is pumped through the Farr Pumping Plant and the Granby 

Pump Canal into Shadow Mountain Lake. Shadow Mountain 

Lake and Grand Lake are similar (hydraulically connected) 

and relatively nonfluctuating. Their function is to convey 

project water into the Adams Tunnel, the largest transcontinental diversion in Colorado (average 230,000 acre-ft diverted 

annually).

On the eastern side of the Continental Divide, project 

water is conveyed through tunnels and eventually to Lake 

Estes, where it mixes with water from the Big Thompson 

River, a watershed with headwaters located primarily 

within Rocky Mountain National Park. The Olympus 

Tunnel then conveys water from Lake Estes to a series of 

tunnels and small reservoirs to Carter Lake or the Hansen 

Feeder Canal. The Hansen Feeder Canal can convey the water 

(and accept water diverted at the mouth of Big Thompson 

Canyon through the Dille Tunnel) to Horsetooth Reservoir. 

Boulder Creek

Boulder 

Reservoir

St. Vrain Creek

Little Thompson River

Big Thompson River

Big Thompson River

South Platte River

Carter Lake

Olympus Tunnel

Adams Tunnel CONTINENTAL DIVIDE

Dille Tunnel

Diversion

Hansen

Feeder Canal

Cache La Poudre River

Lake Estes

Mary's Lake

Lake Granby

Shadow Mountain

and Grand Lakes

Colorado River

Colorado River

Willow Creek

Granby Pump Canal

Windy Gap

Reservoir

Willow Creek

Reservoir

Willow Creek

Canal

Fraser River

Horsetooth

Reservoir

Figure 2. Schematic diagram of CBT Project facilities (from Mueller, 1990). 
Introduction 5

Carter Lake (112,230 acre-ft total capacity) and Horsetooth 

Reservoir (156,735 acre-ft total capacity) are the primary 

storage facilities on the eastern slope and were constructed 

by damming valleys of intermittent drainages along the Front 

Range. Project water is then conveyed to water users by way 

of streams, canals, and pipelines within the NCWCD boundaries to Boulder Creek, Saint Vrain Creek, Big Thompson River, 

Cache La Poudre River, and the main-stem South Platte River. 

Drinking water is supplied to 30 Front Range cities and towns, 

and water for supplemental irrigation is provided to more 

than 600,000 acres of agricultural land. Project water was 

initially used primarily for agriculture, but municipal uses 

have increased in recent years (fig. 3).

Primary land uses within the parts of the watersheds 

where CBT project water is diverted and stored are undeveloped forest and rangelands under the management of the 

U.S. Department of Agriculture Forest Service and National 

Park Service (figs. 4 and 5). The facilities on the western slope 

and those near Estes Park are located in mountainous terrain 

(greater than 6,000-ft elevation) covered in coniferous forest, 

underlain by mostly crystalline bedrock. Annual precipitation 

is in the range of 16 to more than 40 inches per year (Colorado 

Climate Center, 1984). Horsetooth Reservoir and Carter Lake 

are located in more arid foothills on the edge of the mountain 

front, are surrounded by grass/shrublands, and are underlain 

by sedimentary rock (fig. 6). While one of the primary uses of 

water resources is recreation, there are two developed urban 

areas that could affect the water quality within the study area: 

Grand Lake and Estes Park. Grand Lake is a town of approximately 250 people, on the shores of Grand Lake. Similar, 

though less dense, residential development is present along 

the shores of Shadow Mountain and Granby Lakes (fig. 4). 

Before 1982, individual sewage disposal systems (ISDS) were 

used for sanitation. Due to increasing water-quality concerns, 

a municipal sanitation system that pumps waste downstream 

from Lake Granby for treatment was completed in 1982. 

The town of Estes Park is located around Lake Estes and 

has a population of approximately 3,000 (fig. 5). Less dense 

development is located along the Big Thompson Canyon from 

Estes Park to the mouth of the canyon west of Loveland. Water 

quality is potentially affected by treated sewage effluent from 

Estes Park discharged into the Big Thompson River upstream 

from Lake Estes. Smaller sewage-treatment facilities and 

ISDS’s located along the Big Thompson River downstream 

from Lake Estes could affect the quality of water diverted to 

Horsetooth Reservoir through the Dille Tunnel (located just 

upstream from the mouth of Big Thompson Canyon) and 

Hansen Feeder Canal. Development around Carter Lake is 

minimal, but residential development around Horsetooth 

Reservoir could affect water quality (fig. 6).

Figure 3. Percentage of Colorado–Big Thompson (CBT) water supplied for municipal use.

EXPLANATION

TOTAL WATER YEAR

 DELIVERY OF CBT WATER

PERCENTAGE OF CBT WATER

 TO MUNICIPAL USE

350,000

50,000

100,000

150,000

200,000

250,000

300,000

TOTAL WATER YEAR DELIVERY

OF CBT WATER, IN ACRE-FEET

1955 1960 1965 1970 1975 1980 1985 1990 1995 2000 2005

WATER YEAR

0

100

10

20

30

40

50

60

70

80

90

PERCENTAGE OF CBT PROJECT WATER

DELIVERED FOR MUNICIPAL USE 

1950

0
6 Water Quality and Trend Analysis of Colorado–Big Thompson System Reservoirs and Related Conveyances

Hydrology

The study area, because of its location in high mountains, 

derives annual streamflow primarily from snowmelt during 

May through July as shown in the streamflow statistics for 

the period of record at the Colorado River below Baker Gulch 

stream gage (fig. 7). The watershed area upstream from Lake 

Granby outlet is about 310 square miles. The watershed area 

upstream from the mouth of Big Thompson Canyon is about 

306 square miles. The high-elevation areas of these catchments receive large amounts of snow from September to 

May. Maximum snowpack accumulation generally occurs 

about the beginning of May and is mostly melted by about 

mid-June when daily streamflow runoff peaks. Daily fluctuations in streamflow, which can substantially affect streamwater quality by influencing dilution and particle transport, 

may be largely due to diurnal variations in air temperatures, 

affecting snowmelt. 

Operation of reservoirs and conveyances can affect seasonal water quality, streamflow, and limnological characteristics of storage reservoirs. Some streamflow and reservoir data 

were available from USGS sources, but most was obtained 

from either NCWCD or the Colorado State Engineers Office. 

Seasonal and annual operational characteristics of selected 

conveyances and reservoirs are shown in figure 8. Release and 

storage patterns are relatively similar over the period of record 

Open water, 4 percent

Developed, less than 1 percent

Forest, 71 percent

Grass/shrubland, 24 percent

Agriculture, less than 1 percent

Land-use type and percentage of basin

EXPLANATION

Grand Lake

Lake Granby

Shadow Mountain Lake

0 5 10 MILES

0 5 10 KILOMETERS

N

Figure 4. Land uses in watershed upstream from Lake Granby.
Hydrology 7

EXPLANATION

N

Open water, less than 1 percentDeveloped, not greater than 1 percentForest, 68 percentGrass/shrubland, 30 percentAgriculture, less than 1 percent

Land-use type and percentage of basin

EstesPark

LakeEstes

0 5 10 MILES0 5 10 KILOMETERS

Figure 5. Land uses in watershed upstream from the mouth of Big Thompson Canyon.
8 Water Quality and Trend Analysis of Colorado–Big Thompson System Reservoirs and Related Conveyances

on a year-to-year basis at Shadow Mountain Lake, Horsetooth 

Reservoir, and Carter Lake (fig. 9). The contents of Shadow 

Mountain Lake, and consequently Grand Lake, vary little 

because the reservoir is controlled to minimize fluctuations 

in water-surface elevation. Multiyear patterns of storage can 

be seen in plots of contents for Lake Granby. Storage accumulated in wet years can persist for several years. Similarly, 

when depleted, Lake Granby can take several years to refill. 

The effects of drought years such as 1977 can be seen clearly 

in plots of reservoir contents for Lake Granby and Horsetooth 

Reservoir. 

Seasonal flow patterns among the conveyances are evident in the plots. Highest flow rates occur at the Willow Creek 

Canal and the Windy Gap pipeline during snowmelt season 

(fig. 10). Flow rates are highest in Granby Pump Canal, Adams 

Tunnel, and Carter Lake inflow during the winter months. 

Hansen Feeder Canal conveys flow mostly in winter and 

summer and generally maintains lower rates of flow in early 

spring and fall. On average, Lake Granby reaches maximum 

elevation in July of each year, falling steadily through summer and winter until April, when storage of the next snowmelt 

begins (fig. 11). Storage in Horsetooth Reservoir is somewhat different from Lake Granby in that average storage in 

Horsetooth increases from November through May or June 

and is then depleted during the summer season. In Carter 

Lake, storage increases from about October through April.

Water Quality

In this report, water quality will be described in two 

general categories: chemical characteristics and limnology. 

The discussion of chemical characteristics will present summary statistics, boxplots of chemical constituents, and descriptions of general water-quality conditions at the sampling sites. 

The number of samples may differ from boxplot to summary 

statistics because of the automated nature of the summary 

statistics/database and the use of estimated values in the 

Land-use type and percentage of basin

Open water, 17 percent

Developed, 1 percent

Forest, 21 percent

Grass/shrubland, 59 percent

Agriculture, 1 percent

EXPLANATION

Horsetooth

Reservoir

0 2 4 MILES

0 2 4 KILOMETERS

N

Figure 6. Land uses in watershed draining directly to Horsetooth Reservoir.
Water Quality 9

boxplots. The discussion of limnology will present reservoir 

field-measurement profiles, hypolimnetic oxygen dynamics, 

trophic indicators, and nutrient limitation at reservoir sites. 

Chemical Characteristics

The types of water-quality data collected at most sites 

that are part of the scope of this report include field measurements (water temperature, specific conductance, pH, and 

dissolved oxygen), major ions (calcium, magnesium, sodium, 

potassium, acid-neutralizing capacity or alkalinity, sulfate, 

chloride, fluoride), nutrients (ammonia plus organic nitrogen, ammonia, nitrite, nitrite plus nitrate, total phosphorus, 

dissolved phosphorus, orthophosphorus), and selected trace 

elements (iron, manganese, barium, strontium, and zinc). Summary statistics were compiled for the 10 sites in this study and 

are listed in tables 4 to 20 in the “Hydrologic Data” section 

(appendices at back of this report).

Surface and near-bottom sampling from each reservoir 

included field-measurement profiles. Some reservoir sites 

during the 1970’s had only a surface sample collected or an 

additional intermediate depth sample collected in addition to 

the surface and near-bottom samples. Sampling frequency has 

not remained constant throughout the history of the program. 

The frequency history described here includes only the sites 

considered for this report. Other discontinued sampling sites 

on Carter Lake and Horsetooth Reservoir are not described in 

this report but are described in Mueller (1990).

Lake Granby (site 1, fig. 1) near the dam was sampled 

quarterly at the surface from 1973 to 1975. During 1979 to 

1988, annual samples were collected from the surface and 

near bottom. From 1989 to 1990, samples were collected 

at surface and bottom, three times during the growing season (May to October). From 1991 to 1997, samples were 

collected approximately monthly from May to October. In 

1998, sampling frequency decreased to four times per year 

during the growing season. Sampling at Lake Granby (site 2) 

in Rainbow Bay was started in 1989 and followed a schedule of one per month during May to September until 2000. 

Sampling at Granby Pump Canal (site 3) was approximately 

bimonthly on average from 1970 to 2000. The exact months 

sampled varied because samples were not collected in a 

particular month if there was no flow at the time of the visit. 

Sampling at Shadow Mountain Lake (site 4) began consistently in 1989 at a frequency of three per year during the 

growing season, increased to six per year from 1991 to 1997, 

and decreased to four per year from 1998 to 2000. Sampling 

at the Adams Tunnel East Portal (site 5) began in 1970 at a 

monthly frequency until 1988, when the bimonthly schedule 

that continued through 2000 was implemented. Sampling at 

Lake Estes (site 6) was begun in 1998 at a frequency of two 

per year, with samples collected at the surface and near the 

bottom in May and August. Monthly sampling at Olympus 

Tunnel (site 7) began in the fall of 1970 until 1986. By 1989, 

frequency had dropped to three samples per year, one each in 

spring, summer, and fall, and continued at the same frequency 

until 2000. Sampling at Carter Lake Reservoir (site 8) began 

in 1970 at a frequency of 9 or 10 per year, April to November 

until 1979. By 1983, sampling frequency was reduced to three 

per year during the spring through fall and continued to 2000. 

Sampling at Horsetooth Reservoir (Soldier Canyon Dam) 

(site 9) began in 1969, and samples were collected at surface, 

bottom, and an intermediate depth from April to October or 

November until 1978. By 1982, frequency was down to one 

surface and one near-bottom sample in spring, summer, and 

fall. Sample collection began at the Spring Canyon Dam 

(site 10) on Horsetooth in 1983 at a frequency of three per 

year, surface and near bottom.

Figure 7. Maximum, mean, and minimum daily streamflows for the period of record (1953 to 2000) at Colorado River below 

Baker Gulch (09010500).

EXPLANATION

MAXIMUM DAILY 

DAILY MEAN 

MINIMUM DAILY

J FMAM J J A SOND

1,000

200

400

600

800

STREAMFLOW, IN CUBIC FEET 

PER SECOND

0
10 Water Quality and Trend Analysis of Colorado–Big Thompson System Reservoirs and Related Conveyances

0

40,000

20,000

MONTHLY MEAN STREAMFLOW, IN ACRE-FEET

0

40,000

20,000

0

40,000

20,000

0

40,000

20,000

0

40,000

20,000

0

40,000

20,000

1974 1976 1978 1980 1982 1984 1986 1988 1990 1992 1994 1996 1998 2000

1974 1976 1978 1980 1982 1984 1986 1988 1990 1992 1994 1996 1998 2000

HANSEN FEEDER CANAL

CARTER LAKE INFLOW

1974 1976 1978 1980 1982 1984 1986 1988 1990 1992 1994 1996 1998 2000

WILLOW CREEK CANAL

1974 1976 1978 1980 1982 1984 1986 1988 1990 1992 1994 1996 1998 2000

WINDY GAP CANAL

1974 1976 1978 1980 1982 1984 1986 1988 1990 1992 1994 1996 1998 2000

GRANBY PUMP CANAL

1974 1976 1978 1980 1982 1984 1986 1988 1990 1992 1994 1996 1998 2000

ADAMS TUNNEL

Figure 8. Time-series plots of operation of selected Colorado–Big Thompson conveyances.
Water Quality 11

R

1948 1950 1952 1954 1956 1958 1960 1962 1964 1966 1968 1970 1972 1974 1976 1978 1980 1982 1984 1986 1988 1990 1992 1994 1996 1998 2000

0

150,000

50,000

100,000

CARTER LAKE

1948 1950 1952 1954 1956 1958 1960 1962 1964 1966 1968 1970 1972 1974 1976 1978 1980 1982 1984 1986 1988 1990 1992 1994 1996 1998 2000

0

200,000100,000

HORSETOOTH RESERVOI

0

800,000600,000400,000200,000

19,00018,00017,00016,000

SHADOW MOUNTAIN LAKE

RESERVOIR CONTENTS, IN ACRE-FEET

LAKE GRANBY

1948 1950 1952 1954 1956 1958 1960 1962 1964 1966 1968 1970 1972 1974 1976 1978 1980 1982 1984 1986 1988 1990 1992 1994 1996 1998 20001948 1950 1952 1954 1956 1958 1960 1962 1964 1966 1968 1970 1972 1974 1976 1978 1980 1982 1984 1986 1988 1990 1992 1994 1996 1998 2000

Figure 9. Time-series plots of operation of selected Colorado–Big Thompson reservoirs.
12 Water Quality and Trend Analysis of Colorado–Big Thompson System Reservoirs and Related Conveyances

EXPLANATION

ADAMS TUNNEL

GRANBY PUMP CANAL

HANSEN FEEDER CANAL

CARTER LAKE INFLOW

WILLOW CREEK CANAL

WINDY GAP PIPELINE

0

30,000

5,000

10,000

15,000

20,000

25,000

MONTHLY AVERAGE DISCHARGE, 

IN ACRE-FEET

JAN FEB MAR APR MAY JUNE JULY AUG SEPT OCT NOV DEC

EXPLANATION

LAKE GRANBY

SHADOW MOUNTAIN LAKE

CARTER LAKE

HORSETOOTH RESERVOIR

0

450,000

50,000

100,000

150,000

200,000

250,000

300,000

350,000

400,000

MONTHLY AVERAGE RESERVOIR CONTENTS, 

IN ACRE-FEET 

JAN FEB MAR APR MAY JUNE JULY AUG SEPT OCT NOV DEC

Figure 10. Monthly average discharge characteristics of selected Colorado–Big Thompson conveyances.

Figure 11. Monthly average reservoir contents of selected Colorado–Big Thompson reservoirs.
Water Quality 13

Field Measurements and Major-Ion 

Concentrations

In snowmelt-dominated mountain streams, one of the 

primary processes controlling specific-conductance and majorion concentrations is dilution; minimum values of specific 

conductance occur during peak snowmelt discharge and maximum values occur during winter low-flow conditions (Stevens, 

2001; Clark and others, 2000). Daily fluctuations of specific 

conductance in streams not affected by reservoirs also reflect 

dilution by the daily diurnal fluctuation of flow during high 

snowmelt (Stevens, 2001). In reservoirs and conveyances that 

move water from reservoirs, these fluctuations are tempered 

by mixing that occurs during storage. Thus, concentrations are 

smoothed substantially in a reservoir like Lake Granby that 

can potentially store multiple years of runoff.

Water at CBT sampling sites was dilute and typical of 

high-elevation snowmelt runoff (fig. 12). Median concentrations of specific conductance at sites on the western slope 

(such as Lake Granby and Shadow Mountain Lake) and the 

high-elevation sites on the eastern slope (such as Adams 

Tunnel, Lake Estes, and Olympus Tunnel) were generally 

below 60 microsiemens per centimeter at 25 degrees Celsius 

(μS/cm). These sites are indicative of a hydrologic setting that 

is typical of snowmelt-source water—low rates of evapotranspiration (ET) and crystalline bedrock geology. Specific 

conductance at sites at Carter Lake and Horsetooth Reservoir 

were generally greater than 70 μS/cm. These sites possibly 

show the cumulative effects of development and ET from the 

rest of the storage and conveyance system. These sites are at 

lower elevations (higher ET) in areas of sedimentary rocks 

that can increase dissolved-solids content. Even so, all water 

sampled in this system should be considered to be high quality 

because of its low dissolved-solids content.

Water temperature varied seasonally, with reservoir sites 

developing strong stratification patterns. The pH at CBT sites 

was generally in a neutral range of 7 to 8. Dissolved oxygen was generally in the range of 6 to 10 mg/L except in the 

hypolimnetic water of the reservoirs, where seasonal stratification sometimes produced dissolved-oxygen concentrations 

approaching 2 mg/L or less.

Major-ion composition of selected waters was plotted 

on a Piper diagram (fig. 13) to determine if the composition 

of water is changing as it moves from one part of the system 

to another. Adams Tunnel east portal was chosen because it 

represents the composition of water that is diverted from the 

western slope and reflects any urban influences present on the 

west side of the Continental Divide. Olympus Tunnel represents water with urban influences from Estes Park. Horsetooth 

Reservoir represents water with urban influences around the 

reservoir and sedimentary substrate, and any influences of 

wastewater treatment, septic tanks, and highway runoff in 

water diverted from the Dille Tunnel near the mouth of Big 

Thompson Canyon. Cation compositions in water from all 

three sites are somewhat mixed calcium-magnesium-sodium, 

but calcium is predominant (fig. 13). Anion compositions are 

primarily bicarbonate. Adams and Olympus Tunnel water had 

the most variability, with some samples containing relatively 

less bicarbonate and more sulfate. Horsetooth Reservoir 

composition was less variable but is mostly within the range of 

water sampled from the tunnels. Despite having larger concentration of dissolved solids, Horsetooth Reservoir water seemed 

to be a more concentrated version of the tunnel water. 

Nutrients

Compounds of nitrogen and phosphorus are referred to as 

“nutrients” because they are the major building blocks of plant 

growth. In excess quantities, nutrients can promote nuisance 

algae growth in streams and reservoirs. Natural sources of 

nutrients include precipitation and biochemical processes in 

the watershed. Possible anthropogenic sources of nutrients in 

the CBT system include the following:

1. Urban runoff.—Associated with developed areas and 

roads, storm runoff may have higher nutrient concentrations and, as road runoff, may bypass natural removal 

processes in soil by rapid delivery to a receiving stream 

or reservoir;

2. Sewage effluent.—Septic-tank effluent can move with 

ground water into surface-water systems. Rural areas are 

often developed beyond the practical extent of municipal 

sewer lines, necessitating the use of septic leach fields. 

Although the town of Grand Lake has installed a sewer 

system (Three Lakes Sanitation District), not all residences within the watershed are connected to it. Estes 

Park effluent is treated (Estes Park Sanitation District 

and Upper Thompson Sanitation District) and returned to 

the Big Thompson River. Other scattered development in 

the Big Thompson watershed is not connected to municipal sewer systems; and

3. Erosion of soil and plant materials exposed near roads 

or streams can cause loading of organic nitrogen and 

particulate phosphorus when transported to a stream.

Nutrient concentrations at CBT sites generally were low 

(fig. 12). Data censoring levels changed many times throughout the study, and data analyses at times were of insufficient 

precision to fully characterize concentration. Median concentrations of ammonia plus organic nitrogen were less than 

0.3 mg/L at all sites. Median concentrations of dissolved 

nitrite plus nitrate concentrations were generally less than 

0.2 mg/L, and median concentrations of total phosphorus were 

less than 0.05 mg/L at all sites. Concentrations of nitrite and 

orthophosphorus were rarely reported. The largest concentrations of nutrients tended to be at Carter Lake and Horsetooth 

Reservoir. The smallest concentrations were generally measured at Lake Granby and Shadow Mountain Lake. Bottom 

samples tended to have larger concentrations of most nutrients 

than surface samples at the same site.
14 Water Quality and Trend Analysis of Colorado–Big Thompson System Reservoirs and Related Conveyances EXPLANATION 90th percentile 75th percentile Median 25th percentile 10th percentile Reporting limit Data point plotted when number of observations is less than 10 (202) Number of observations Adams Tunnel Carter Lake top Carter Lake bottom Lake Granby top Lake Granby bottom Granby Pump Canal Lake Granby Rainbow Bay top Lake Granby Rainbow Bay bottom Horsetooth Reservoir Soldier Dam top Horsetooth Reservoir Soldier Dam bottom Horsetooth Reservoir Spring Dam top Horsetooth Reservoir Spring Dam bottom Olympus Tunnel Shadow Mountain Lake top Shadow Mountain Lake bottom Lake Estes top Lake Estes bottom

0

15

5

10

OXYGEN, DISSOLVED,

IN MILLIGRAMS PER LITER

3210

CHLORIDE, DISSOLVED,

IN MILLIGRAMS PER LITER

840

SULFATE, DISSOLVED,

IN MILLIGRAMS PER LITER

0

200100

SPECIFIC CONDUCTANCE,

IN MICROSIEMENS 

PER CENTIMETER

(52) (173) (34) (37) (60) (57) (8) (35) (58) (34) (58) (58) (8) (59) (194) (35) (56)

(51) (181) (35) (37) (60) (57) (8) (35) (58) (34) (58) (58) (8) (59) (202) (35) (56)

(158) (244) (125) (124) (78) (61) (8) (50) (65) (50) (58) (60) (8) (69) (279) (156) (179)

(146) (241) (133) (129) (80) (60) (8) (49) (65) (49) (58) (59) (8) (72) (276) (143) (170)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17

123456789

1011121314151617

Figure 12. Boxplots of selected water-quality constituents.
Water Quality 15

)

EXPLANATION

Number of observations90th percentile75th percentileMedian25th percentile10th percentileReporting limit

(188)

0

1.00.5

NITROGEN, AMMONIA PLUS 

ORGANIC, TOTAL, IN 

MILLIGRAMS PER LITER

0

0.150.100.05

NITROGEN, AMMONIA, 

DISSOLVED, IN 

MILLIGRAMS PER LITER

0.3

NITROGEN, NITRITE PLUS 

NITRATE, DISSOLVED, 

IN MILLIGRAMS PER LITER

(53) (40) (35) (42) (78) (59) (8) (42) (65) (42) (58) (59) (8) (70) (188) (35) (139)

(91) (38) (64) (76) (59) (59) (8) (42) (59) (41) (58) (59) (8 (58) (70) (66) (53)

(127) (121) (98) (102) (53) (54) (8) (33) (54) (35) (53) (53) (8) (56) (122) (108) (48)

0

0.20.1

(126) (178) (104) (98) (73) (54) (5) (36) (60) (39) (55) (55) (5) (66) (179) (131) (134)

0.001

1

0.0050.002

0.050.020.01

0.50.20.1

PHOSPHORUS, TOTAL,

IN MILLIGRAMS PER LITER

Adams TunnelCarter Lake topCarter Lake bottomLake Granby topLake Granby bottomGranby Pump CanalLake Granby Rainbow Bay topLake Granby Rainbow Bay bottomHorsetooth Reservoir Soldier Dam topHorsetooth Reservoir Soldier Dam bottomHorsetooth Reservoir Spring Dam topHorsetooth Reservoir Spring Dam bottomOlympus TunnelShadow Mountain Lake topShadow Mountain Lake bottomLake Estes topLake Estes bottom

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17

123456789

1011121314151617

Data point plotted when number of observations is less than 10

Figure 12. Boxplots of selected water-quality constituents.—Continued
16 Water Quality and Trend Analysis of Colorado–Big Thompson System Reservoirs and Related Conveyances EXPLANATION 90th percentile 75th percentile Median 25th percentile 10th percentile Reporting limit

100

0.150.100.05

0

0.30.20.1

0.001

10

0.01

0.1

1

note: log scale

Adams TunnelCarter Lake topCarter Lake bottomLake Granby topLake Granby bottomGranby Pump CanalLake Granby Rainbow Bay topLake Granby Rainbow Bay bottomHorsetooth Reservoir Soldier Dam topHorsetooth Reservoir Soldier Dam botto

m

Horsetooth Reservoir Spring Dam topHorsetooth Reservoir Spring Dam bottomOlympus TunnelShadow Mountain Lake topShadow Mountain Lake bottomLake Estes topLake Estes bottom

AMMONIA, DISSOLVED,

IN MILLIGRAMS PER LITER

NITRITE PLUS NITRATE, 

DISSOLVED, IN 

MILLIGRAMS PER LITER

PHOSPHORUS, TOTAL

IN MILLIGRAMS PER LITER AMMONIA PLUS ORGANIC

NITROGEN, TOTAL,

IN MILLIGRAMS PER LITER

1

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17

23456789

1011121314151617

Data point plotted when number of observations is less than 10

Figure 12. Boxplots of selected water-quality constituents.—Continued
Water Quality 17

Trace Elements

Trace elements, for the purpose of this report, are metal 

and transition metal elements commonly found in small (less 

than 1 milligram per liter) concentrations. Analytes commonly 

detected at most sites in this study included barium, iron, manganese, strontium, and zinc. Most samples also were analyzed 

for barium, beryllium, boron, cadmium, chromium, cobalt, 

copper, lead, lithium, molybdenum, nickel, silver, and vanadium. Trace elements are important indicators of water quality 

because, in large concentrations, they are toxic to aquatic life; 

a large proportion of Colorado’s regulated constituents are 

trace elements (Colorado Department of Public Health and 

Environment, Water Quality Control Commission, 2001).

Concentrations of common dissolved trace elements 

(such as iron and manganese) were commonly above 

Minimum Reporting Limits (MRL’s) but usually less than 

1 milligram per liter (fig. 12). Others, such as beryllium, 

chromium, cobalt, lithium, molybdenum, nickel, silver, and 

vanadium, were almost never reported. Zinc, lead, cadmium, 

CALCIUM

SODIUM PLUS POTASSIUM

CARBONATE PLUS BICARBONATE

 100 

 80 

 60 

 40 

 20 

 0 

 0 

 20 

 40 

 60 

 80 

 100 0 

 20 

 40 

 60 

 80 

 100 

 0 

 20 

 40 

 60 

 80 

 100 

 100 

 80 

 60 

 40 

 20 

 0 100 

 80 

 60 

 40 

 20 

 0 

 0 

 20 

 40 

 60 

 80 

 100 

 0 

 20 

 40 

 60 

 80 

 100 100 

 80 

 60 

 40 

 20 

 0 

 100 

 80 

 60 

 40 

 20 

 0 

PERCENT

PERCENT

MAGNESIUM

SULFATE PLUS CHLORIDE

CALCIUM PLUS MAGNESIUM

SULFATE

CHLORIDE, FLUORIDE, NITRITE PLUS NITRATE

PERCENT

EXPLANATION

Adams Tunnel

Olympus Tunnel

Horsetooth Reservoir

Figure 13. Piper diagram showing major-ion composition of samples at selected sites.
18 Water Quality and Trend Analysis of Colorado–Big Thompson System Reservoirs and Related Conveyances

0

0

20406080

0

OCTOBER

180

20406080

100120140160

0

180

20406080

100120140160

DEPTH BELOW WATER SURFACE, IN FEET

MAY

OCTOBER

MAY

OCTOBER OCTOBER

MAY

100 50

SPECIFIC CONDUCTANCE,

AT 25 DEGREES

CELSIUS

180

20406080

100120140160

DISSOLVED OXYGEN,

IN MILLIGRAMS PER LITER

180

20406080

100120140160

0 5 10

WATER TEMPERATURE,

IN DEGREES CELSIUS

180

20406080

100120140160

180

6 7

pH, IN STANDARD UNITS

180

20406080

100120140160

OCTOBER

0

20406080

100120140160

MAY

100120140160180

0 0

10 5 0 15 0 8 9

0

Figure 14. May and October field-measurement profiles at Lake Granby (dam).
Water Quality 19

DEPTH BELOW WATER SURFACE, IN FEET

SPECIFIC CONDUCTANCE,

AT 25 DEGREES

CELSIUS

DISSOLVED OXYGEN,

IN MILLIGRAMS PER LITER

WATER TEMPERATURE,

IN DEGREES CELSIUS

pH, IN STANDARD UNITS

0

35

5

1015202530

0

35

5

1015202530

0

35

5

1015202530

0

30

5

10152025

0

35

5

1015202530

0

30

5

10152025

0

30

5

10152025

0

30

5

10152025

80 20 0 40 60 12 0 2 4 6 8 10 14 2 0 4 6 8 10 12 678 9

OCTOBER OCTOBER OCTOBER OCTOBER

MAY MAY MAY MAY

Figure 15. May and October field-measurement profiles at Shadow Mountain Lake.
20 Water Quality and Trend Analysis of Colorado–Big Thompson System Reservoirs and Related Conveyances

Figure 16.

0

 May and October field-measurement profiles at Carter Lake.

180

20406080

100120140160

DEPTH BELOW WATER SURFACE, IN FEET

0

140

20406080

100120

OCTOBER

MAY

OCTOBER

MAY

OCTOBER

MAY

OCTOBER

MAY

15 0 5 10 0 100 50 20 5 10 15 6 7 8 9

SPECIFIC CONDUCTANCE,

AT 25 DEGREES

CELSIUS

DISSOLVED OXYGEN,

IN MILLIGRAMS PER LITER

WATER TEMPERATURE,

IN DEGREES CELSIUS

pH, IN STANDARD UNITS
Water Quality 21 SPECIFIC CONDUCTANCE, AT 25 DEGREES CELSIUS

DEPTH BELOW WATER SURFACE, IN FEET

DISSOLVED OXYGEN,

IN MILLIGRAMS PER LITER

WATER TEMPERATURE,

IN DEGREES CELSIUS

pH, IN STANDARD UNITS

0

180

20406080

100120140160

0

160

20406080

100120140

MAY

OCTOBER

MAY

OCTOBER

MAY

OCTOBER

MAY

OCTOBER

0 100 50 12 2 0 4 6 8 10 20 5 0 10 15 7 6 89

Figure 17. May and October field-measurement profiles at Horsetooth Reservoir (Soldier Canyon Dam).
22 Water Quality and Trend Analysis of Colorado–Big Thompson System Reservoirs and Related Conveyances

boron, and copper concentrations were reported infrequently, 

and when reported, ranged from a few micrograms per liter to 

a few tens of micrograms per liter (zinc). Sources of the more 

toxic trace elements (such as copper, cadmium, lead, and zinc) 

are generally associated with ore deposits or urban runoff. The 

relative lack of such ore deposits and the relatively low density 

of development in the drainage areas of the CBT project will 

tend to limit concentrations of these toxic trace elements to 

relatively low levels.

Colorado Water-Quality Standards

In Colorado, water-quality standards are determined by 

the Colorado Water Quality Control Commission and consist 

of a narrative or numeric restriction established to protect the 

beneficial uses of water. Water-quality standards for streams in 

Colorado are based on use classifications such as aquatic life, 

recreation, water supply, and agriculture. Some water-quality 

standards for streams are set to specific values or concentrations (such as dissolved oxygen, pH, nitrite, nitrate, chloride, 

sulfate, iron, and manganese). Others, such as un-ionized 

ammonia and some trace elements, are usually site specific 

and use hardness data and equations for each element to calculate standards (Colorado Department of Public Health and 

Environment, Water Quality Control Commission, 2001). In 

certain cases, fixed numeric standards have been established 

that reflect site-specific conditions and may specify traceelement phases and analytical techniques (such as dissolved 

or total recoverable methods).

Trace-element concentrations are assumed to be 

dissolved unless designated otherwise by the numeric 

standards (Colorado Department of Public Health and 

Environment, Water Quality Control Commission, 2001). 

Acute standards are values or concentrations not to be 

exceeded for 1 day. Chronic standards are not to be exceeded 

by the 85th percentile of sample concentrations in a representative period. Extraordinary ephemeral or seasonal conditions 

do not constitute violations. Stream-standard concentrations and values are not to be exceeded more than once every 

3 years on average (Colorado Department of Public Health 

and Environment, Water Quality Control Commission, 2001). 

Thus, continued systematic monitoring is needed to determine 

compliance or noncompliance with a standard. 

The State of Colorado specifies that hardness concentrations used to calculate the trace-element standards be computed from the lower 95-percent confidence limit of the mean 

hardness at the periodic low-flow criteria determined from 

regression analysis of site-specific or regional data. In this 

study, however, hardness concentrations were determined from 

analysis of water samples collected at the time of sampling. 

The comparison of standards in this report, therefore, is a 

general indicator.

The use classifications for CBT sampling sites for this 

report are Aquatic life cold 1, Recreation 1, Water supply, 

and Agriculture (Colorado Department of Public Health and 

Environment, Water Quality Control Commission, 2001). 

Estimates of the Colorado water-quality standards for samples 

collected at the 10 sampling sites were calculated on the 

basis of numeric fixed standards or equations (Table of Value 

Standards [TVS]) for each element (Colorado Department 

of Public Health and Environment, Water Quality Control 

Commission, 2001). Laboratory MRL’s for trace elements 

such as cadmium, mercury, silver, and lead were commonly 

too high to determine concentrations at the level of the standard. These constituents might be present in the sample at 

concentrations lower than the laboratory MRL but higher than 

the standard. Samples were not analyzed for some constituents 

listed in the Colorado water-quality standards (for example, 

arsenic, chlorine, cyanide, selenium, and sulfide). 

Based on all samples collected over the period of record, 

standards for chloride, sulfate, un-ionized ammonia, nitrate, 

dissolved chromium, and dissolved nickel were not exceeded 

at any of the sampling sites in the study area. Dissolved 

oxygen exceeded minimum concentrations, and pH was not 

below or above standards. Silver, lead, and cadmium concentrations occasionally exceeded computed standard values. The 

fixed standard for dissolved iron (300 μg/L) was exceeded at 

Lake Granby (two samples), Adams Tunnel (one sample), and 

Shadow Mountain Lake (one sample). The fixed standard for 

dissolved manganese (50 μg/L) was exceeded at Lake Granby 

(10 samples), Horsetooth Reservoir (17 samples), and Shadow 

Mountain Lake (17 samples). Many of the manganese exceedances were in the early 1990’s, mainly in near-bottom samples. 

The fixed standard for manganese was not exceeded at Carter 

Lake. The presence of manganese is a common problem in the 

bottom water of lakes and reservoirs and can adversely affect 

drinking-water treatment, cause staining of laundry when used 

for washing, and when it precipitates in excessive quantities, 

can smother aquatic life (Niyogi and others, 1999). 

Limnology

The following sections of the report will discuss issues 

of water quality that pertain to the reservoir sites. Conditions 

in the reservoirs affect not only that particular water body but 

the conveyances and streams into which that water is released, 

diverted, or pumped.

Reservoir Profile Characteristics

Reservoir profile data are routinely measured when 

water-quality samples are collected. These measurements are 

typically made with an instrument that contains sensors for 

water temperature, specific conductance, pH, and dissolved 

oxygen. The instrument is lowered beneath the surface from 

a boat that is held as stationary as possible over the sampling 

location. Successive readings are taken throughout the depth 

profile when the instrument stabilizes at a particular depth. 
Water Quality 23

Plots of profiles measured in May and October at 

Lake Granby, Shadow Mountain Lake, Carter Lake, and 

Horsetooth Reservoir are shown in figures 14 to 17. The profiles are intended to show how these measurements vary with 

depth at the beginning of the ice-free season (May) and the 

end of the ice-free season (October). May and October 

also were the two months commonly measured at all sites 

a number of times during the study period.

By May, water-temperature stratification had already 

begun at most of the reservoirs, particularly at the lower 

elevations of Carter Lake and Horsetooth Reservoir, where 

conditions may be ice-free a couple of months earlier. Specific 

conductance, pH, and dissolved oxygen have not yet become 

strongly stratified with depth in May, and dissolved-oxygen 

concentrations are relatively high throughout the profiles. 

In the fall (October), conditions at the deeper reservoirs 

usually remained strongly stratified from summer heating 

except at Shadow Mountain Lake, where shallow depths 

(mean depth approximately 15 ft) interfere with the full 

development of a thermocline. In the other reservoirs, the top 

of the thermocline was at approximately 30 to 60 ft of depth 

most years in October (figs. 14–17). During stratification, 

specific conductance sometimes increased slightly with depth 

in Lake Granby and Horsetooth Reservoir. Dissolved oxygen 

was sometimes hypoxic in October profiles in the hypolimnion of the deeper reservoirs, and pH generally decreased 

with depth in May and October profiles at Lake Granby, 

Horsetooth Reservoir, and Carter Lake. The lowest hypolimnetic dissolved-oxygen concentrations in October were from 

Lake Granby and Horsetooth Reservoir, where near-bottom 

concentrations in some years were less than 2 mg/L.

Hypolimnetic Oxygen Dynamics

Dissolved-oxygen profiles show the distribution of 

oxygen concentrations with depth, but the shape of the profiles, their seasonal characteristics, reservoir water level, and 

hydraulic residence time (flushing rate) can be highly variable. This variation makes it difficult to compare from year 

to year or site to site. One approach to using the dissolvedoxygen profile data involves making computations that take 

into account reservoir bathymetry, reservoir contents, and the 

development of the oxygen profile during the growing season. 

In this way, there is the potential to compare dissolved-oxygen 

dynamics from year to year. Because low concentrations of 

dissolved oxygen can be cited as a consequence of eutrophic 

conditions, a comprehensive measure of those concentrations 

can contribute to the understanding of trends in eutrophication.

The zone in a reservoir where oxygen depletion is 

generally greatest is in the hypolimnion. The hypolimnion is 

the deep portion of the water-temperature profile, below the 

thermocline, that becomes isolated from the epilimnion and 

metalimnion during temperature stratification. After becoming isolated from the atmosphere and other sources of oxygen, 

the oxygen concentration begins to decrease steadily throughout the stratification season as a result of decay processes 

that steadily consume oxygen. The rate at which oxygen is 

consumed is indicative of the rate of biological activity, which 

is indicative of the level of eutrophication (Green, 1996). 

The hypolimnion does not remix and reoxygenate until fall 

overturn in lakes that are stratified in summer. These lakes 

restratify in winter and remix again in spring. 

The technique of computing and using areal hypolimnetic 

oxygen deficits for analysis of eutrophication is described in 

detail in Green (1996) and will be described briefly in this 

report. Through the use of one of the PROFILE routines in the 

U.S. Army Corps of Engineers BATHTUB model (Walker, 

1996), areal hypolimnetic oxygen deficit (AHOD) was computed for each growing season in each of four reservoirs (Lake 

Granby, Shadow Mountain Lake, Carter Lake, and Horsetooth 

Reservoir) when sufficient profile data were available. The 

model can be used with data inputs of bathymetry, profile 

data, estimates of the elevation of the hypolimnion for each 

profile, and time series of reservoir water levels to make computations of AHOD, which is defined as the change in oxygen 

per square meter of hypolimnetic surface per day through 

the stratification season (approximately May to September or 

October). To ensure that fall overturn reoxygenation has not 

begun, Walker (1996) recommends that the profile period of 

computation for AHOD begin with a spring profile in which 

stratification has already been established (usually May in this 

study) and end with a profile that is not beyond the peak of 

near-bottom oxygen depletion (usually July or August in this 

study). AHOD is computed as the rate of change (slope) of the 

areal hypolimnetic oxygen content (AHOC) computed by the 

model through the stratification season.

Time-series plots of computed annual AHOD are shown 

in figure 18 for Shadow Mountain Lake, Lake Granby, Carter 

Lake, and Horsetooth Reservoir. There may be patterns present 

in the AHOD data, and the cycles are not necessarily the same 

for each reservoir. These patterns suggest that climatic, biological, or operational cycles may influence the rates of oxygen 

consumption in these reservoirs. The AHODs for each reservoir 

should not be directly compared to one another because they 

include the influences of water temperature and morphological 

characteristics, which may vary in each reservoir.

Trophic Indicators and Nutrient Limitation

The relative fertility of a lake or reservoir can be evaluated by assessing the trophic status. Oligotrophic (nutrientpoor) lakes have characteristics such as high transparency, 

small organic-matter content, relatively large dissolvedoxygen concentrations, small nutrient concentrations, and 

small algal biomass. Eutrophic (nutrient-rich) lakes have the 

opposite characteristics (Woods, 1992). On the basis of data 

collected over the period of record at each site, CBT reservoirs 

were assessed using the method developed by Carlson (1977). 
24 Water Quality and Trend Analysis of Colorado–Big Thompson System Reservoirs and Related Conveyances

Trophic-state index (TSI) values were calculated from the 

following equations:

 TSI(SD) = 60 – 14.41(ln SD) (1)

 TSI(TP) = 14.42(ln TP) + 4.15 (2)

 TSI(CHLA) = 9.81(ln CHLA) + 30.6 (3)

where

SD is Secchi-disk depth, in meters;

TP is total phosphorus concentration, in micrograms 

per liter;

CHLA is chlorophyll-a concentration (high-performance liquid chromatography was used), in 

micrograms per liter;

and

ln is the natural logarithm of the indicated variable.

Secchi-disk depth, total phosphorus (surface only), and 

chlorophyll-a values were averaged for each year to compute 

mean values. The annual mean values were then used to compute annual TSI values plotted in figure 19. Censored values of 

total phosphorus were common in the 1990’s, and not all TSI’s 

could be computed. The Carlson (1977) TSI index assumes 

phosphorus limitation. The boundary between oligotrophic 

(nutrient-poor) and mesotrophic (moderate nutrients) is a TSI 

value of 30. The boundary between mesotrophic and eutrophic 

(nutrient-rich) is a TSI value of 50. Most of the TSI values 

for the reservoirs in this study are in the mesotrophic range. 

A tendency toward decreasing trophic status in the 1990’s at 

Lake Granby and Shadow Mountain Lake is based primarily 

on Secchi depth, and chlorophyll-a concentrations and can be 

seen in the plots. 

AREAL HYPOLIMNETIC OXYGEN DEFICIT, IN MILLIGRAMS PER SQUARE METER PER DAY

0

100

20

40

60

80

0

1,000

200

400

600

800

0

2,000

500

1,000

1,500

1976 1978 1980 1982 1984 1986 1988 1990 1992 1994 1996 1998 2000 2002

0

1,500

500

1,000

SHADOW MOUNTAIN LAKE

LAKE GRANBY

CARTER LAKE

HORSETOOTH RESERVOIR

Figure 18. Time-series plots of annual areal hypolimnetic oxygen deficit in reservoirs.
Water Quality 25

Figure 19. Annual trophic status index (TSI) computations for Secchi depth, total phosphorus, and 

chlorophyll-a by the Carlson (1977) method.

The indices are meant to normalize the three indicators 

so that any of the three could describe the conditions in the 

water body. At Lake Granby and Shadow Mountain Lake, the 

three indices agree fairly well. At Carter Lake and Horsetooth 

Reservoir, the index for phosphorus does not compare well 

with the indices for Secchi depth and chlorophyll-a. Turbid 

conditions caused by suspended sediment could raise both the 

total phosphorus and Secchi-depth TSI’s relatively but might 

not affect or might even decrease the chlorophyll-a index 

due to reduced light penetration. Other cyclic factors such as 

grazing of phytoplankton by zooplankton can make interpretation of chlorophyll-a indices problematic. The use of TSI can 

give a qualitative indication of relative status of water bodies 

but should not be used exclusively to evaluate whether lakes 

and reservoirs are meeting the criteria of their water-quality 

classifications.

SHADOW MOUNTAIN LAKE

OLIGOTROPHIC

MESOTROPHIC

EUTROPHIC

0

100

20

40

60

80

LAKE GRANBY RAINBOW BAY

OLIGOTROPHIC

MESOTROPHIC

EUTROPHIC

CARTER LAKE

OLIGOTROPHIC

MESOTROPHIC

EUTROPHIC

HORSETOOTH RESERVOIR SPRING 

CANYON DAM

MESOTROPHIC

EUTROPHIC

1980 1990 2000 1970 2000 1980 1990

HORSETOOTH RESERVOIR SOLDIER 

CANYON DAM

OLIGOTROPHIC

TROPHIC STATUS INDEX, DIMENSIONLESS

0

100

20

40

60

80

1970

0

100

20

40

60

80

OLIGOTROPHIC

MESOTROPHIC

EUTROPHIC

LAKE GRANBY NEAR DAM

OLIGOTROPHIC

MESOTROPHIC

EUTROPHIC

EXPLANATION

TSI, Secchi depth

TSI, Total phosphorus

TSI, Chlorophyll-a
26 Water Quality and Trend Analysis of Colorado–Big Thompson System Reservoirs and Related Conveyances

Nitrogen (N) to phosphorus (P) mass ratios have been 

used to characterize the limiting nutrient for algal growth 

present in water (Britton and Gaggiani, 1987; Woods, 1992). 

If a nutrient is limiting, the hypothesis is that the addition of 

that nutrient would then cause an increase in algal production (in the absence of other limiting factors) (Britton and 

Gaggiani, 1987; Woods, 1992). Due to the large number of 

censored nitrogen and phosphorus concentrations, the N/P 

ratios were not analyzed. An analysis of N/P ratios would 

indicate only certain conditions when nutrient concentrations 

were high (greater than minimum reporting limits) and not the 

full range of ratios for different concentration conditions in the 

reservoirs.

Trend Analysis

Time-series trend analysis is a common approach to 

evaluating the temporal changes in long-term data sets. The 

method chosen for the CBT data is the seasonal Kendall 

nonparametric test. The test is robust in that it does not require 

normality, can be used with censored data, and seasonal effects 

that can obscure the detection of a trend can be removed. The 

test is computed on the ranks of the data and will indicate 

the significance of monotonic trends (generally increasing or 

decreasing with time) and provide an estimate of the slope of 

the change in the median of the data. Further details on the 

method and applications can be found in publications such as 

Helsel and Hirsch (1995), Middelburg (1993), Clark and others (2000), and Mueller (1990).

Time-Series Data

Time-series data were compiled from the USGS database 

for the period of record at each site. To help with visual assimilation of these large data sets, scatterplots of each constituent of 

interest were made and a trace of a LOWESS smooth regression 

was drawn unless periods of data were missing or a series of 

values were less than MRL (see figs. 23–37 in “Hydrologic and 

Water-Quality Data” section at back of report). The LOWESS 

smooth curve was estimated using the MRL in cases where data 

were censored. Total ammonia and total nitrate were considered 

similar to dissolved ammonia and dissolved nitrite plus nitrate 

for plotting and trend-testing purposes. Mueller (1990) made a 

comparison of the total and dissolved data for these constituents 

and found no statistical difference. If a field specific conductance or pH value was missing or erroneous, the laboratory 

value was substituted.

Certain characteristics of the data are visible in the plots 

that are not evident in the application of the statistical trend 

test. Variability, nonmonotonic trends, and nonseasonal cycles 

are important information that are not conveyed by a p-value 

or slope. 

A few general observations about the data from the timeseries plots (see figs. 23–37 in “Hydrologic and Water-Quality 

Data” section at back of report) are listed here:

1. Many time-series data are not exclusively monotonic, 

a condition that could obscure small-scale trends in the 

trend testing. For example, a normal distribution curve 

would represent strong trends if it were a time series, 

but the symmetrical qualities would cancel out the 

monotonic properties and no significant trends would 

be computed. This problem is particularly evident in 

the nutrient data at some of the sites that have long 

periods of record. Sites such as Granby Pump Canal, 

Olympus Tunnel, Adams Tunnel east portal, Horsetooth 

Reservoir at Soldier Canyon Dam, and Lake Granby 

Dam show clearly some peaking of nutrient concentrations in the early 1980’s (despite the problematic changes 

in minimum reporting limits) that might not be assessed 

adequately by monotonic trend testing of the full period 

of record (fig. 20). Other constituents are well characterized as monotonic distributions (fig. 21).

2. In general, specific conductance, dissolved solids, 

and most of the major ions at most sites have similar 

LOWESS curve shapes, indicating that some of the data 

for constituents and properties may be providing redundant trend information.

3. Because the data at most sites are collected in a very 

limited number of seasons, some of the variability and 

representativeness of the annual water-quality changes 

at the sites are potentially lost. Trend results apply to 

seasons represented in the data set.

4. Apparent decreases in nutrient concentrations throughout the system are accompanied by decreases in iron 

(Fe) and manganese (Mn) concentrations. This pattern 

could indicate a relation between fertility and bottomwater hypoxia (which can cause reducing conditions that 

release Fe and Mn).

5. The degree of variability is high in the chlorophyll-a

data, indicating that sampling frequency may not be sufficient to adequately describe algal population dynamics 

that occur in short time scales.

Some qualitative temporal trend information based on 

visual inspection of the time-series plots is summarized here:

1. Field measurements of dissolved oxygen and pH are 

relatively stable through time at most sites. Small fluctuations are probably related to factors such as changes 

in water temperature, water depth, collection date, and 

reservoir contents. Slight decreases in dissolved oxygen 

are shown by the LOWESS curves in the near-bottom 

samples at Lake Granby (Rainbow Bay) (fig. 26), Carter 

Lake (fig. 33), and Horsetooth Reservoir (fig. 35).

2. Specific conductance, dissolved solids, and most major 

ions show decreases in concentrations at most sites 

over time. Since 1998, however, downward trends have 

flattened at sites on the western slope (Lake Granby, 

Shadow Mountain Lake, Granby Pump Canal), and since 

1998, upward trends are evident at Horsetooth Reservoir. 

Some decreases might be related to high runoff years 

when large volumes of water containing low dissolved 

solids are stored, influencing concentrations for many 
Trend Analysis 27

EXPLANATION

LOWESS CURVE

 ALL DATA

LOWESS CURVE

 SEASONAL

UNCENSORED

CENSORED

NITROGEN, AMMONIA PLUS ORGANIC,TOTAL,

IN MILLIGRAMS PER LITER

1970 1972 1974 1976 1978 1980 1982 1984 1986 1988 1990 1992 1994 1996 1998 2000

0

2.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

1.8

Pre-1983 data

Upward trend

Slope = 0.1008 mg/L per year

p-value = less than 0.0001 

All data 1978 to 2000

Downward trend

Slope = –0.006888 mg/L per year

p-value = less than 0.0001

Post-1982 data

Downward trend

Slope = –0.00857 mg/L per year

p-value = less than 0.0001

1970 1972 1974 1976 1978 1980 1982 1984 1986 1988 1990 1992 1994 1996 1998 2000

0

300

50

100

150

200

250

IRON, DISSOLVED, IN MICROGRAMS PER LITER

All data 1978 to 2000

Downward trend

Slope = –1.791 micrograms per year

p-value = less than 0.0001

EXPLANATION

LOWESS CURVE

 ALL DATA

LOWESS CURVE

 SEASONAL

UNCENSORED

CENSORED

Figure 20. Nonmonotonic distribution and seasonal Kendall test results for pre-1983 and post-1983 of ammonia plus 

organic nitrogen (total) at Adams Tunnel east portal.

Figure 21. Monotonic distribution and seasonal Kendall test results for all samples of dissolved iron at Adams 

Tunnel east portal.
28 Water Quality and Trend Analysis of Colorado–Big Thompson System Reservoirs and Related Conveyances

Constituents chosen for analysis included field measurements, 

major ions, nutrients, chlorophyll-a and -b, fecal and total coliform bacteria, and trace elements that were routinely reported 

above censoring levels.

Trend for a particular constituent was only computed 

when less than 50 percent of the data was below the MRL and 

more than 5 years of data was available. Lake Estes had less 

than 5 years of data and was therefore excluded. If more than 

5 percent of the data was censored, the slope of the median 

was not reported. Small breaks in the time series were acceptable if they occurred in the middle one-third of the data. Many 

of the nutrient species at particular sites were mostly censored 

data and were not tested. Top- and bottom-sample constituents were tested separately. No flow adjustments were made 

because all sites were reservoirs or conveyances receiving 

water from reservoirs. Because multiple MRL’s were common, 

all censored data were recensored to the highest MRL. Or, in a 

few cases, the highest MRL data in a series were ignored if the 

data could be eliminated by minor shortening of the beginning 

or ending of the period of record for the analysis. Estimated 

data were treated as legitimate. Most outliers were allowed to 

remain, as the data sets commonly were very large, and the 

seasonal Kendall method is relatively unaffected by outliers. 

The choice of available seasons was limited as sampling 

frequency at most sites was down to three samples per year 

near the end of the period of record. Seasons were chosen to 

maximize the length of record for testing. The seasons and 

periods of record used for testing each site are listed in table 2. 

The period of record tested may differ slightly depending on 

the particular constituent or property and the sampling history. 

When multiple samples were available for a particular season, 

the most complete set of sample constituents was first criterion, 

and the sample closest to the beginning of the target month was 

the next criterion for choosing among multiple samples.

Summary results of trend testing are summarized in 

table 3. Detailed summaries by site that include p-values and 

any trend slopes that were computed are listed in Hydrologic 

and Water-Quality Data tables 21 to 35 at back of report. 

Three alpha-significance levels were used in the summaries: 

greater than 90 percent was considered significant; greater 

than 95 percent was moderately significant; and greater than 

99 percent was highly significant. The slope is the median of 

all slopes between data pairs in the same season and is reported 

in the tables in units of change per year for the median. Positive slopes indicate upward trends (increasing concentrations 

or values with time); negative slopes indicate downward trends 

(decreasing concentrations or values) with time.

Among field measurements, a downward (decreasing) 

trend in specific conductance was common at all sites except 

surface samples in Shadow Mountain Lake (site 4S). An 

upward (increasing) temporal trend for pH was noted at all the 

conveyances and the surface samples of Carter Lake (site 8S) 

and Horsetooth Reservoirs (sites 9S only). A negative trend 

in pH was computed for the near-bottom samples from Lake 

Granby Rainbow Bay (site 2B). Trends for dissolved-oxygen 

concentrations decreased in the near-bottom samples at Lake 

Granby Rainbow Bay (site 2B), Carter Lake (site 8B), and 

years afterward, as that dilute water is pumped throughout the system from the Lake Granby storage. The dilutional effects of high runoff in 1995 can be seen in many 

plots (figs. 23–37).

3. At sites with 20 years or more years of record, nutrient 

concentrations have decreased (sometimes substantially) 

from early 1980’s levels. Multiple MRL’s and large 

amounts of data below the MRL in recent years have 

made it difficult to characterize the most recent trends. 

It is clear, however, that nutrient concentrations seem 

to have peaked sometime in the 1970’s and early 1980’s 

and have stayed at relatively low levels during the 1990’s. 

Good examples of this pattern can be seen in some plots 

of ammonia plus organic nitrogen, ammonia, nitrite plus 

nitrate, and total phosphorus for Adams Tunnel (fig. 30), 

Olympus Tunnel (fig. 31), Granby Pump Canal (fig. 27), 

Horsetooth Reservoir (Soldier Canyon Dam) (figs. 34 

and 35), and Carter Lake (figs. 32 and 33). This pattern 

is much less definitive in Lake Granby (figs. 23 and 24).

4. Trace elements such as iron (Fe) and manganese (Mn) 

have a high degree of variability that is probably caused 

by seasonal reducing conditions. Reducing conditions 

can cause the release of Fe and Mn from bottom sediments and are proportional to the degree of hypolimnetic 

oxygen depletion in a given growing season and the proximity to which the near-bottom sample can be collected 

close to the reservoir bottom sediment (a variable and 

uncertain exercise). Similar to nutrients, concentrations of 

Fe and Mn in the long records at Adams Tunnel (fig. 30), 

Olympus Tunnel (fig. 31), and Horsetooth Reservoir 

(figs. 33 and 35, iron only) peak in the early 1980’s 

and generally decline through 2000. 

5. Miscellaneous measurements such as transparency, 

chlorophyll-a, and bacteria also show patterns that 

can be observed on time-series plots. Records of transparency generally show increasing clarity in CBT system 

reservoirs from 1970 to 2000, although seasonal variability 

is fairly high (often 50 to 100 inches or more over a few 

months). The record for chlorophyll-a is relatively short 

(about 11 years). Two peaks of concentration can be seen 

at Lake Granby (spillway) (fig. 23), Shadow Mountain 

Reservoir (fig. 28), and Horsetooth Reservoir (figs. 34 

and 36) corresponding to approximately 1990–91 and 

1995–96. Concentrations of bacteria are generally very 

low (less than 10 colonies per 100 milliliters) for all sites. 

Some large spikes in concentrations of fecal and total coliform bacteria (more than 100 colonies per 100 milliliters) 

fecal coliform and more than 1,000 colonies per 100 milliliters total coliform were reported in the early 1980’s at 

Olympus Tunnel (fig. 31).

Statistical Testing

Seasonal Kendall test (Helsel and Hirsch, 1995) was 

used to determine the significance of monotonic temporal 

trends over the period of record for selected constituents. 
Trend Analysis 29

Horsetooth Reservoir (sites 9B and 10B), and increased in the 

near-bottom samples of Lake Granby spillway (site 1B) and 

Shadow Mountain Lake (site 4B).

 In general, major ions and hardness at all sites showed 

significant downward trends. The exceptions were silica, 

which indicated upward trends at Shadow Mountain (site 4), 

Adams Tunnel (site 5), and all eastern slope sites (sites 6–10), 

and chloride, which indicated upward trends at Carter Lake 

(site 8) and Horsetooth Reservoir (sites 9 and 10). Trends for 

fluoride and sodium adsorption ratio were not significant. 

When significant, dissolved-solids trends were downward.

Among nutrients, a small number of significant trends 

were mostly stable (no trend) or downward. Downward trends 

were computed for total phosphorus at the Granby Pump Canal 

(site 3), Olympus Tunnel (site 7), Carter Lake (site 8), and 

the surface samples at Horsetooth Reservoir (site 9S). Downward trends also are noted for ammonia plus organic nitrogen 

(total) at Granby Pump Canal (site 3), Shadow Mountain Lake 

(site 4S), and Adams Tunnel east portal (site 5). Nitrite plus 

nitrate had a significant downward trend at the Granby Pump 

Canal (site 3) and an upward trend at Horsetooth Reservoir 

(site 10B) in the near-bottom sample.

Transparency and chlorophyll-a variables, which are 

directly related to perceptions of eutrophication, showed some 

improvement with time in Lake Granby (sites 1S and 2S). 

Transparency showed an upward trend and chlorophyll-a a 

downward trend at both of the surface sites in Lake Granby. 

The only other significant trend in transparency was an 

upward trend at Horsetooth Reservoir (site 9S). No significant 

trends in fecal coliform bacteria were detected. 

Among trace elements, only barium, iron, manganese, 

strontium, and zinc had sufficient uncensored data for trend 

testing. Downward trends in iron and manganese were notable 

at Lake Granby (sites 1 and 2, Fe-Mn), Granby Pump Canal 

(site 3, Fe-Mn), and Shadow Mountain Lake (site 4S and 

4B, Mn only). On the eastern slope, iron showed downward 

trends at Adams Tunnel (site 5), Olympus Tunnel (site 7), and 

Carter Lake (site 8S and 8B). The trend in manganese was 

downward at Olympus Tunnel (site 7). Strontium, a minor 

ion with similar chemistry to the major ions, showed a 

downward trend at all sites. Zinc was a downward trend 

at Olympus Tunnel (site 7), the only significant trend for 

zinc. The trend in barium was downward at Carter Lake 

(site 8S and 8B) and upward in the near-bottom samples at 

Horsetooth Reservoir (site 9B).

The AHOD data computed for Lake Granby, Shadow 

Mountain Lake, Carter Lake, and Horsetooth Reservoir at 

the dam locations (related to hypolimnetic oxygen dynamics described earlier) also were tested for temporal trends. 

First, the annual AHOD data were correlated with year 

(temporal variable) by using a parametric Pearson correlation. R-squared values ranged from less than 0.1 (Carter 

Lake Reservoir) to 0.14 (Granby Reservoir), indicating 

no linear correlation. Green (1996) established a correlation between AHOD and reciprocal hydraulic residence 

time (RHRT) that explained additional variance in the 

temporal correlation. Plots of time-series RHRT (May to 

August each year) at four CBT reservoirs were computed 

using annual inflow or outflow data and average reservoir 

contents for each year (fig. 22). With CBT reservoir data, 

a correlation of log(AHOD) with log(RHRT) yielded 

r-squared values ranging from 0.11 to 0.65. Encouraged 

by the potential of the residence time to improve the 

temporal correlation, a regression model for log(AHOD) 

that included log(RHRT) and the year was computed and 

yielded r-squared values ranging from 0.12 to 0.65. Residuals from a logAHOD/logRHRT regression (effects of 

RHRT removed) were then regressed with year. The largest 

r-squared obtained was 0.13, indicating little evidence of 

AHOD changes with time (temporal trend). The AHOD 

data and RHRT data were then tested for trend using 

Kendall’s tau and testing correlation with time. None of 

the AHOD, RHRT, or residuals from the log(AHOD) and 

log(RHRT) relation data had significant temporal trends.

Table 2. Seasonal periods and period of record tested by seasonal Kendall test.

[Seasons are shown as ranges in months of the year; the hyphens connect months that represent ranges of the seasonal periods]

Site

number

(fig. 1)

Site name Seasons Period 

of record

1 Lake Granby near Granby, Colo. (near spillway) May–June, July–Aug, Sept, Oct 1973–present

2 Lake Granby near Granby, Colo. (near dam in Rainbow Bay) May–June, July–Aug, Sept, Oct 1989–present

3 Granby Pump Canal near Grand Lake, Colo. No seasonal partition 1970–present

4 Shadow Mountain Lake near Grand Lake, Colo. May, July–Aug, Sept 1989–present

5 Alva B. Adams Tunnel east portal, near Estes Park, Colo. Bimonthly 1970–present

6 Lake Estes near dam near Estes Park, Colo. Insufficient record (less than 5 years) 1998–present

7 Olympus Tunnel at Lake Estes, Colo. Mar–Apr, July, Nov 1970–present

8 Carter Lake near Berthoud, Colo. Apr–May, July–Aug, Sept–Oct 1970–present

9 Horsetooth Reservoir near Fort Collins, Colo. (Soldier Canyon Dam) May–June, July–Aug, Sept–Oct 1970–present

10 Horsetooth Reservoir near Fort Collins, Colo. (Spring Canyon Dam) May–June, July–Aug, Sept–Oct 1983–present
30 Water Quality and Trend Analysis of Colorado–Big Thompson System Reservoirs and Related Conveyances Table 3. Summary of significant upward and downward temporal trends at Colorado-Big Thompson sampling sites. [--, insufficient data; (+), upward trend; (–, downward trend; 0, trend not significant; significance: *, greater than 90 percent; **, greater than 95 percent; ***, greater than 99 percent; A, no trend computation because censored values exceeded 50 percent of data; site 6 was not tested because period of record was less than 5 years; S, surface sample; B, near-bottom sample; site names listed in table 1; mg/L, milli- grams per liter; μS/cm, microsiemens per centimeter at 25 degrees Celsius; °, degree Celsius; mL, milliliter; μg/L, micrograms per liter] Property or constituent Significant trends by study site Site 1(S) Site 1(B) Site 2(S) Site 2(B) Site 

3

Site 

4(S)

Site 

4(B)

Site 

5

Site 

7

Site 

8(S)

Site 

8(B)

Site 

9(S)

Site 

9(B)

Site 

10(S)

Site 

10(B)

Oxygen, dissolved (mg/L) (–)** (–)*** 0 (–)* 0 0 (+)*** 0 (+)** 0 (–)*** 0 (–)*** 0 (–)*pH, field (standard units) 0 0 0 (–)** (+)*** 0 0 (+)*** (+)* (+)** 0 (+)*** 0 0(–)*** 0Specific conductance (μS/cm) (–)*** (–)*** (–)*** (–)*** (–)*** 0 (–)*** (–)*** (–)*** (–)*** (–)*** (–)*** (–)*** (–)*** (–)***

Water temperature (°C) 0 (–)*** 0 (–)*** (+)* -- -- (+)*** 0 0 (+)** 0 (+)* (–)* (+)**Hardness, total (mg/L) (–)*** (–)*** (–)*** (–)*** (–)*** (–)* (–)*** 0 (–)** (–)*** (–)*** (–)*** (–)*** (–)** (–)***Calcium (mg/L) (–)*** (–)*** (–)*** (–)*** (–)*** (–)** (–)*** 0 (–)** (–)*** (–)*** (–)*** (–)*** (–)** (–)***Magnesium (mg/L) (–)*** (–)*** (–)*** (–)*** (–)*** 0 0 (–)*** (–)*** (–)*** (–)*** (–)* (–)*** (–)***Potassium (mg/L) (–)*** (–)*** (–)*** (–)*** (–)*** 0 0 (–)*** 0 0 0 0 0 0 0Sodium adsorption ratio 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0Sodium, dissolved (mg/L) (–)*** (–)*** (–)*** (–)*** (–)*** (–)*** (–)*** (–)*** 0 (–)*** (–)*** (–)** (–)** (–)** (–)*Sodium (percent) (–)*** (–)*** (–)*** (–)*** (–)*** (–)*** (–)*** 0 0 0 0 0 0 0 (+)**Acid-neuralizing capacity (mg/L as CaCO3) (–)*** (–)*** (–)*** (–)*** (–)*** (–)* (–)*** 0 0 (–)*** (–)*** (–)*** (–)*** (–)* (–)***

Chloride, dissolved (mg/L) 0 0 0 0 (–)** 0 0 0 0 (+)*** (+)*** (+)*** (+)*** (+)*** (+)***Fluoride, dissolved (mg/L) 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0Silica, dissolved (mg/L) 0 0 0 0 0 (+)** 0 (+)*** (+)*** (+)*** (+)*** (+)** (+)* (+)* (+)***Sulfate, dissolved (mg/L) (–)*** (–)*** (–)*** (–)*** (–)*** (–)*** (–)*** (–)*** (–)*** (–)*** *** -- 0 (–)* (–)***Nitrogen, ammonia plus organic, total (mg/L as N) 0 A 0 0 (–)*** (–)*** 0 (–)*** 0 0 A 0 A 0 ANitrogen, ammonia, dissolved (mg/L as N) A A A A A A 0 A 0 0 0 0 0 A 0Nitrogen, nitrite plus nitrate, dissolved (mg/L as N) A A A A (–)*** A A 0 0 A A A 0 A (+)*Nitrogen, nitrite, dissolved (mg/L as N) A A A A A A A A A A A A A A APhosphorus, dissolved (mg/L as P) A A A A A A A 0 (–)*** A A A A A APhosphorus, ortho, dissolved (mg/L as P) A A A A A A A 0 A A A A A A APhosphorus, total (mg/L as P) A 0 0 0 (–)*** 0 0 A (–)*** (–)*** (–)*** (–)*** 0 A 0Residue, dissolved at 180°C (mg/L) (–)** 0 0 (–)* (–)* 0 (–)* 0 0 (–)*** (–)*** (–)*** (–)*** 0 0Dissolved solids, sum of constituents (–)*** (–)*** (–)*** (–)*** (–)*** 0 (–)** 0 (–)** (–)*** (–)*** (–)** (–)*** 0 (–)*Transparency, Secchi disk (inches) (–)*** -- (+)*** -- -- 0 -- -- -- 0 -- (+)*** -- 0 --Coliform, fecal (colonies per 100 mL) A -- A -- -- A -- -- -- A -- A -- 0 --Chlorophyll-a (μg/L) (–)*** -- (–)*** -- -- 0 -- -- -- 0 -- 0 -- 0 --

Chlorophyll-b (μg/L) A -- A -- -- A -- -- -- -- -- A -- A --

Barium, dissolved (μg/L) 0 0 0 0 0 0 (0)** 0 0 (–)* (–)* 0 (+)* 0 0Iron, dissolved (μg/L) (–)*** (–)*** (–)*** (–)*** (–)*** 0 (–)*** (–)*** (–)*** (–)** 0 0 0 0Manganese, dissolved (μg/L) A (–)*** (–)*** (–)*** (–)*** (–)*** (–)*** -- (–)** A 0 0 A 0 0Strontium, dissolved (μg/L) (–)*** (–)*** (–)*** (–)*** (–)*** (–)** (–)*** (–)*** (–)** (–)*** (–)*** (–)*** (–)*** (–)*** (–)***Zinc, dissolved (μg/L) A 0 A A 0 A 0 0 (–)** 0 0 0 0 A A
Summary and Conclusions 31

Summary and Conclusions

The U.S. Geological Survey (USGS), in an ongoing cooperative monitoring program with the Northern 

Colorado Water Conservancy District (NCWCD), Bureau 

of Reclamation (BOR), and City of Fort Collins, has collected water-quality data in reservoirs and conveyances such 

as canals and tunnels related to the Colorado–Big Thompson 

(CBT) Project in north-central Colorado since 1969. 

This report assessed and summarized (1) waterquality and field-measurement profile data collected 

by USGS and stored in the USGS National Water 

Information System, (2) time-series trends of chemical 

constituents and physical properties by using the seasonal 

Kendall trend test method, (3) oxygen deficits in the hypolimnion (reservoirs only), (4) trophic status indicators, and 

(5) water-quality data in terms of Colorado water-quality 

standards.

0

R

0.015

0.005

0.010

0.02

0.08

0.04

0.06

PROPORTION OF RESERVOIR 

VOLUME LEAVING RESERVOIR PER

DAY THROUGH OUTLET AND

GRANBY PUMP CANAL, MAY

THROUGH AUGUST EACH YEAR

PROPORTION OF RESERVOIR 

VOLUME PUMPED INTO

RESERVOIR BY GRANBY

PUMP CANAL PER DAY, MAY 

THROUGH AUGUST EACH YEAR

0.01

10

0.02

0.05

0.1

0.2

0.5

1

2

5

1976

0

0.06

0.02

0.04

PROPORTION OF RESERVOIR VOLUME ENTERING 

RESERVOIR PER DAY,

MAY THROUGH AUGUST EACH YEAR 

SHADOW MOUNTAIN LAKE

LAKE GRANBY

CARTER LAKE

HORSETOOTH RESERVOI

1978 1980 1982 1984 1986 1988 1990 1992 1994 1996 1998 2000 2002

Figure 22. Time-series plots of reservoir reciprocal hydraulic residence time in terms of the proportion of water entering or 

leaving the water body.
32 Water Quality and Trend Analysis of Colorado–Big Thompson System Reservoirs and Related Conveyances

Water at CBT sites is dilute and typical of high-elevation 

snowmelt runoff. Median concentrations of specific conductance at sites on the western slope (Lake Granby and Shadow 

Mountain Lake) and the high-elevation sites on the eastern 

slope (Adams Tunnel east portal, Lake Estes, and Olympus 

Tunnel) were generally below 60 microsiemens per centimeter 

at 25 degrees Celsius. These sites are indicative of a hydrologic setting that is typical of snowmelt source water—low 

rates of evapotranspiration (ET) and crystalline bedrock 

geology. Sites with large specific conductances (generally 

greater than 70 microsiemens per centimeter at 25 degrees 

Celsius) and concentrations of major ions are Carter Lake and 

Horsetooth Reservoir. These sites show the cumulative effects 

of urban development and ET from the rest of the storage and 

conveyance system. These sites are in lower elevations (higher 

ET) in areas of sedimentary rocks that can increase dissolvedsolids content. Even so, all water sampled in this system 

should be considered to be of high quality in terms of low 

dissolved-solids content.

Water temperature varied seasonally, with reservoir sites 

developing strong stratification patterns. The pH at CBT sites 

was generally in a neutral range of 7 to 8. Dissolved oxygen 

was generally in the range of 6 to 10 milligrams per liter, 

except in the hypolimnetic water of the reservoirs, where 

seasonal stratification sometimes produced dissolved-oxygen 

concentrations approaching 2 milligrams per liter or less.

Cation compositions at Adams Tunnel, Olympus 

Tunnel, and Horsetooth Reservoir are somewhat mixed 

calcium-magnesium-sodium, but calcium is predominant. 

Anion compositions are primarily bicarbonate. Despite having 

larger concentrations of dissolved solids, Horsetooth Reservoir 

water seems to be simply a more concentrated version of the 

tunnel waters. 

Nutrient concentrations at CBT sites generally were 

low. Data censoring levels changed many times throughout 

the study and at times had insufficiently high reporting levels 

to fully characterize concentration. Largest nutrient concentrations tended to occur at Carter Lake and Horsetooth 

Reservoirs. The smallest nutrient concentrations were generally measured at Lake Granby and Shadow Mountain Lake. 

Bottom samples tended to have larger concentrations of most 

nutrients than did surface samples at the same site.

Concentrations of common dissolved trace elements 

such as iron and manganese commonly exceeded MRL’s, but 

concentrations were usually less than 1 milligram per liter. 

Beryllium, chromium, cobalt, lithium, molybdenum, nickel, 

silver, and vanadium were almost never detected. Zinc, lead, 

cadmium, boron, and copper concentrations were reported 

infrequently, and concentrations ranged from a few micrograms per liter to a few tens of micrograms per liter (zinc). 

Based on all samples collected over the period of 

record, standards for dissolved oxygen, pH, chloride, sulfate, 

un-ionized ammonia, nitrate, dissolved chromium, and dissolved nickel were not exceeded at any of the sampling sites 

in the study area. Silver, lead, copper, cadmium, and zinc 

concentrations occasionally exceeded computed standard 

values. The fixed standard for dissolved iron (300 micrograms per liter) was exceeded in two samples at Lake Granby, 

one sample at Adams Tunnel east portal, and one sample at 

Shadow Mountain Lake. The fixed standard for dissolved 

manganese (50 micrograms per liter) was exceeded at Lake 

Granby (10 samples), Horsetooth Reservoir (17 samples), and 

Shadow Mountain Lake (17 samples). Many of the exceedances of manganese were in the early 1990’s in mainly nearbottom samples. The fixed standard for manganese was not 

exceeded at Carter Lake.

Time-series of computed annual areal hypolimnetic oxygen deficit (AHOD) were plotted for Shadow Mountain Lake, 

Lake Granby, Carter Lake, and Horsetooth Reservoir. Multiyear patterns are evident in the AHOD data, and the cycles 

are not necessarily the same for each reservoir. This pattern 

suggests that climatic, biological, or operational cycles may 

influence the rates of oxygen consumption in these reservoirs. 

On the basis of data collected over the period of record 

at each site, CBT reservoirs were assessed for trophic status 

using the Trophic-State Index (TSI) method developed by 

Carlson (1977). Most of the index values for the reservoirs 

are in the mesotrophic range most years. A tendency toward 

decreasing trophic status in the 1990’s at all reservoirs based 

mostly on Secchi depth and chlorophyll-a can be seen in timeseries plots of index values. The use of TSI can give a qualitative indication of relative status of water bodies but should not 

be used exclusively to evaluate whether lakes and reservoirs 

are meeting the criteria of their water-quality classifications. 

Time-series data were compiled from the USGS database 

for the period of record at each site. To help with visual assimilation of these large data sets, scatterplots of each constituent of interest were made and a trace of a LOWESS smooth 

regression was drawn when sufficient data over the minimum 

reporting limit were present. 

A few general observations and interpretations from the 

time-series plots were made:

1. Many time-series data are not exclusively monotonic, 

a condition that could obscure small-scale trends in the 

trend testing. Other constituents are well characterized 

as monotonic distributions.

2. Specific conductance, dissolved solids, and most of the 

major ions have similar LOWESS curve shapes, indicating that some of the constituent concentrations may be 

providing redundant trend information.

3. Because the data at most sites are collected in a very limited number of seasons, some of the variability and representativeness of the annual water-quality changes at the 

sites are potentially lost. Trend results should be applied 

cautiously to seasons not represented in the sampling.

4. Apparent decreases in nutrient concentrations throughout the system are accompanied by decreases in iron 

(Fe) and manganese (Mn) concentrations. This pattern 
References Cited 33

could indicate a relation between fertility and bottomwater hypoxia (which can cause reducing conditions that 

release Fe and Mn).

5. The degree of variability in the chlorophyll-a data is 

high, indicating that sampling frequency is not dense 

enough to adequately describe algal population dynamics 

that occur on short time scales.

Seasonal Kendall test was used to determine the significance of monotonic temporal trends over the period of record 

for field measurements, major ions, nutrients, chlorophyll-a

and -b, fecal and total coliform bacteria, and trace elements 

that were routinely reported above censoring levels.

Among field measurements, a downward trend in specific 

conductance was common to all sites except surface samples 

in Shadow Mountain Lake. In general, major ions and hardness at all sites showed significant downward trends. The 

exceptions were (1) silica, which had an upward trend at 

Shadow Mountain Lake, Adams Tunnel east portal, and all 

eastern slope sites, and (2) chloride, which indicated upward 

trends at Carter Lake and Horsetooth Reservoir. Fluoride and 

sodium adsorption ratio had no significant trends. When significant, dissolved-solids trends were downward.

Among nutrients that met testing criteria, most either had 

no evidence of trend or a downward trend. Transparency and 

chlorophyll-a, variables that are directly related to perceptions of eutrophication, showed some improvement with time 

in Lake Granby. The trend for transparency was upward and 

chlorophyll-a was downward at both of the surface sites in 

Lake Granby. The only other significant trend in these variables was an upward trend in transparency at the Horsetooth 

Reservoir (Soldier Canyon Dam). No significant trends in 

fecal coliform bacteria were detected. Among trace elements, 

only barium, iron, manganese, strontium, and zinc had sufficient uncensored data for trend testing. Downward trends in 

iron and manganese were notable at Lake Granby (Fe-Mn), 

Granby Pump Canal (Fe-Mn), and Shadow Mountain Lake 

(Mn only). On the eastern slope, the trend in iron was upward 

at Adams Tunnel, Olympus Tunnel, and Carter Lake. Manganese trend was downward at Adams and Olympus Tunnels. 

The zinc trend was downward at Olympus Tunnel, the only 

significant trend for zinc.

The AHOD data (computed for Lake Granby, Shadow 

Mountain Lake, Carter Lake, and Horsetooth Reservoir, at 

the dam locations) related to hypolimnetic oxygen dynamics 

also were tested for temporal trend. A regression model that 

included log(AHOD), log(RHRT), and the year, was computed. The model yielded r-squared values that ranged from 

0.12 to 0.65. Residuals from a logAHOD/logRHRT regression (effects of RHRT removed) were then regressed with 

year. The largest r-squared obtained was 0.13, indicating little 

evidence of AHOD changes with time (temporal trend). The 

AHOD data and RHRT data were then tested for trend by 

using Kendall’s tau and testing correlation with time. None 

of the AHOD, RHRT, or residuals from the log(AHOD) and 

log(RHRT) relation data had significant temporal trends.

Major conclusions of the study were as follows:

1. Water quality was generally good throughout the 

CBT system over the period of record, which ranged 

from 1969 to 2000. Dissolved solids and nutrient 

concentrations were low and typical of a forested/

mountainous/crystalline bedrock hydrologic setting. 

Most of the more toxic trace elements were rarely 

detected or were found in low concentrations, 

which was due at least in part to a relative lack of oremineral deposits within the drainage areas of the CBT 

project.

2. Constituent concentrations consistently met water-quality 

standard thresholds set by the State of Colorado.

3. Trophic-state index values indicated mesotrophic conditions generally prevail at reservoirs based on available 

Secchi depth, total phosphorus concentrations, and 

chlorophyll-a concentrations. 

4. Based on plots of time-series values and concentrations 

and seasonal Kendall nonparametric trend testing, 

concentrations of dissolved solids and most major ions 

were decreasing at most sites. Many of the nutrient timeseries data did not meet the minimum criteria for testing. 

But for those that did, nutrient concentrations had insufficient evidence of trends or downward temporal trends 

(ammonia plus organic nitrogen and total phosphorus). 

Iron and manganese data either had insufficient evidence 

of trend or indicated downward trends. Chlorophyll-a

data were collected for only 11 years but generally indicated insufficient evidence of trend or downward temporal trends. 

References Cited

Britton, L.J., and Gaggiani, N.G., 1987, Water-quality assessment of Arvada Reservoir, Denver Metropolitan area, Colorado: U.S. Geological Survey Water-Resources Investigations Report 87–4107, 66 p.

Carlson, R.E., 1977, A trophic state index for lakes: Limnology and Oceanography, v. 22, no. 2, p. 361–369.

Clark, M.L., Eddy-Miller, C.A., and Mast, M.A., 2000, 

Environmental characteristics and water quality of Hydrologic Benchmark Network stations in West-Central United 

States, 1963–95: U.S. Geological Survey Circular 1173–C, 

115 p.

Colorado Climate Center, 1984, Colorado average annual precipitation map 1951–1980: Fort Collins, Colorado Climate 

Center map, scale 1:500,000.
34 Water Quality and Trend Analysis of Colorado–Big Thompson System Reservoirs and Related Conveyances

Colorado Department of Public Health and Environment, 

Water Quality Control Commission, 2001, Classifications 

and numeric standards for South Platte River Basin; Laramie River Basin; Republican River Basin; Smoky Hill River 

Basin: Denver, Colorado Department of Public Health and 

Environment, Water Quality Control Commission Report, 

3.8.0 (5 CCR 1002–8), variously paginated.

Green, W.R., 1996, Eutrophication trends inferred from hypolimnetic dissolved-oxygen dynamics within selected White 

River reservoirs, northern Arkansas-southern Missouri, 

1974–94: U.S. Geological Survey Water-Resources Investigations Report 96–4096, 22 p.

Greve, A.I., 2002, Data-quality measures of stakeholderimplemented watershed-monitoring programs: U.S. Geological Survey Open-File Report 02–141, 19 p.

Helsel, D.R., and Cohn, T.A., 1988, Estimation of descriptive 

statistics for multiply censored water-quality data: Water 

Resources Research, v. 24, p. 1997–2004.

Helsel, D.R., and Hirsch, R.M., 1995, Statistical methods 

in water resources, studies in environmental science 49, 

3d ed.: New York, Elsevier, 529 p.

Jassby, A.D., and Goldman, C.R., 1999, Horsetooth and Carter 

Lake Reservoirs, water quality comparisons: Davis, Calif., 

Report of Ecological Research Associates, 49 p.

Johnson, B.M., and Goettl, J.P., 1999, Food web changes over 

fourteen years following introduction of rainbow smelt into 

a Colorado reservoir: North American Journal of Fisheries 

Management, v. 19, p. 629–642.

Middelburg, R.F., 1993, Colorado stream water quality, 

in Paulson, R.W., and others, National water summary, 

Hydrologic events and stream water quality: U.S. 

Geological Survey Water-Supply Paper 2400, 

p. 204.

Mueller, D.K., 1990, Analysis of water-quality data and sampling programs at selected sites in north-central Colorado: 

U.S. Geological Survey Water-Resources Investigations 

Report 90–4005, 79 p.

Niyogi, D.K., McKnight, D.M., and Lewis, W.M., 1999, 

Influences of water and substrate quality for periphyton in a montane stream affected by acid mine drainage: Limnology and Oceanography v. 44, no. 3, 

p. 804–814.

Stevens, M.R., 2001, Assessment of water quality, road runoff, 

and bulk atmospheric deposition, Guanella Pass area, Clear 

Creek and Park counties, Colorado, water years 1995–97: 

U.S. Geological Survey Water-Resources Investigations 

Report 00–4186, 183 p.

Walker, W.W., 1996, Simplified procedures for eutrophication 

assessment and prediction, user manual: U.S. Army Corps 

of Engineers, Waterways Experiment Station, Instruction 

Report W–96–2, 227 p.

Woods, P.F., 1992, Limnology of Big Lake, south-central 

Alaska, 1983–84: U.S. Geological Survey Water-Supply 

Paper 2382, 108 p.
Hydrologic and Water-Quality Data
Hydrologic and Water-Quality Data 37

Abbreviations

CaCO3 Calcium carbonate

Ca Calcium

Mg Magnesium

K Potassium

Na Sodium

Cl Chloride

F Fluoride

SiO2 Silicon dioxide

SO4 Sulfate

N Nitrogen

P Phosphorus

As Arsenic

Ba Barium

Be Beryllium

B Boron

Cd Cadmium

Cr Chromium

Co Cobalt

Cu Copper

Fe Iron

Pb Lead

Li Lithium

Mn Manganese

Mo Molybdenum

Ni Nickel

Ag Silver

Sr Strontium

V Vanadium

Zn Zinc
38 Water Quality and Trend Analysis of Colorado–Big Thompson System Reservoirs and Related Conveyances

0

20

5

10

15

OXYGEN, DISSOLVED,

IN MILLIGRAMS PER LITER

5

9

6

7

8

pH, IN STANDARD UNITS

0

100

20

40

60

80

SPECIFIC CONDUCTANCE, IN

MICROSIEMENS PER 

CENTIMETER AT 

25 DEGREES CELSIUS

0

10

20

WATER TEMPERATURE,

IN DEGREES CELSIUS

EXPLANATION

LOWESS CURVE

 ALL DATA

LOWESS CURVE

 SEASONAL

UNCENSORED

CENSORED

1970 1975 1980 1985 1990 1995 2000 1970 1975 1980 1985 1990 1995 2000

Figure 23. Temporal variations in periodic measurements and concentrations at Lake Granby near spillway, sampled 

near the surface.
Hydrologic and Water-Quality Data 39

EXPLANATION

LOWESS CURVE

 ALL DATA

LOWESS CURVE

 SEASONAL

UNCENSORED

CENSORED

0

4

1

2

3

MAGNESIUM, DISSOLVED,

IN MILLIGRAMS PER LITER

0

2

1

POTASSIUM, DISSOLVED,

IN MILLIGRAMS PER LITER

0

0.4

0.1

0.2

0.3

SODIUM ADSORPTION RATIO

0

4

1

2

3

SODIUM, DISSOLVED,

IN MILLIGRAMS PER LITER

0

40

10

20

30

SODIUM, IN PERCENT

1970 1975 1980 1985 1990 1995 2000

1970 1975 1980 1985 1990 1995 2000

0

10

2

4

6

8

CALCIUM, DISSOLVED,

IN MILLIGRAMS PER LITER

0

40

10

20

30

HARDNESS, TOTAL,

IN MILLIGRAMS PER LITER AS

CACO3

Figure 23. Temporal variations in periodic measurements and concentrations at Lake Granby near spillway, sampled 

near the surface.—Continued
40 Water Quality and Trend Analysis of Colorado–Big Thompson System Reservoirs and Related Conveyances

EXPLANATION

LOWESS CURVE

 ALL DATA

LOWESS CURVE

 SEASONAL

UNCENSORED

CENSORED

0

80

20

40

60

SOLIDS, RESIDUE AT 180 

DEGREES CELSIUS, 

DISSOLVED, IN MILLIGRAMS 

PER LITER 

0

250

50

100

150

200

TRANSPARENCY, SECCHI,

IN INCHES

0

15

5

10

CHLOROPHYLL-a, 

PHYTOPLANKTON, IN 

MICROGRAMS PER LITER

0

80

20

40

60

SOLIDS, DISSOLVED, SUM

OF CONSTITUENTS,

IN MILLIGRAMS PER LITER

0

5

1

2

3

4

COLIFORM, FECAL, IN

COLONIES PER 100 MILLILITERS

0

0.4

0.1

0.2

0.3

CHLOROPHYLL-b, PHYTOPLANKTON,

IN MICROGRAMS PER LITER

1970 1975 1980 1985 1990 1995 2000 1970 1975 1980 1985 1990 1995 2000

Figure 23. Temporal variations in periodic measurements and concentrations at Lake Granby near spillway, sampled 

near the surface.—Continued
Hydrologic and Water-Quality Data 41

0

0.10

0.02

0.04

0.06

0.08

NITROGEN, AMMONIA, 

DISSOLVED,

IN MILLIGRAMS PER LITER

0

0.20

0.05

0.10

0.15

NITROGEN, NITRITE PLUS 

NITRATE, DISSOLVED, 

IN MILLIGRAMS PER LITER

0

0.10

0.02

0.04

0.06

0.08

NITROGEN, NITRITE, 

DISSOLVED,

IN MILLIGRAMS PER LITER

0

0.10

0.02

0.04

0.06

0.08

PHOSPHORUS, DISSOLVED,

IN MILLIGRAMS PER LITER

1970 1975 1980 1985 1990 1995 2000

0

0.10

0.02

0.04

0.06

0.08

PHOSPHORUS, ORTHO, 

DISSOLVED,

IN MILLIGRAMS PER LITER

1970 1975 1980 1985 1990 1995 2000

0

0.10

0.02

0.04

0.06

0.08

PHOSPHORUS, TOTAL,

IN MILLIGRAMS PER LITER

0

4

1

2

3

NITROGEN, AMMONIA PLUS 

ORGANIC, TOTAL, 

IN MILLIGRAMS PER LITER

EXPLANATION

LOWESS CURVE

 ALL DATA

LOWESS CURVE

 SEASONAL

UNCENSORED

CENSORED

Figure 23. Temporal variations in periodic measurements and concentrations at Lake Granby near spillway, sampled 

near the surface.—Continued
42 Water Quality and Trend Analysis of Colorado–Big Thompson System Reservoirs and Related Conveyances

EXPLANATION

LOWESS CURVE

 ALL DATA

LOWESS CURVE

 SEASONAL

UNCENSORED

CENSORED

0

2

1

CHLORIDE, DISSOLVED,

IN MILLIGRAMS PER LITER

1970

0

10

2

4

6

8

SILICA, DISSOLVED,

IN MILLIGRAMS PER LITER

1975 1980 1985 1990 1995 2000

0

40

10

20

30

ACID-NEUTRALIZING CAPACITY,

IN MILLIGRAMS PER

 LITER AS CACO3

0

0.4

0.1

0.2

0.3

FLUORIDE, DISSOLVED,

IN MILLIGRAMS PER LITER

1970 1975 1980 1985 1990 1995 2000

0

10

2

4

6

8

SULFATE, DISSOLVED,

IN MILLIGRAMS PER LITER

Figure 23. Temporal variations in periodic measurements and concentrations at Lake Granby near spillway, sampled 

near the surface.—Continued
Hydrologic and Water-Quality Data 43

EXPLANATION

LOWESS CURVE

 ALL DATA

LOWESS CURVE

 SEASONAL

UNCENSORED

CENSORED

BARIUM, DISSOLVED,

IN MICROGRAMS PER LITER

MANGANESE, DISSOLVED,

IN MICROGRAMS PER LITER

ZINC, DISSOLVED,

IN MICROGRAMS PER LITER

IRON, DISSOLVED,

IN MICROGRAMS PER LITER

STRONTIUM, DISSOLVED,

IN MICROGRAMS PER LITER

0

100

20

40

60

80

0

80

20

40

60

1970 1975 1980 1985 1990 1995 2000

1970 1975 1980 1985 1990 1995 2000

0

20

5

10

15

0

20

5

10

15

0

40

10

20

30

Figure 23. Temporal variations in periodic measurements and concentrations at Lake Granby near spillway, sampled 

near the surface.—Continued
44 Water Quality and Trend Analysis of Colorado–Big Thompson System Reservoirs and Related Conveyances

EXPLANATION

LOWESS CURVE

 ALL DATA

LOWESS CURVE

 SEASONAL

UNCENSORED

CENSORED

0

10

2

4

6

8

OXYGEN, DISSOLVED,

IN MILLIGRAMS PER LITER

0

100

20

40

60

80

SPECIFIC CONDUCTANCE,

IN MICROSIEMENS 

PER CENTIMETER

0

100

20

40

60

80

SOLIDS, RESIDUE AT 180 

DEGREES CELSIUS, DISSOLVED,

IN MILLIGRAMS PER LITER

1970 1975 1980 1985 1990 1995 2000

5

9

6

7

8

pH, IN STANDARD UNITS

0

20

5

10

15

WATER TEMPERATURE,

IN DEGREES CELSIUS

0

100

20

40

60

80

SOLIDS, DISSOLVED,

SUM OF CONSTITUENTS,

IN MILLIGRAMS PER LITER

1970 1975 1980 1985 1990 1995 2000

Figure 24. Temporal variations in periodic measurements and concentrations at Lake Granby near spillway, sampled 

near the bottom.
Hydrologic and Water-Quality Data 45

Figure 24. Temporal variations in periodic measurements and concentrations at Lake Granby near spillway, sampled 

near the bottom.—Continued

0

40

10

20

30

EXPLANATION

LOWESS CURVE

 ALL DATA

LOWESS CURVE

 SEASONAL

UNCENSORED

CENSORED

0

4

1

2

3

MAGNESIUM, DISSOLVED,

IN MILLIGRAMS PER LITER

0

0.4

0.1

0.2

0.3

SODIUM ADSORPTION RATIO

0

40

10

20

30

SODIUM, IN PERCENT

1970 1975 1980 1985 1990 1995 2000

0

20

5

10

15

CALCIUM, DISSOLVED,

IN MILLIGRAMS PER LITER

0

2

1

POTASSIUM, DISSOLVED,

IN MILLIGRAMS PER LITER

1970

0

4

1

2

3

SODIUM, DISSOLVED

1975 1980 1985 1990 1995 2000

HARDNESS, TOTAL,

IN MILLIGRAMS PER LITER

AS CACO3
46 Water Quality and Trend Analysis of Colorado–Big Thompson System Reservoirs and Related Conveyances

EXPLANATION

LOWESS CURVE

 ALL DATA

LOWESS CURVE

 SEASONAL

UNCENSORED

CENSORED

0

1.5

0.5

1.0

CHLORIDE, DISSOLVED,

IN MILLIGRAMS PER LITER

1970 1975 1980 1985 1990 1995 2000

10

ACID-NEUTRALIZING CAPACITY,

IN MILLIGRAMS PER LITER

0

15

5

10

SILICA, DISSOLVED,

IN MILLIGRAMS PER LITER

40

20

30

0

6

2

4

SULFATE, DISSOLVED,

IN MILLIGRAMS PER LITER

0

0.4

0.1

0.2

0.3

FLUORIDE, DISSOLVED,

IN MILLIGRAMS PER LITER

1970 1975 1980 1985 1990 1995 2000

Figure 24. Temporal variations in periodic measurements and concentrations at Lake Granby near spillway, sampled 

near the bottom.—Continued
Hydrologic and Water-Quality Data 47

Figure 24. Temporal variations in periodic measurements and concentrations at Lake Granby near spillway, sampled 

near the bottom.—Continued

EXPLANATION

LOWESS CURVE

 ALL DATA

LOWESS CURVE

 SEASONAL

UNCENSORED

CENSORED

0

0.10

0.02

0.04

0.06

0.08

NITROGEN, AMMONIA,

DISSOLVED, IN

MILLIGRAMS PER LITER

0

0.10

0.02

0.04

0.06

0.08

NITROGEN, NITRITE, 

DISSOLVED, IN

MILLIGRAMS PER LITER

1970

0

0.4

0.1

0.2

0.3

PHOSPHORUS, TOTAL,

IN MILLIGRAMS PER LITER

1975 1980 1985 1990 1995 2000

0

1.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

NITROGEN, AMMONIA PLUS 

ORGANIC, TOTAL, IN

MILLIGRAMS PER LITER

0

0.4

0.1

0.2

0.3

NITROGEN, NITRITE PLUS 

NITRATE, DISSOLVED, IN

MILLIGRAMS PER LITER

0

0.10

0.02

0.04

0.06

0.08

PHOSPHORUS, DISSOLVED,

IN MILLIGRAMS PER LITER

1970 1975 1980 1985 1990 1995 2000
48 Water Quality and Trend Analysis of Colorado–Big Thompson System Reservoirs and Related Conveyances

Figure 24. Temporal variations in periodic measurements and concentrations at Lake Granby near spillway, sampled 

near the bottom.—Continued

EXPLANATION

LOWESS CURVE

 ALL DATA

LOWESS CURVE

 SEASONAL

UNCENSORED

CENSORED

1970 1975 1980 1985 1990 1995 2000

0

80

20

40

60

STRONTIUM, DISSOLVED,

IN MICROGRAMS PER LITER

0

100

20

40

60

80

IRON, DISSOLVED,

IN MICROGRAMS PER LITER

0

20

5

10

15

BARIUM, DISSOLVED,

IN MICROGRAMS PER LITER

0

200

50

100

150

MANGANESE, DISSOLVED,

IN MICROGRAMS PER LITER

1970 1975 1980 1985 1990 1995 2000

0

60

20

40

ZINC, DISSOLVED,

IN MICROGRAMS PER LITER
Hydrologic and Water-Quality Data 49

EXPLANATION

LOWESS CURVE

 ALL DATA

LOWESS CURVE

 SEASONAL

UNCENSORED

CENSORED

5

9

6

7

8

pH, IN STANDARD UNITS

0

10

20

WATER TEMPERATURE,

IN DEGREES CELSIUS

1970 1975 1980 1985 1990 1995 2000

0

100

20

40

60

80

SOLIDS, DISSOLVED,

SUM OF CONSTITUENTS,

IN MILLIGRAMS PER LITER

1970 1975 1980 1985 1990 1995 2000

0

0

100

20

40

60

80

SOLIDS, DISSOLVED,

RESIDUE AT 180 DEGREES

CELSIUS, IN MILLIGRAMS

PER LITER

100

20

40

60

80

SPECIFIC CONDUCTANCE, IN

MICROSIEMENS PER 

CENTIMETER AT 25 DEGREES 

CELSIUS

5

15

6

7

8

9

10

11

12

13

14

OXYGEN, DISSOLVED,

IN MILLIGRAMS PER LITER

Figure 25. Temporal variations in periodic measurements and concentrations at Lake Granby near Rainbow Bay, sampled 

near the surface.
50 Water Quality and Trend Analysis of Colorado–Big Thompson System Reservoirs and Related Conveyances

0

EXPLANATION

LOWESS CURVE

 ALL DATA

LOWESS CURVE

 SEASONAL

UNCENSORED

CENSORED

40

10

20

30

HARDNESS, TOTAL,

IN MILLIGRAMS PER LITER

AS CACO3

0

4

1

2

3

MAGNESIUM, DISSOLVED,

IN MILLIGRAMS PER LITER

0

0.4

0.1

0.2

0.3

SODIUM ADSORPTION RATIO

10

30

15

20

25

SODIUM, IN PERCENT

1970 1975 1980 1985 1990 1995 2000

0

20

5

10

15

CALCIUM, DISSOLVED,

IN MILLIGRAMS PER LITER

0

2

1

POTASSIUM, DISSOLVED,

IN MILLIGRAMS PER LITER

0

4

1

2

3

SODIUM, DISSOLVED,

IN MILLIGRAMS PER LITER

1970 1975 1980 1985 1990 1995 2000

Figure 25. Temporal variations in periodic measurements and concentrations at Lake Granby near Rainbow Bay, sampled 

near the surface.—Continued
Hydrologic and Water-Quality Data 51

EXPLANATION

LOWESS CURVE

 ALL DATA

LOWESS CURVE

 SEASONAL

UNCENSORED

CENSORED

0

2

1

CHLORIDE, DISSOLVED,

IN MILLIGRAMS PER LITER

1970 1975 1980 1985 1990 1995 2000

0

10

2

4

6

8

SILICA, DISSOLVED,

IN MILLIGRAMS PER LITER

ACID-NEUTRALIZING CAPACITY,

IN MILLIGRAMS PER LITER 

AS CACO3

0

0.4

0.1

0.2

0.3

FLUORIDE, DISSOLVED,

IN MILLIGRAMS PER LITER

1970 1975 1980 1985 1990 1995 2000

5

2

1

3

4

SULFATE, DISSOLVED,

IN MILLIGRAMS PER LITER

40

0

10

20

30

0

Figure 25. Temporal variations in periodic measurements and concentrations at Lake Granby near Rainbow Bay, sampled 

near the surface.—Continued
52 Water Quality and Trend Analysis of Colorado–Big Thompson System Reservoirs and Related Conveyances

EXPLANATION

LOWESS CURVE

 ALL DATA

LOWESS CURVE

 SEASONAL

UNCENSORED

CENSORED

0

1.0

0.5

NITROGEN, AMMONIA PLUS 

ORGANIC, TOTAL, IN

MILLIGRAMS PER LITER

0

0.20

0.05

0.10

0.15

PHOSPHORUS, DISSOLVED,

IN MILLIGRAMS PER LITER

1970

0

0.08

0.02

0.04

0.06

PHOSPHORUS, TOTAL,

IN MILLIGRAMS PER LITER

0

0.20

0.05

0.10

0.15

NITROGEN, NITRITE PLUS 

NITRATE, DISSOLVED, IN

MILLIGRAMS PER LITER

1975 1980 1985 1990 1995 2000

0

0.10

0.02

0.04

0.06

0.08

NITROGEN, AMMONIA,

DISSOLVED, IN

MILLIGRAMS PER LITER

0

0.04

0.01

0.02

0.03

NITROGEN, NITRITE,

DISSOLVED, IN

MILLIGRAMS PER LITER

0

0.04

0.01

0.02

0.03

PHOSPHORUS, ORTHO, 

DISSOLVED, IN

MILLIGRAMS PER LITER

1970 1975 1980 1985 1990 1995 2000

Figure 25. Temporal variations in periodic measurements and concentrations at Lake Granby near Rainbow Bay, sampled 

near the surface.—Continued
Hydrologic and Water-Quality Data 53

EXPLANATION

LOWESS CURVE

 ALL DATA

LOWESS CURVE

 SEASONAL

UNCENSORED

CENSORED

0

300

100

200

TRANSPARENCY, SECCHI,

IN INCHES

0

10

2

4

6

8

CHLOROPHYLL-a, 

PHYTOPLANKTON,

IN MICROGRAMS PER LITER

1970 1975 1980 1985 1990 1995 2000

0

80

20

40

60

COLIFORM, FECAL, IN 

COLONIES PER 100 MILLILITERS

1970 1975 1980 1985 1990 1995 2000

0

0.4

0.1

0.2

0.3

CHLOROPHYLL-b, 

PHYTOPLANKTON,

IN MICROGRAMS PER LITER

Figure 25. Temporal variations in periodic measurements and concentrations at Lake Granby near Rainbow Bay, sampled 

near the surface.—Continued
54 Water Quality and Trend Analysis of Colorado–Big Thompson System Reservoirs and Related Conveyances

EXPLANATION

LOWESS CURVE

 ALL DATA

LOWESS CURVE

 SEASONAL

UNCENSORED

CENSORED

0

30

10

20

MANGANESE, DISSOLVED,

IN MICROGRAMS PER LITER

1970 1975 1980 1985 1990 1995 2000

0

30

10

20

ZINC, DISSOLVED, IN 

MICROGRAMS PER LITER

0

20

5

10

15

BARIUM, DISSOLVED, IN

MICROGRAMS PER LITER

0

100

20

40

60

80

IRON, DISSOLVED,

IN MICROGRAMS PER LITER

1970 1975 1980 1985 1990 1995 2000

0

80

20

40

60

STRONTIUM, DISSOLVED,

IN MICROGRAMS PER LITER

Figure 25. Temporal variations in periodic measurements and concentrations at Lake Granby near Rainbow Bay, sampled 

near the surface.—Continued
Hydrologic and Water-Quality Data 55

EXPLANATION

LOWESS CURVE

 ALL DATA

LOWESS CURVE

 SEASONAL

UNCENSORED

CENSORED

0

100

20

40

60

80

SPECIFIC CONDUCTANCE, IN

MICROSIEMENS PER

CENTIMETER AT 25 

DEGREES CELSIUS

0

100

20

40

60

80

DEPTH, IN FEET

0

80

20

40

60

SOLIDS, DISSOLVED, SUM

OF CONSTITUENTS, IN

MILLIGRAMS PER LITER

0

10

2

4

6

8

OXYGEN, DISSOLVED, IN

MILLIGRAMS PER LITER

1970 1975 1980 1985 1990 1995 2000

5

9

6

7

8

pH, IN STANDARD UNITS

0

10

20

WATER TEMPERATURE,

IN DEGREES CELSIUS

0

100

20

40

60

80

SOLIDS, RESIDUE AT 180 

DEGREES CELSIUS,

DISSOLVED, IN

MILLIGRAMS PER LITER

1970 1975 1980 1985 1990 1995 2000

Figure 26. Temporal variations in periodic measurements and concentrations at Lake Granby near Rainbow Bay, sampled 

near the bottom.
56 Water Quality and Trend Analysis of Colorado–Big Thompson System Reservoirs and Related Conveyances

EXPLANATION

LOWESS CURVE

 ALL DATA

LOWESS CURVE

 SEASONAL

UNCENSORED

CENSORED

0

3

1

2

MAGNESIUM, DISSOLVED, IN

MILLIGRAMS PER LITER

0

0.4

0.1

0.2

0.3

SODIUM ADSORPTION RATIO

1970 1975 1980 1985 1990 1995 2000

0

30

10

20

SODIUM, IN PERCENT

0

40

10

20

30

HARDNESS, TOTAL, IN

MILLIGRAMS PER LITER

AS CACO3

0

20

5

10

15

CALCIUM, DISSOLVED, IN

MILLIGRAMS PER LITER

0

2.0

0.5

1.0

1.5

POTASSIUM, DISSOLVED, IN

MILLIGRAMS PER LITER

1970 1975 1980 1985 1990 1995 2000

0

4

1

2

3

SODIUM, DISSOLVED, IN

MILLIGRAMS PER LITER

Figure 26. Temporal variations in periodic measurements and concentrations at Lake Granby near Rainbow Bay, sampled 

near the bottom.—Continued
Hydrologic and Water-Quality Data 57

EXPLANATION

LOWESS CURVE

 ALL DATA

LOWESS CURVE

 SEASONAL

UNCENSORED

CENSORED

0

1.0

0.5

CHLORIDE, DISSOLVED,

IN MILLIGRAMS PER LITER

0

0.4

0.1

0.2

0.3

FLUORIDE, DISSOLVED,

IN MILLIGRAMS PER LITER

1970 1975 1980 1985 1990 1995 2000

0

20

5

10

15

SILICA, DISSOLVED, IN

MILLIGRAMS PER LITER

1970 1975 1980 1985 1990 1995 2000

0

5

1

2

3

4

SULFATE, DISSOLVED, IN

MILLIGRAMS PER LITER

0

40

10

20

30

ACID-NEUTRALIZING CAPACITY,

 IN MILLIGRAMS PER LITER

AS CACO3

Figure 26. Temporal variations in periodic measurements and concentrations at Lake Granby near Rainbow Bay, sampled 

near the bottom.—Continued
58 Water Quality and Trend Analysis of Colorado–Big Thompson System Reservoirs and Related Conveyances

EXPLANATION

LOWESS CURVE

 ALL DATA

LOWESS CURVE

 SEASONAL

UNCENSORED

CENSORED

0

0.08

0.02

0.04

0.06

PHOSPHORUS, DISSOLVED,

IN MILLIGRAMS PER LITER

1970 1975 1980 1985 1990 1995 2000

0

0.08

0.02

0.04

0.06

PHOSPHORUS, TOTAL, IN

MILLIGRAMS PER LITER

0

0.8

0.2

0.4

0.6

NITROGEN, AMMONIA PLUS 

ORGANIC, TOTAL, IN

MILLIGRAMS PER LITER

0

0.20

0.05

0.10

0.15

NITROGEN, NITRITE PLUS 

NITRATE, DISSOLVED, IN

MILLIGRAMS PER LITER

1970 1975 1980 1985 1990 1995 2000

0

0.08

0.02

0.04

0.06

PHOSPHORUS, ORTHO, 

DISSOLVED, IN

MILLIGRAMS PER LITER

0

0.02

NITROGEN, AMMONIA, 

DISSOLVED,IN

MILLIGRAMS PER LITER

0

0.10

0.02

0.04

0.06

0.08

NITROGEN, NITRITE,

DISSOLVED, IN

MILLIGRAMS PER LITER

0.10

0.04

0.06

0.08

Figure 26. Temporal variations in periodic measurements and concentrations at Lake Granby near Rainbow Bay, sampled 

near the bottom.—Continued
Hydrologic and Water-Quality Data 59

EXPLANATION

LOWESS CURVE

 ALL DATA

LOWESS CURVE

 SEASONAL

UNCENSORED

CENSORED

0

100

20

40

60

80

MANGANESE, DISSOLVED, IN

MICROGRAMS PER LITER

1970 1975 1980 1985 1990 1995 2000

0

30

10

20

ZINC, DISSOLVED, IN

MICROGRAMS PER LITER

0

20

5

10

15

BARIUM, DISSOLVED, IN

MICROGRAMS PER LITER

0

100

20

40

60

80

IRON, DISSOLVED, IN

MICROGRAMS PER LITER

1970 1975 1980 1985 1990 1995 2000

0

80

20

40

60

STRONTIUM, DISSOLVED, IN

MICROGRAMS PER LITER

Figure 26. Temporal variations in periodic measurements and concentrations at Lake Granby near Rainbow Bay, sampled 

near the bottom.—Continued
60 Water Quality and Trend Analysis of Colorado–Big Thompson System Reservoirs and Related Conveyances

EXPLANATION

LOWESS CURVE

 ALL DATA

UNCENSORED

CENSORED

0

1,000

200

400

600

800

INSTANTANEOUS DISCHARGE,

IN CUBIC FEET PER SECOND

0

20

5

10

15

OXYGEN, DISSOLVED,

IN MILLIGRAMS PER LITER

5

9

6

7

8

pH, IN STANDARD UNITS

0

100

20

40

60

80

SPECIFIC CONDUCTANCE, IN

MICROSIEMENS PER

CENTIMETER AT

25 DEGREES CELSIUS

0

10

20

WATER TEMPERATURE,

IN DEGREES CELSIUS

1970 1975 1980 1985 1990 1995 2000

0

60

20

40

SOLIDS, RESIDUE AT 180

DEGREES CELSIUS,

DISSOLVED, IN

MILLIGRAMS PER LITER

1970 1975 1980 1985 1990 1995 2000

0

60

20

40

SOLIDS, DISSOLVED, SUM

OF CONSTITUENTS, IN

MILLIGRAMS PER LITER

Figure 27. Temporal variations in periodic measurements and concentrations at Granby Pump Canal.
Hydrologic and Water-Quality Data 61

EXPLANATION

LOWESS CURVE

 ALL DATA

UNCENSORED

CENSORED

0

40

10

20

30

HARDNESS, TOTAL, IN

MILLIGRAMS PER LITER

AS CACO3

0

2.0

0.4

0.8

1.2

1.6

MAGNESIUM, DISSOLVED,

IN MILLIGRAMS PER LITER

0

0.4

0.1

0.2

0.3

SODIUM ADSORPTION RATIO

1970 1975 1980 1985 1990 1995 2000

0

30

10

20

SODIUM, IN PERCENT

0

15

5

10

CALCIUM, DISSOLVED, IN

MILLIGRAMS PER LITER

0

2.0

0.4

0.8

1.2

1.6

POTASSIUM, DISSOLVED,

IN MILLIGRAMS PER LITER

1970 1975 1980 1985 1990 1995 2000

0

4

1

2

3

SODIUM, DISSOLVED, IN

MILLIGRAMS PER LITER

Figure 27. Temporal variations in periodic measurements and concentrations at Granby Pump Canal.—Continued
62 Water Quality and Trend Analysis of Colorado–Big Thompson System Reservoirs and Related Conveyances

EXPLANATION

LOWESS CURVE

 ALL DATA

UNCENSORED

CENSORED

0

40

10

20

30

ACID-NEUTRALIZING CAPACITY,

IN MILLIGRAMS PER LITER

AS CACO3

0

1

FLUORIDE, DISSOLVED,

IN MILLIGRAMS PER LITER

1970 1975 1980 1985 1990 1995 2000

0

6

2

4

SULFATE, DISSOLVED,

IN MILLIGRAMS PER LITER

0

2

1

CHLORIDE, DISSOLVED,

IN MILLIGRAMS PER LITER

1970 1975 1980 1985 1990 1995 2000

0

10

2

4

6

8

SILICA, DISSOLVED,

IN MILLIGRAMS PER LITER

Figure 27. Temporal variations in periodic measurements and concentrations at Granby Pump Canal.—Continued
Hydrologic and Water-Quality Data 63

EXPLANATION

LOWESS CURVE

 ALL DATA

UNCENSORED

CENSORED

0

1.5

0.5

1.0

NITROGEN, AMMONIA PLUS 

ORGANIC, TOTAL, IN

MILLIGRAMS PER LITER

0

0.4

0.1

0.2

0.3

NITROGEN, NITRITE PLUS 

NITRATE, DISSOLVED, IN

MILLIGRAMS PER LITER

0

0.10

0.02

0.04

0.06

0.08

PHOSPHORUS, DISSOLVED,

IN MILLIGRAMS PER LITER

1970 1975 1980 1985 1990 1995 2000

0

0.10

0.02

0.04

0.06

0.08

PHOSPHORUS, TOTAL,

IN MILLIGRAMS PER LITER

0

0.10

0.02

0.04

0.06

0.08

NITROGEN, AMMONIA, 

DISSOLVED, IN MILLIGRAMS

PER LITER

0

0.10

0.02

0.04

0.06

0.08

NITROGEN, NITRITE, 

DISSOLVED, IN

MILLIGRAMS PER LITER

1970 1975 1980 1985 1990 1995 2000

0

0.04

0.01

0.02

0.03

PHOSPHORUS, ORTHO, 

DISSOLVED, IN

MILLIGRAMS PER LITER

Figure 27. Temporal variations in periodic measurements and concentrations at Granby Pump Canal.—Continued
64 Water Quality and Trend Analysis of Colorado–Big Thompson System Reservoirs and Related Conveyances

EXPLANATION

LOWESS CURVE

 ALL DATA

UNCENSORED

CENSORED

0

20

5

10

15

BARIUM, DISSOLVED,

IN MICROGRAMS PER LITER

0

100

20

40

60

80

IRON, DISSOLVED,

IN MICROGRAMS PER LITER

0

100

20

40

60

80

MANGANESE, DISSOLVED,

IN MICROGRAMS PER LITER

1970 1975 1980 1985 1990 1995 2000

0

80

20

40

60

STRONTIUM, DISSOLVED,

IN MICROGRAMS PER LITER

1970 1975 1980 1985 1990 1995 2000

0

40

10

20

30

ZINC, DISSOLVED,

IN MICROGRAMS PER LITER

Figure 27. Temporal variations in periodic measurements and concentrations at Granby Pump Canal.—Continued
Hydrologic and Water-Quality Data 65

EXPLANATION

LOWESS CURVE

 ALL DATA

LOWESS CURVE

 SEASONAL

UNCENSORED

CENSORED

1970 1975 1980 1985 1990 1995 2000

0

100

20

40

60

80

SPECIFIC CONDUCTANCE, 

IN MICROSIEMENS PER 

CENTIMETER

AT 25 DEGREES CLESIUS

0

20

5

10

15

OXYGEN, DISSOLVED,

IN MILLIGRAMS PER LITER

1970 1975 1980 1985 1990 1995 2000

5

9

6

7

8

pH, IN STANDARD UNITS

Figure 28. Temporal variations in periodic measurements and concentrations at Shadow Mountain Lake, sampled at 

the surface.
66 Water Quality and Trend Analysis of Colorado–Big Thompson System Reservoirs and Related Conveyances

EXPLANATION

LOWESS CURVE

 ALL DATA

LOWESS CURVE

 SEASONAL

UNCENSORED

CENSORED

0

2

1

MAGNESIUM, DISSOLVED,

IN MILLIGRAMS PER LITER

0

0.4

0.1

0.2

0.3

SODIUM ADSORPTION RATIO

1970 1975 1980 1985 1990 1995 2000

0

30

10

20

SODIUM, IN PERCENT

0

40

10

20

30

HARDNESS, TOTAL,

IN MILLIGRAMS PER LITER

AS CACO3

0

15

5

10

CALCIUM, DISSOLVED,

IN MILLIGRAMS PER LITER

0

2

1

POTASSIUM, DISSOLVED,

IN MILLIGRAMS PER LITER

1970 1975 1980 1985 1990 1995 2000

0

4

1

2

3

SODIUM, DISSOLVED,

IN MILLIGRAMS PER LITER

Figure 28. Temporal variations in periodic measurements and concentrations at Shadow Mountain Lake, sampled at 

the surface.—Continued
Hydrologic and Water-Quality Data 67

EXPLANATION

LOWESS CURVE

 ALL DATA

LOWESS CURVE

 SEASONAL

UNCENSORED

CENSORED

0

2

1

CHLORIDE, DISSOLVED,

IN MILLIGRAMS PER LITER

0

0.4

0.1

0.2

0.3

FLUORIDE, DISSOLVED,

IN MILLIGRAMS PER LITER

1970 1975 1980 1985 1990 1995 2000

0

10

2

4

6

8

SILICA, DISSOLVED,

IN MILLIGRAMS PER LITER

1970 1975 1980 1985 1990 1995 2000

0

6

2

4

SULFATE, DISSOLVED,

IN MILLIGRAMS PER LITER

0

40

10

20

30

ACID-NEUTRALIZING CAPACITY,

IN MILLIGRAMS PER LITER

Figure 28. Temporal variations in periodic measurements and concentrations at Shadow Mountain Lake, sampled at 

the surface.—Continued
68 Water Quality and Trend Analysis of Colorado–Big Thompson System Reservoirs and Related Conveyances

EXPLANATION

LOWESS CURVE

 ALL DATA

LOWESS CURVE

 SEASONAL

UNCENSORED

CENSORED

0

0.15

0.05

0.10

NITROGEN, AMMONIA, 

DISSOLVED, IN

MILLIGRAMS PER LITER

0

0.20

0.05

0.10

0.15

NITROGEN, NITRITE PLUS 

NITRATE, DISSOLVED, IN

MILLIGRAMS PER LITER

0

0.10

0.02

0.04

0.06

0.08

NITROGEN, NITRITE, 

DISSOLVED, IN

MILLIGRAMS PER LITER

0

0.10

0.02

0.04

0.06

0.08

PHOSPHORUS, DISSOLVED,

IN MILLIGRAMS PER LITER

1970 1975 1980 1985 1990 1995 2000

0

0.04

0.01

0.02

0.03

PHOSPHORUS, ORTHO, 

DISSOLVED, IN MILLIGRAMS

PER LITER

1970 1975 1980 1985 1990 1995 2000

0

0.06

0.02

0.04

PHOSPHORUS, TOTAL,

IN MILLIGRAMS PER LITER

0

0.5

1.0

NITROGEN, AMMONIA PLUS 

ORGANIC, TOTAL, IN

MILLIGRAMS PER LITER

Figure 28. Temporal variations in periodic measurements and concentrations at Shadow Mountain Lake, sampled at 

the surface.—Continued
Hydrologic and Water-Quality Data 69

EXPLANATION

LOWESS CURVE

 ALL DATA

LOWESS CURVE

 SEASONAL

UNCENSORED

CENSORED

0

80

20

40

60

SOLIDS, DISSOLVED, SUM OF

CONSTITUENTS, IN

MILLIGRAMS PER LITER

0

200

50

100

150

TRANSPARENCY, SECCHI,

IN INCHES

0

20

5

10

15

COLIFORM, FECAL, IN

COLONIES PER 100 MILLILITERS

1970 1975 1980 1985 1990 1995 2000

0

30

10

20

CHLOROPHYLL-a, 

PHYTOPLANKTON,

IN MILLIGRAMS PER LITER

1970 1975 1980 1985 1990 1995 2000

0

1.0

0.2

0.4

0.6

0.8

0

80

20

40

60

SOLIDS, DISSOLVED,

RESIDUE AT 180 DEGREES

CELSIUS, IN MILLIGRAMS

PER LITER

CHLOROPHYLL-b, 

PHYTOPLANKTON, IN

MICROGRAMS PER LITER

Figure 28. Temporal variations in periodic measurements and concentrations at Shadow Mountain Lake, sampled at 

the surface.—Continued
70 Water Quality and Trend Analysis of Colorado–Big Thompson System Reservoirs and Related Conveyances

EXPLANATION

LOWESS CURVE

 ALL DATA

LOWESS CURVE

 SEASONAL

UNCENSORED

CENSORED

0

200

50

100

150

IRON, DISSOLVED,

IN MICROGRAMS PER LITER

0

100

20

40

60

80

MANGANESE, DISSOLVED,

IN MICROGRAMS PER LITER

1970 1975 1980 1985 1990 1995 2000

0

80

20

40

60

STRONTIUM, DISSOLVED,

IN MICROGRAMS PER LITER

1970 1975 1980 1985 1990 1995 2000

0

30

10

20

ZINC, DISSOLVED,

IN MICROGRAMS PER LITER

15

10

BARIUM, DISSOLVED,

IN MICROGRAMS PER LITER

0

5

Figure 28. Temporal variations in periodic measurements and concentrations at Shadow Mountain Lake, sampled at 

the surface.—Continued
Hydrologic and Water-Quality Data 71

EXPLANATION

LOWESS CURVE

 ALL DATA

LOWESS CURVE

 SEASONAL

UNCENSORED

CENSORED

5

9

6

7

8

pH, IN STANDARD UNITS

0

80

20

40

60

SOLIDS, DISSOLVED, RESIDUE AT 

180 DEGREES CELSIUS, IN

MILLIGRAMS PER LITER

0

40

10

20

30

SAMPLING DEPTH BELOW

 WATER SURFACE, IN FEET

1970 1975 1980 1985 1990 1995 2000

0

15

5

10

OXYGEN, DISSOLVED,

IN MILLIGRAMS PER LITER

0

100

20

40

60

80

SPECIFIC CONDUCTANCE,

IN MICROSIEMENS PER CENTIMETER

AT 25 DEGREE CELSIUS

1970 1975 1980 1985 1990 1995 2000

0

80

20

40

60

SOLIDS, DISSOLVED,

SUM OF CONSTITUENTS,

IN MILLIGRAMS PER LITER

Figure 29. Temporal variations in periodic measurements and concentrations at Shadow Mountain Lake, sampled near 

the bottom.
72 Water Quality and Trend Analysis of Colorado–Big Thompson System Reservoirs and Related Conveyances

EXPLANATION

LOWESS CURVE

 ALL DATA

LOWESS CURVE

 SEASONAL

UNCENSORED

CENSORED

0

10

2

4

6

8

CALCIUM, DISSOLVED,

IN MILLIGRAMS PER LITER

0

3

1

2

MAGNESIUM, DISSOLVED,

IN MILLIGRAMS PER LITER

0

2

1

POTASSIUM, DISSOLVED,

IN MILLIGRAMS PER LITER

0

0.4

0.1

0.2

0.3

SODIUM ADSORPTION RATIO

1970 1975 1980 1985 1990 1995 2000

0

4

1

2

3

SODIUM, DISSOLVED,

IN MILLIGRAMS PER LITER

1970 1975 1980 1985 1990 1995 2000

0

30

10

20

SODIUM, IN PERCENT

0

40

10

20

30

HARDNESS, TOTAL,IN

MILLIGRAMS PER LITER

AS CACO3

Figure 29. Temporal variations in periodic measurements and concentrations at Shadow Mountain Lake, sampled near 

the bottom.—Continued
Hydrologic and Water-Quality Data 73

0

1.5

0.5

1.0

CHLORIDE, DISSOLVED,

IN MILLIGRAMS PER LITER

0

0.4

0.1

0.2

0.3

FLUORIDE, DISSOLVED,

IN MILLIGRAMS PER LITER

0

10

2

4

6

8

SILICA, DISSOLVED,

IN MILLIGRAMS PER LITER

1970 1975 1980 1985 1990 1995 2000

1970 1975 1980 1985 1990 1995 2000

0

6

2

4

SULFATE, DISSOLVED,

IN MILLIGRAMS PER LITER

0

40

10

20

30

ACID-NEUTRALIZING CAPACITY,

IN MILLIGRAMS PER LITER

EXPLANATION

LOWESS CURVE

 ALL DATA

LOWESS CURVE

 SEASONAL

UNCENSORED

CENSORED

Figure 29. Temporal variations in periodic measurements and concentrations at Shadow Mountain Lake, sampled near 

the bottom.—Continued
74 Water Quality and Trend Analysis of Colorado–Big Thompson System Reservoirs and Related Conveyances

EXPLANATION

LOWESS CURVE

 ALL DATA

LOWESS CURVE

 SEASONAL

UNCENSORED

CENSORED

0

0.20

0.05

0.10

0.15

NITROGEN, AMMONIA, 

DISSOLVED, IN 

MILLIGRAMS PER LITER

0

0.20

0.05

0.10

0.15

NITROGEN, NITRITE PLUS 

NITRATE, DISSOLVED, IN

MILLIGRAMS PER LITER

0

0.04

0.01

0.02

0.03

NITROGEN, NITRITE, 

DISSOLVED, IN MILLIGRAMS

PER LITER

0

0.08

0.02

0.04

0.06

PHOSPHORUS, DISSOLVED,

IN MILLIGRAMS PER LITER

1970 1975 1980 1985 1990 1995 2000

0

0.08

0.02

0.04

0.06

PHOSPHORUS, ORTHO, 

DISSOLVED, IN

MILLIGRAMS PER LITER

1970 1975 1980 1985 1990 1995 2000

0

0.08

0.02

0.04

0.06

PHOSPHORUS, TOTAL, IN

MILLIGRAMS PER LITER

0

1.0

0.5

1.0

NITROGEN, AMMONIA PLUS 

ORGANIC, TOTAL, IN

MILLIGRAMS PER LITER

Figure 29. Temporal variations in periodic measurements and concentrations at Shadow Mountain Lake, sampled near 

the bottom.—Continued
Hydrologic and Water-Quality Data 75

EXPLANATION

LOWESS CURVE

 ALL DATA

LOWESS CURVE

 SEASONAL

UNCENSORED

CENSORED

0

600

200

400

MANGANESE,DISSOLVED,

IN MICROGRAMS PER LITER

1970 1975 1980 1985 1990 1995 2000

0

40

10

20

30

ZINC, DISSOLVED,

IN MICROGRAMS PER LITER

0

20

5

10

15

BARIUM, DISSOLVED,

IN MICROGRAMS PER LITER

0

300

100

200

IRON, DISSOLVED,

IN MICROGRAMS PER LITER

1970 1975 1980 1985 1990 1995 2000

0

100

20

40

60

80

STRONTIUM, DISSOLVED,

IN MICROGRAMS PER LITER

Figure 29. Temporal variations in periodic measurements and concentrations at Shadow Mountain Lake, sampled near 

the bottom.—Continued
76 Water Quality and Trend Analysis of Colorado–Big Thompson System Reservoirs and Related Conveyances

5

15

6

8

9

7

10

12

12

14

13

OXYGEN, DISSOLVED,

IN MILLIGRAMS PER LITER

5

9

6

7

8

pH, IN STANDARD UNITS

0

100

20

40

60

80

SPECIFIC CONDUCTANCE,

IN MICROSIEMENS PER CENTIMETER

0

20

5

10

15

WATER TEMPERATURE,

IN DEGREES CELSIUS

1970 1975 1980 1985 1990 1995 2000

0

80

20

40

60

SOLIDS, RESIDUE, DISSOLVED

AT 180 DEGREES CELSIUS

1970 1975 1980 1985 1990 1995 2000

0

80

20

40

60

SOLIDS, DISSOLVED, 

SUM OF CONSTITUENTS

EXPLANATION

LOWESS CURVE

 ALL DATA

LOWESS CURVE

 SEASONAL

UNCENSORED

CENSORED

Figure 30. Temporal variations in periodic measurements and concentrations at Adams Tunnel, east portal, near 

Estes Park.
Hydrologic and Water-Quality Data 77

EXPLANATION

LOWESS CURVE

 ALL DATA

LOWESS CURVE

 SEASONAL

UNCENSORED

CENSORED

0

40

10

20

30

HARDNESS, TOTAL,

IN MILLIGRAMS PER LITER

0

10

2

4

6

8

CALCIUM, DISSOLVED,

IN MILLIGRAMS PER LITER

0

2

1

MAGNESIUM, DISSOLVED,

IN MILLIGRAMS PER LITER

0

2

1

POTASSIUM, DISSOLVED,

IN MILLIGRAMS PER LITER

0

0.5

1.0

SODIUM ADSORPTION RATIO

1970 1975 1980 1985 1990 1995 2000

0

8

2

4

6

SODIUM, DISSOLVED,

IN MILLIGRAMS PER LITER

1970 1975 1980 1985 1990 1995 2000

0

40

10

20

30

SODIUM, PERCENT

Figure 30. Temporal variations in periodic measurements and concentrations at Adams Tunnel, east portal, near 

Estes Park.—Continued
78 Water Quality and Trend Analysis of Colorado–Big Thompson System Reservoirs and Related Conveyances

EXPLANATION

LOWESS CURVE

 ALL DATA

LOWESS CURVE

 SEASONAL

UNCENSORED

CENSORED

0

40

10

20

30

ACID NEUTRALIZING CAPACITY,

IN MILLIGRAMS PER LITER

0

2

1

CHLORIDE, DISSOLVED,

IN MILLIGRAMS PER LITER

0

1.0

0.5

FLUORIDE, DISSOLVED,

IN MILLIGRAMS PER LITER

1970 1975 1980 1985 1990 1995 2000

0

10

2

4

6

8

SILICA, DISSOLVED, 

IN MILLIGRAMS PER LITER

1970 1975 1980 1985 1990 1995 2000

0

10

2

4

6

8

SULFATE, DISSOLVED,

IN MILLIGRAMS PER LITER

Figure 30. Temporal variations in periodic measurements and concentrations at Adams Tunnel, east portal, near 

Estes Park.—Continued
Hydrologic and Water-Quality Data 79

EXPLANATION

LOWESS CURVE

 ALL DATA

LOWESS CURVE

 SEASONAL

UNCENSORED

CENSORED

0

2

1

NITROGEN, AMMONIA PLUS 

ORGANIC, TOTAL, IN

MILLIGRAMS PER LITER

0

0.10

0.02

0.04

0.06

0.08

NITROGEN, AMMONIA, 

DISSOLVED, IN

MILLIGRAMS PER LITER

0

0.4

0.1

0.2

0.3

NITROGEN, NITRITE PLUS 

NITRATE, DISSOLVED, IN

MILLIGRAMS PER LITER

0

0.10

0.02

0.04

0.06

0.08

NITROGEN, NITRITE,

DISSOLVED, IN

MILLIGRAMS PER LITER

1970 1975 1980 1985 1990 1995 2000

0

0.10

0.02

0.04

0.06

0.08

PHOSPHORUS, ORTHO,

DISSOLVED, IN

MILLIGRAMS PER LITER

0

0.10

0.02

0.04

0.06

0.08

PHOSPHORUS, DISSOLVED,

IN MILLIGRAMS PER LITER

1970 1975 1980 1985 1990 1995 2000

0

0.20

0.05

0.10

0.15

PHOSPHORUS, TOTAL,

IN MILLIGRAMS PER LITER

Figure 30. Temporal variations in periodic measurements and concentrations at Adams Tunnel, east portal, near 

Estes Park.—Continued
80 Water Quality and Trend Analysis of Colorado–Big Thompson System Reservoirs and Related Conveyances

0

300

100

200

IRON, DISSOLVED,

IN MICROGRAMS PER LITER

1970 1975 1980 1985 1990 1995 2000

0

80

20

40

60

STRONTIUM, DISSOLVED,

IN MICROGRAMS PER LITER

1970 1975 1980 1985 1990 1995 2000

0

40

10

20

30

ZINC, DISSOLVED,

IN MICROGRAMS PER LITER

0

20

5

10

15

MANGANESE, DISSOLVED,

IN MICROGRAMS PER LITER

EXPLANATION

LOWESS CURVE

 ALL DATA

LOWESS CURVE

 SEASONAL

UNCENSORED

CENSORED

Figure 30. Temporal variations in periodic measurements and concentrations at Adams Tunnel, east portal, near 

Estes Park.—Continued
Hydrologic and Water-Quality Data 81

EXPLANATION

LOWESS CURVE

 ALL DATA

LOWESS CURVE

 SEASONAL

UNCENSORED

CENSORED

0

100

20

40

60

80

SPECIFIC CONDUCTANCE, IN

MICROSIEMENS PER

CENTIMETER AT

25 DEGREES CELSIUS

0

80

20

40

60

SOLIDS, DISSOLVED, RESIDUE 

AT 180 DEGREES CELSIUS, IN

MILLIGRAMS PER LITER

1970 1975 1980 1985 1990 1995 2000

0

3,000

1,000

2,000

COLIFORM, TOTAL, IN

COLONIES PER 100 MILLILITERS

1970 1975 1980 1985 1990 1995 2000

0

20

5

10

15

OXYGEN, DISSOLVED,

IN MILLIGRAMS PER LITER

5

9

6

7

8

pH, IN STANDARD UNITS

0

10

20

WATER TEMPERATURE,

IN DEGREES CELSIUS

0

80

20

40

60

SOLIDS, DISSOLVED,

SUM OF CONSTITUENTS, 

IN MILLIGRAMS PER LITER

0

1,500

500

1,000

COLIFORM, FECAL, IN

COLONIES PER 100 MILLILITERS

Figure 31. Temporal variations in periodic measurements and concentrations at Olympus Tunnel.
82 Water Quality and Trend Analysis of Colorado–Big Thompson System Reservoirs and Related Conveyances

Figure 31. Temporal variations in periodic measurements and concentrations at Olympus Tunnel.—Continued

EXPLANATION

LOWESS CURVE

 ALL DATA

LOWESS CURVE

 SEASONAL

UNCENSORED

CENSORED

0

4

1

2

3

MAGNESIUM, DISSOLVED,

IN MILLIGRAMS PER LITER

0

40

10

20

30

HARDNESS, TOTAL, IN

MILLIGRAMS PER LITER

AS CACO3

0

0.5

0.1

0.2

0.3

0.4

SODIUM ADSORPTION RATIO

0

40

10

20

30

SODIUM, IN PERCENT

1970 1975 1980 1985 1990 1995 2000

1970 1975 1980 1985 1990 1995 2000

0

6

2

4

SODIUM, DISSOLVED,

IN MILLIGRAMS PER LITER

0

2.

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

1.8

2.0

POTASSIUM, DISSOLVED,

IN MILLIGRAMS PER LITER

0

10

2

4

6

8

CALCIUM, DISSOLVED,

IN MILLIGRAMS PER LITER
Hydrologic and Water-Quality Data 83

Figure 31. Temporal variations in periodic measurements and concentrations at Olympus Tunnel.—Continued

EXPLANATION

LOWESS CURVE

 ALL DATA

LOWESS CURVE

 SEASONAL

UNCENSORED

CENSORED

0

5

1

2

3

4

CHLORIDE, DISSOLVED,

IN MILLIGRAMS PER LITER

0

0.5

0.1

0.2

0.3

0.4

FLUORIDE, DISSOLVED,

IN MILLIGRAMS PER LITER

0

8

2

4

6

SILICA, DISSOLVED,

IN MILLIGRAMS PER LITER

1970 1975 1980 1985 1990 1995 2000

1970 1975 1980 1985 1990 1995 2000

0

15

5

10

SULFATE, DISSOLVED,

IN MILLIGRAMS PER LITER

0

40

10

20

30

ACID-NEUTRALIZING CAPACITY,

IN MILLIGRAMS PER LITER

AS CACO3
84 Water Quality and Trend Analysis of Colorado–Big Thompson System Reservoirs and Related Conveyances

0

EXPLANATION

LOWESS CURVE

 ALL DATA

LOWESS CURVE

 SEASONAL

UNCENSORED

CENSORED

0

0.4

0.1

0.2

0.3

NITROGEN, NITRITE PLUS 

NITRATE, DISSOLVED, IN

MILLIGRAMS PER LITER

0

0.10

0.02

0.04

0.06

0.08

PHOSPHORUS, DISSOLVED,

IN MILLIGRAMS PER LITER

1970 1975 1980 1985 1990 1995 2000

0

0.6

0.2

0.4

PHOSPHORUS, TOTAL,

IN MILLIGRAMS PER LITER

0

0.8

0.2

0.4

0.6

NITROGEN, AMMONIA PLUS 

ORGANIC, IN MILLIGRAMS 

PER LITER

0

0.3

0.1

0.2

NITROGEN, AMMONIA, 

DISSOLVED, IN MILLIGRAMS

PER LITER

1970 1975 1980 1985 1990 1995 2000

0

0.10

0.02

0.04

0.06

0.08

PHOSPHORUS, ORTHO, 

DISSOLVED, IN

MILLIGRAMS PER LITER

0.04

0.01

0.02

0.03

NITROGEN, NITRITE, 

DISSOLVED, IN

MILLIGRAMS PER LITER

Figure 31. Temporal variations in periodic measurements and concentrations at Olympus Tunnel.—Continued
Hydrologic and Water-Quality Data 85

0

300

100

200

IRON, DISSOLVED,

IN MICROGRAMS PER LITER

0

40

10

20

30

MANGANESE, DISSOLVED,

IN MICROGRAMS PER LITER

1970 1975 1980 1985 1990 1995 2000

0

80

20

40

60

STRONTIUM, DISSOLVED,

IN MICROGRAMS PER LITER

1970 1975 1980 1985 1990 1995 2000

0

40

10

20

30

ZINC, DISSOLVED,

IN MICROGRAMS PER LITER

0

15

5

10

BARIUM, DISSOLVED,

IN MICROGRAMS PER LITER

EXPLANATION

LOWESS CURVE

 ALL DATA

LOWESS CURVE

 SEASONAL

UNCENSORED

CENSORED

Figure 31. Temporal variations in periodic measurements and concentrations at Olympus Tunnel.—Continued
86 Water Quality and Trend Analysis of Colorado–Big Thompson System Reservoirs and Related Conveyances

Figure 32. Temporal variations in periodic measurements and concentrations at Carter Lake, sampled near the surface.

0

5

0

0

EXPLANATION

LOWESS CURVE

 ALL DATA

LOWESS CURVE

 SEASONAL

UNCENSORED

CENSORED

20

10

15

OXYGEN, DISSOLVED,

IN MILLIGRAMS PER LITER

5

9

6

7

8

pH, IN STANDARD UNITS

150

50

100

SPECIFIC CONDUCTANCE, IN

MICROSIEMENS PER 

CENTIMETER

0

3

10

20

WATER TEMPERATURE, 

IN DEGREES CELSIUS

1970 1975 1980 1985 1990 1995 2000

30

90

40

50

60

70

80

SOLIDS, RESIDUE AT 

180 DEGREES CELSIUS,

DISSOLVED, IN

MILLIGRAMS PER LITER

1970 1975 1980 1985 1990 1995 2000

30

90

40

50

60

70

80

SOLIDS, DISSOLVED, 

SUM OF CONSTITUENTS,

IN MILLIGRAMS PER LITER
Hydrologic and Water-Quality Data 87

Figure 32. Temporal variations in periodic measurements and concentrations at Carter Lake, sampled near the surface. 

—Continued

EXPLANATION

LOWESS CURVE

 ALL DATA

LOWESS CURVE

 SEASONAL

UNCENSORED

CENSORED

0

20

5

10

15

CALCIUM, DISSOLVED,

IN MILLIGRAMS PER LITER

0

2.0

0.5

1.0

1.5

POTASSIUM, DISSOLVED,

IN MILLIGRAMS PER LITER

1970 1975 1980 1985 1990 1995 2000

0

4

1

2

3

SODIUM, DISSOLVED,

IN MILLIGRAMS PER LITER

0

3

1

2

MAGNESIUM, DISSOLVED,

IN MILLIGRAMS PER LITER

0

0.4

0.1

0.2

0.3

SODIUM ADSORPTION RATIO

1970 1975 1980 1985 1990 1995 2000

0

20

5

10

15

SODIUM, IN PERCENT

0

60

20

40

HARDNESS, TOTAL, IN

MILLIGRAMS PER LITER

AS CACO3
88 Water Quality and Trend Analysis of Colorado–Big Thompson System Reservoirs and Related Conveyances

0

EXPLANATION

LOWESS CURVE

 ALL DATA

LOWESS CURVE

 SEASONAL

UNCENSORED

CENSORED

0

2

1

CHLORIDE, DISSOLVED,

IN MILLIGRAMS PER LITER

0

0.6

0.2

0.4

FLUORIDE, DISSOLVED,

IN MILLIGRAMS PER LITER

1970 1975 1980 1985 1990 1995 2000

0

8

2

4

6

SILICA, DISSOLVED,

IN MILLIGRAMS PER LITER

1970 1975 1980 1985 1990 1995 2000

0

8

2

4

6

SULFATE, DISSOLVED,

IN MILLIGRAMS PER LITER

60

20

40

ACID-NEUTRALIZING 

CAPACITY, IN MILLIGRAMS 

PER LITER AS CACO3

Figure 32. Temporal variations in periodic measurements and concentrations at Carter Lake, sampled near the surface.

—Continued
Hydrologic and Water-Quality Data 89

0

EXPLANATION

LOWESS CURVE

 ALL DATA

LOWESS CURVE

 SEASONAL

UNCENSORED

CENSORED

NITROGEN, NITRITE PLUS 

NITRATE, DISSOLVED, IN

MILLIGRAMS PER LITER

NITROGEN, AMMONIA PLUS 

ORGANIC, TOTAL, IN

MILLIGRAMS PER LITER

0

0.3

0.1

0.2

0

0

0.08

0.02

0.04

0.06

PHOSPHORUS, DISSOLVED

IN MILLIGRAMS PER LITER

1970 1975 1980 1985 1990 1995 2000

0

0.7

0.1

0.2

0.3

0.4

0.5

0.6

PHOSPHORUS, TOTAL,

IN MILLIGRAMS PER LITER

1970 1975 1980 1985 1990 1995 2000

0

0.08

0.02

0.04

0.06

PHOSPHORUS, ORTHO, 

DISSOLVED, IN

MILLIGRAMS PER LITER

0

0.08

0.02

0.04

0.06

NITROGEN, NITRITE, 

DISSOLVED, IN

MILLIGRAMS PER LITER

0

0.5

0.1

0.2

0.3

0.4

NITROGEN, AMMONIA, 

DISSOLVED, IN

MILLIGRAMS PER LITER

1.

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.

Figure 32. Temporal variations in periodic measurements and concentrations at Carter Lake, sampled near the surface.

—Continued
90 Water Quality and Trend Analysis of Colorado–Big Thompson System Reservoirs and Related Conveyances

EXPLANATION

LOWESS CURVE

 ALL DATA

LOWESS CURVE

 SEASONAL

UNCENSORED

CENSORED

0

200

50

100

150

TRANSPARENCY, SECCHI DISC,

IN INCHES

1970 1975 1980 1985 1990 1995 2000

0

8

2

4

6

CHLOROPHYLL-a, 

PHYTOPLANKTON, IN 

MICROGRAMS PER LITER

0

10

2

4

6

8

COLIFORM, FECAL, IN COLONIES 

PER 100 MILLILITER

1970 1975 1980 1985 1990 1995 2000

0

0.4

0.1

0.2

0.3

CHLOROPHYLL-b, 

PHYTOPLANKTON, IN

MICROGRAMS PER LITER

Figure 32. Temporal variations in periodic measurements and concentrations at Carter Lake, sampled near the surface.

—Continued
Hydrologic and Water-Quality Data 91

EXPLANATION

LOWESS CURVE

 ALL DATA

LOWESS CURVE

 SEASONAL

UNCENSORED

CENSORED

0

30

10

20

IRON, DISSOLVED,

IN MICROGRAMS PER LITER

0

15

5

10

MANGANESE, DISSOLVED,

IN MICROGRAMS PER LITER

1970 1975 1980 1985 1990 1995 2000

0

80

20

40

60

STRONTIUM, DISSOLVED,

IN MICROGRAMS PER LITER

1970 1975 1980 1985 1990 1995 2000

0

40

10

20

30

ZINC, DISSOLVED,

IN MICROGRAMS PER LITER

0

40

10

20

30

BARIUM, DISSOLVED,

IN MICROGRAMS PER LITER

Figure 32. Temporal variations in periodic measurements and concentrations at Carter Lake, sampled near the surface.

—Continued
92 Water Quality and Trend Analysis of Colorado–Big Thompson System Reservoirs and Related Conveyances

Figure 33. Temporal variations in periodic measurements and concentrations at Carter Lake, sampled near the bottom.

0

EXPLANATION

LOWESS CURVE

 ALL DATA

LOWESS CURVE

 SEASONAL

UNCENSORED

CENSORED

0

20

5

10

15

OXYGEN, DISSOLVED,

IN MILLIGRAMS PER LITER

5

9

6

7

8

pH, IN STANDARD UNITS

0

200

50

100

150

SPECIFIC CONDUCTANCE, 

IN MICROSIEMENS PER

CENTIMETER AT

25 DEGREES CELSIUS

0

10

20

WATER TEMPERATURE,

IN DEGREES CELSIUS

1970 1975 1980 1985 1990 1995 2000

0

100

20

4

60

80

SOLIDS, RESIDUE AT 

180 DEGREES CELSIUS,

DISSOLVED, IN

MILLIGRAMS PER LITER

1970 1975 1980 1985 1990 1995 2000

0

100

20

40

60

80

SOLIDS, DISSOLVED,

SUM OF CONSTITUENTS,

IN MILLIGRAMS PER LITER
Hydrologic and Water-Quality Data 93

Figure 33. Temporal variations in periodic measurements and concentrations at Carter Lake, sampled near the bottom.

—Continued

0

5

EXPLANATION

LOWESS CURVE

 ALL DATA

LOWESS CURVE

 SEASONAL

UNCENSORED

CENSORED

0

2

1

MAGNESIUM, DISSOLVED,

IN MILLIGRAMS PER LITER

0

0.4

0.1

0.2

0.3

SODIUM ADSORPTION RATIO

0

20

5

10

15

SODIUM, IN PERCENT

0

40

10

20

30

HARDNESS, TOTAL, IN

MILLIGRAMS PER LITER

AS CACO3

1970 1975 1980 1985 1990 1995 2000

0

1.0

0.5

POTASSIUM, DISSOLVED,

IN MILLIGRAMS PER LITER

1970 1975 1980 1985 1990 1995 2000

0

4

1

2

3

SODIUM, DISSOLVED,

IN MILLIGRAMS PER LITER

15

10

CALCIUM, DISSOLVED,

IN MILLIGRAMS PER LITER
94 Water Quality and Trend Analysis of Colorado–Big Thompson System Reservoirs and Related Conveyances

0

EXPLANATION

LOWESS CURVE

 ALL DATA

LOWESS CURVE

 SEASONAL

UNCENSORED

CENSORED

0

0.4

0.1

0.2

0.3

FLUORIDE, DISSOLVED,

IN MILLIGRAMS PER LITER

1970 1975 1980 1985 1990 1995 2000

0

6

2

4

SULFATE, DISSOLVED,

IN MILLIGRAMS PER LITER

60

20

40

ACID-NEUTRALIZING CAPACITY,

IN MILLIGRAMS PER

LITER AS CACO3

0

2

1

CHLORIDE, DISSOLVED,

IN MILLIGRAMS PER LITER

0

8

2

4

6

SILICA, DISSOLVED,

IN MILLIGRAMS PER LITER

1970 1975 1980 1985 1990 1995 2000

Figure 33. Temporal variations in periodic measurements and concentrations at Carter Lake, sampled near the bottom.

—Continued
Hydrologic and Water-Quality Data 95

EXPLANATION

LOWESS CURVE

 ALL DATA

LOWESS CURVE

 SEASONAL

UNCENSORED

CENSORED

0

1.0

0.5

NITROGEN, AMMONIA PLUS 

ORGANIC, TOTAL, IN

MILLIGRAMS PER LITER

0.4

0.1

0.2

0.3

NITROGEN, NITRITE PLUS 

NITRATE, DISSOLVED, IN

MILLIGRAMS PER LITER

0

0.08

0.02

0.04

0.06

NITROGEN, NITRITE, 

DISSOLVED, IN MILLIGRAMS

PER LITER

0

0.10

0.02

0.04

0.06

0.08

PHOSPHORUS, DISSOLVED,

IN MILLIGRAMS PER LITER

1970 1975 1980 1985 1990 1995 2000

0

0.10

0.02

0.04

0.06

0.08

PHOSPHORUS, ORTHO, 

DISSOLVED, IN MILLIGRAMS

PER LITER

1970 1975 1980 1985 1990 1995 2000

0

0.4

0.1

0.2

0.3

PHOSPHORUS, TOTAL, 

IN MILLIGRAMS PER LITER

0

0.3

0.1

0.2

NITROGEN, AMMONIA, 

DISSOLVED, IN MILLIGRAMS

PER LITER

0

Figure 33. Temporal variations in periodic measurements and concentrations at Carter Lake, sampled near the bottom.

—Continued
96 Water Quality and Trend Analysis of Colorado–Big Thompson System Reservoirs and Related Conveyances

EXPLANATION

LOWESS CURVE

 ALL DATA

LOWESS CURVE

 SEASONAL

UNCENSORED

CENSORED

0

40

10

20

30

IRON, DISSOLVED,

IN MICROGRAMS PER LITER

1970 1975 1980 1985 1990 1995 2000

0

80

20

40

60

STRONTIUM, DISSOLVED,

IN MICROGRAMS PER LITER

0

40

10

20

30

BARIUM, DISSOLVED,

IN MICROGRAMS PER LITER

0

40

10

20

30

MANGANESE, DISSOLVED,

IN MICROGRAMS PER LITER

1970 1975 1980 1985 1990 1995 2000

0

40

10

20

30

ZINC, DISSOLVED,

IN MICROGRAMS PER LITER

Figure 33. Temporal variations in periodic measurements and concentrations at Carter Lake, sampled near the bottom.

—Continued
Hydrologic and Water-Quality Data 97

EXPLANATION

LOWESS CURVE

 ALL DATA

LOWESS CURVE

 SEASONAL

UNCENSORED

1965 1970 1975 1980 1985 1990 1995 2000

0

15

5

10

OXYGEN, DISSOLVED,

IN MILLIGRAMS PER LITER

1965 1970 1975 1980 1985 1990 1995 2000

5

9

6

7

8

pH, IN STANDARD UNITS

1970 1975 1980 1985 1990 1995 2000

40

140

60

80

100

120

SPECIFIC CONDUCTANCE,

IN MICROSIEMENS PER

CENTIMETER AT 25 DEGREES

CELSIUS

1970 1975 1980 1985 1990 1995 2000

0

30

10

20

WATER TEMPERATURE,

IN DEGREES CELSIUS

Figure 34. Temporal variations in periodic measurements and concentrations at Horsetooth Reservoir, near Soldier 

Canyon Dam, sampled near the surface.
98 Water Quality and Trend Analysis of Colorado–Big Thompson System Reservoirs and Related Conveyances

Figure 34. Temporal variations in periodic measurements and concentrations at Horsetooth Reservoir, near Soldier 

Canyon Dam, sampled near the surface.—Continued

EXPLANATION

LOWESS CURVE

 ALL DATA

LOWESS CURVE

 SEASONAL

UNCENSORED

6

12

8

10

CALCIUM, DISSOLVED,

IN MILLIGRAMS PER LITER

0.4

1.

0.6

0.8

1.0

1.2

1.4

POTASSIUM, DISSOLVED,

IN MILLIGRAMS PER LITER

1965

1

4

2

3

SODIUM, DISSOLVED,

IN MILLIGRAMS PER LITER

1970 1975 1980 1985 1990 1995 2000

10

40

20

30

HARDNESS, TOTAL, IN

MILLIGRAMS PER LITER

AS CACO3 0.5

2.5

1.0

1.5

2.0

MAGNESIUM, DISSOLVED,

IN MILLIGRAMS PER LITER

0.1

0.4

0.2

0.3

SODIUM ADSORPTION RATIO

10

20

12

14

16

18

SODIUM, IN PERCENT

1965 1970 1975 1980 1985 1990 1995 2000
Hydrologic and Water-Quality Data 99

Figure 34. Temporal variations in periodic measurements and concentrations at Horsetooth Reservoir, near Soldier 

Canyon Dam, sampled near the surface.—Continued

EXPLANATION

LOWESS CURVE

 ALL DATA

LOWESS CURVE

 SEASONAL

UNCENSORED

CENSORED

1965 1970 1975 1980 1985 1990 1995 2000

0

12

2

4

6

8

10

SULFATE, DISSOLVED,

IN MILLIGRAMS PER LITER

0

0.8

0.2

0.4

0.6

FLUORIDE, DISSOLVED,

IN MILLIGRAMS PER LITER

10

50

20

30

40

ACID-NEUTRALIZING CAPACITY,

IN MILLIGRAMS PER LITER

AS CACO3

0

3

1

2

CHLORIDE, DISSOLVED,

IN MILLIGRAMS PER LITER

1965 1970 1975 1980 1985 1990 1995 2000

0

6

2

4

SILICA, DISSOLVED,

IN MILLIGRAMS PER LITER
100 Water Quality and Trend Analysis of Colorado–Big Thompson System Reservoirs and Related Conveyances

Figure 34. Temporal variations in periodic measurements and concentrations at Horsetooth Reservoir, near Soldier 

Canyon Dam, sampled near the surface.—Continued

EXPLANATION

LOWESS CURVE

 ALL DATA

LOWESS CURVE

 SEASONAL

UNCENSORED

CENSORED

0

0.3

0.1

0.2

NITROGEN, NITRITE PLUS 

NITRATE, DISSOLVED, IN

MILLIGRAMS PER LITER

0

0.10

0.02

0.04

0.06

0.08

PHOSPHORUS, DISSOLVED,

IN MILLIGRAMS PER LITER

1970 1975 1980 1985 1990 1995 2000

0

2.5

0.5

1.0

1.5

2.0

PHOSPHORUS, TOTAL,

IN MILLIGRAMS PER LITER

0

2

1

NITROGEN, AMMONIA PLUS 

ORGANIC, TOTAL, IN

MILLIGRAMS PER LITER

0

0.4

0.1

0.2

0.3

NITROGEN, AMMONIA, 

DISSOLVED, IN

MILLIGRAMS PER LITER

0

0.10

0.02

0.04

0.06

0.08

NITROGEN, NITRITE, 

DISSOLVED, IN

MILLIGRAMS PER LITER

1970 1975 1980 1985 1990 1995 2000

0

0.10

0.02

0.04

0.06

0.08

PHOSPHORUS, ORTHO, 

DISSOLVED, IN

MILLIGRAMS PER LITER
Hydrologic and Water-Quality Data 101

EXPLANATION

LOWESS CURVE

 ALL DATA

LOWESS CURVE

 SEASONAL

UNCENSORED

CENSORED

0

100

20

40

60

80

SOLIDS, DISSOLVED, SUM 

OF CONSTITUENTS, IN

MILLIGRAMS PER LITER

0

10

2

4

6

8

COLIFORM, FECAL, IN

COLONIES PER 100 

MILLILITERS

0

1.0

0.5

CHLOROPHYLL-b, 

PHYTOPLANKTON,IN

MICROGRAMS PER LITER

1970 1975 1980 1985 1990 1995 2000

0

100

20

40

60

80

SOLIDS, DISSOLVED, RESIDUE

AT 180 DEGREES CELSIUS,

IN MILLIGRAMS PER LITER

50

200

100

150

TRANSPARENCY, SECCHI, 

IN INCHES

1970 1975 1980 1985 1990 1995 2000

0

8

2

4

6

CHLOROPHYLL-a, 

PHYTOPLANKTON, IN

MICROGRAMS PER LITER

Figure 34. Temporal variations in periodic measurements and concentrations at Horsetooth Reservoir, near Soldier 

Canyon Dam, sampled near the surface.—Continued
102 Water Quality and Trend Analysis of Colorado–Big Thompson System Reservoirs and Related Conveyances

EXPLANATION

LOWESS CURVE

 ALL DATA

LOWESS CURVE

 SEASONAL

UNCENSORED

CENSORED

0

25

5

10

15

20

MANGANESE, DISSOLVED,

IN MICROGRAMS PER LITER

1970 1975 1980 1985 1990 1995 2000

0

30

10

20

ZINC, DISSOLVED,

IN MICROGRAMS PER LITER

0

40

10

20

30

BARIUM, DISSOLVED,

IN MICROGRAMS PER LITER

0

120

20

40

60

80

100

IRON, DISSOLVED,

IN MICROGRAMS PER LITER

1970 1975 1980 1985 1990 1995 2000

0

80

20

40

60

STRONTIUM, DISSOLVED,

IN MICROGRAMS PER LITER

Figure 34. Temporal variations in periodic measurements and concentrations at Horsetooth Reservoir, near Soldier 

Canyon Dam, sampled near the surface.—Continued
Hydrologic and Water-Quality Data 103

Figure 35. Temporal variations in periodic measurements and concentrations at Horsetooth Reservoir, near Soldier 

Canyon Dam, sampled near the bottom.

EXPLANATION

LOWESS CURVE

 ALL DATA

LOWESS CURVE

 SEASONAL

UNCENSORED

CENSORED

10

9

6

5

8

7

pH, IN STANDARD UNITS

1970 1975 1980 1985 1990 1995 2000

0

20

5

10

15

WATER TEMPERATURE,

IN DEGREES CELSIUS

20

120

40

60

80

100

SPECIFIC CONDUCTANCE, 

IN MICROSIEMENS PER

CENTIMETER AT 

25 DEGREES CELSIUS

1970 1975 1980 1985 1990 1995 2000

20

100

40

60

80

SOLIDS, DISSOLVED, RESIDUE 

AT 180 DEGREES CELSIUS, IN

MILLIGRAMS PER LITER

0

20

5

10

15

OXYGEN, DISSOLVED,

IN MILLIGRAMS PER LITER
104 Water Quality and Trend Analysis of Colorado–Big Thompson System Reservoirs and Related Conveyances

Figure 35. Temporal variations in periodic measurements and concentrations at Horsetooth Reservoir, near Soldier 

Canyon Dam, sampled near the bottom.—Continued

EXPLANATION

LOWESS CURVE

 ALL DATA

LOWESS CURVE

 SEASONAL

UNCENSORED

CENSORED

10

40

20

30

HARDNESS, TOTAL, IN

MILLIGRAMS PER LITER

AS CACO3

0

3

1

2

MAGNESIUM, DISSOLVED,

IN MILLIGRAMS PER LITER

0

0.6

0.2

0.4

SODIUM ADSORPTION RATIO

1965 1970 1975 1980 1985 1990 1995 2000

0

30

10

20

SODIUM, IN PERCENT

0

15

5

10

CALCIUM, DISSOLVED,

IN MILLIGRAMS PER LITER

0

1.5

0.5

1.0

POTASSIUM, DISSOLVED,

IN MILLIGRAMS PER LITER

1965 1970 1975 1980 1985 1990 1995 2000

0

4

1

2

3

SODIUM, DISSOLVED,

IN MILLIGRAMS PER LITER
Hydrologic and Water-Quality Data 105

Figure 35. Temporal variations in periodic measurements and concentrations at Horsetooth Reservoir, near Soldier 

Canyon Dam, sampled near the bottom.—Continued

EXPLANATION

LOWESS CURVE

 ALL DATA

LOWESS CURVE

 SEASONAL

UNCENSORED

CENSORED

0

40

10

20

30

ACID-NEUTRALIZING CAPACITY,

IN MILLIGRAMS PER LITER

AS CACO3

0

0.4

0.1

0.2

0.3

FLUORIDE, DISSOLVED,

IN MILLIGRAMS PER LITER

1970 1975 1980 1985 1990 1995 2000

0

6

2

4

SULFATE, DISSOLVED,

IN MILLIGRAMS PER LITER

0

2.0

0.5

1.0

1.5

CHLORIDE, DISSOLVED,

IN MILLIGRAMS PER LITER

1970 1975 1980 1985 1990 1995 2000

0

6

2

4

SILICA, DISSOLVED,

IN MILLIGRAMS PER LITER
106 Water Quality and Trend Analysis of Colorado–Big Thompson System Reservoirs and Related Conveyances

EXPLANATION

LOWESS CURVE

 ALL DATA

LOWESS CURVE

 SEASONAL

UNCENSORED

CENSORED

0

1.0

0.5

NITROGEN, AMMONIA PLUS 

ORGANIC, TOTAL, IN

MILLIGRAMS PER LITER

0

0.5

0.2

0.1

0.4

0.3

NITROGEN, NITRITE PLUS 

NITRATE, DISSOLVED, IN

MILLIGRAMS PER LITER

0

0.10

0.02

0.04

0.06

0.08

PHOSPHORUS, DISSOLVED, 

IN MILLIGRAMS PER LITER

1970 1975 1980 1985 1990 1995 2000

0

1.6

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.

PHOSPHORUS, TOTAL,

IN MILLIGRAMS PER LITER

0

0.4

0.1

0.2

0.3

NITROGEN, AMMONIA, 

DISSOLVED,IN

MILLIGRAMS PER LITER

0

0.10

0.02

0.04

0.06

0.08

NITROGEN, NITRITE, 

DISSOLVED, IN

MILLIGRAMS PER LITER

1970 1975 1980 1985 1990 1995 2000

0

0.04

0.01

0.02

0.03

PHOSPHORUS, ORTHO, 

DISSOLVED, IN

MILLIGRAMS PER LITER

Figure 35. Temporal variations in periodic measurements and concentrations at Horsetooth Reservoir, near Soldier 

Canyon Dam, sampled near the bottom.—Continued
Hydrologic and Water-Quality Data 107

1970 1975 1980 1985 1990 1995 2000

0

30

10

20

ZINC, DISSOLVED,

IN MICROGRAMS PER LITER

0

250

50

100

150

200

MANGANESE, DISSOLVED,

IN MICROGRAMS PER LITER

0

30

10

20

BARIUM, DISSOLVED,

IN MICROGRAMS PER LITER

0

100

20

40

60

80

IRON, DISSOLVED,

IN MICROGRAMS PER LITER

20

60

30

40

50

STRONTIUM, DISSOLVED, 

IN MICROGRAMS PER LITER

1970 1975 1980 1985 1990 1995 2000

EXPLANATION

LOWESS CURVE

 ALL DATA

LOWESS CURVE

 SEASONAL

UNCENSORED

CENSORED

Figure 35. Temporal variations in periodic measurements and concentrations at Horsetooth Reservoir, near Soldier 

Canyon Dam, sampled near the bottom.—Continued
108 Water Quality and Trend Analysis of Colorado–Big Thompson System Reservoirs and Related Conveyances

Figure 36. Temporal variations in periodic measurements and concentrations at Horsetooth Reservoir, near Spring 

Canyon Dam, sampled near the surface.

EXPLANATION

LOWESS CURVE

 ALL DATA

LOWESS CURVE

 SEASONAL

UNCENSORED

CENSORED

5

9

6

7

8

pH, IN STANDARD UNITS

1970 1975 1980 1985 1990 1995 2000

0

100

20

40

60

80

SPECIFIC CONDUCTANCE, IN

MICROSIEMENS PER

CENTIMETER AT

25 DEGREES CELSIUS

1970 1975 1980 1985 1990 1995 2000

0

30

10

20

WATER TEMPERATURE,

IN DEGREES CELSIUS

0

20

5

10

15

OXYGEN, DISSOLVED,

IN MILLIGRAMS PER LITER
Hydrologic and Water-Quality Data 109

Figure 36. Temporal variations in periodic measurements and concentrations at Horsetooth Reservoir, near Spring 

Canyon Dam, sampled near the surface.—Continued

EXPLANATION

LOWESS CURVE

 ALL DATA

LOWESS CURVE

 SEASONAL

UNCENSORED

CENSORED

0

15

5

10

CALCIUM, DISSOLVED,

IN MILLIGRAMS PER LITER

0

3

1

2

MAGNESIUM, DISSOLVED,

IN MILLIGRAMS PER LITER

0

1.0

0.2

0.4

0.6

0.8

POTASSIUM, DISSOLVED,

IN MILLIGRAMS PER LITER

0

0.4

0.1

0.2

0.3

SODIUM ADSORPTION RATIO

1970 1975 1980 1985 1990 1995 2000

0

4

1

2

3

SODIUM, DISSOLVED,

IN MILLIGRAMS PER LITER

1970 1975 1980 1985 1990 1995 2000

0

20

5

10

15

SODIUM, IN PERCENT

0

40

10

20

30

HARDNESS, TOTAL, IN

MILLIGRAMS PER LITER

AS CACO3
110 Water Quality and Trend Analysis of Colorado–Big Thompson System Reservoirs and Related Conveyances

EXPLANATION

LOWESS CURVE

 ALL DATA

LOWESS CURVE

 SEASONAL

UNCENSORED

CENSORED

0

0.4

0.1

0.2

0.3

FLUORIDE, DISSOLVED,

IN MILLIGRAMS PER LITER

1970 1975 1980 1985 1990 1995 2000

0

15

5

10

SULFATE, DISSOLVED,

IN MIILIGRAMS PER LITER

0

50

10

20

30

40

ACID-NEUTRALIZING 

CAPACITY, IN MILLIGRAMS

PER LITER AS CACO3

0

3

1

2

CHLORIDE, DISSOLVED,

IN MILLIGRAMS PER LITER

1970 1975 1980 1985 1990 1995 2000

0

6

2

4

SILICA, DISSOLVED,

IN MILLIGRAMS PER LITER

Figure 36. Temporal variations in periodic measurements and concentrations at Horsetooth Reservoir, near Spring 

Canyon Dam, sampled near the surface.—Continued
Hydrologic and Water-Quality Data 111

EXPLANATION

LOWESS CURVE

 ALL DATA

LOWESS CURVE

 SEASONAL

UNCENSORED

CENSORED

0

0.20

0.05

0.10

0.15

NITROGEN, NITRITE PLUS 

NITRATE, DISSOLVED, IN

MILLIGRAMS PER LITER

0

0.10

0.02

0.04

0.06

0.08

PHOSPHORUS, DISSOLVED,

IN MILLIGRAMS PER LITER

1970 1975 1980 1985 1990 1995 2000

0

0.10

0.02

0.04

0.06

0.08

PHOSPHORUS, TOTAL,

IN MILLIGRAMS PER LITER

0

1.2

0.2

0.4

0.6

0.8

1.0

NITROGEN, AMMONIA PLUS 

ORGANIC, TOTAL, IN

MILLIGRAMS PER LITER

1970 1975 1980 1985 1990 1995 2000

0

0.03

0.01

0.02

PHOSPHORUS, ORTHO, 

DISSOLVED, IN

MILLIGRAMS PER LITER

0

0.10

0.02

0.04

0.06

0.08

NITROGEN, NITRITE, 

DISSOLVED, IN

MILLIGRAMS PER LITER

0

0.10

0.02

0.04

0.06

0.08

NITROGEN, AMMONIA, 

DISSOLVED, IN

MILLIGRAMS PER LITER

Figure 36. Temporal variations in periodic measurements and concentrations at Horsetooth Reservoir, near Spring 

Canyon Dam, sampled near the surface.—Continued
112 Water Quality and Trend Analysis of Colorado–Big Thompson System Reservoirs and Related Conveyances

EXPLANATION

LOWESS CURVE

 ALL DATA

LOWESS CURVE

 SEASONAL

UNCENSORED

CENSORED

0

80

20

40

60

SOLIDS, DISSOLVED,

SUM OF CONSTITUENTS,

IN MILLIGRAMS PER LITER

0

150

50

100

TRANSPARENCY, SECCHI,

IN INCHES

0

15

5

10

COLIFORM, FECAL, 

IN COLONIES

 PER 100 MILLILITERS

1970 1975 1980 1985 1990 1995 2000

0

8

2

4

6

CHLOROPHYLL-a, 

PHYTOPLANKTON, IN

MICROGRAMS PER LITER

1970 1975 1980 1985 1990 1995 2000

0

CHLOROPHYLL-b, 

PHYTOPLANKTON, IN

MICROGRAMS PER LITER

0

80

20

40

60

SOLIDS, DISSOLVED, RESIDUE

AT 180 DEGREE CELSIUS,

IN MILLIGRAMS PER LITER

0.8

0.2

0.4

0.6

Figure 36. Temporal variations in periodic measurements and concentrations at Horsetooth Reservoir, near Spring 

Canyon Dam, sampled near the surface.—Continued
Hydrologic and Water-Quality Data 113

EXPLANATION

LOWESS CURVE

 ALL DATA

LOWESS CURVE

 SEASONAL

UNCENSORED

CENSORED

0

100

20

40

60

80

IRON, DISSOLVED,

IN MICROGRAMS PER LITER

0

10

2

4

6

8

MANGANESE, DISSOLVED,

IN MICROGRAMS PER LITER

0

80

20

40

60

MANGANESE, TOTAL 

RECOVERABLE, IN

MICROGRAMS PER LITER

1970 1975 1980 1985 1990 1995 2000

0

80

20

40

60

STRONTIUM, DISSOLVED,

IN MICROGRAMS PER LITER

1970 1975 1980 1985 1990 1995 2000

0

40

10

20

30

ZINC, DISSOLVED,

IN MICROGRAMS PER LITER

0

40

10

20

30

BARIUM, DISSOLVED,

IN MICROGRAMS PER LITER

Figure 36. Temporal variations in periodic measurements and concentrations at Horsetooth Reservoir, near Spring 

Canyon Dam, sampled near the surface.—Continued
114 Water Quality and Trend Analysis of Colorado–Big Thompson System Reservoirs and Related Conveyances

Figure 37. Temporal variations in periodic measurements and concentrations at Horsetooth Reservoir, near Spring 

Canyon Dam, sampled near the bottom.

EXPLANATION

LOWESS CURVE

 ALL DATA

LOWESS CURVE

 SEASONAL

UNCENSORED

CENSORED

0

15

5

10

OXYGEN, DISSOLVED,

IN MILLIGRAMS PER LITER

5

9

6

7

8

pH, IN STANDARD UNITS

0

150

50

100

SPECIFIC CONDUCTANCE, IN

MICROSIEMENS PER 

CENTIMETER AT 

25 DEGREES CELSIUS

0

15

5

10

WATER TEMPERATURE,

IN DEGREES CELSIUS

1970 1975 1980 1985 1990 1995 2000

0

80

20

40

60

SOLIDS, DISSOLVED,

RESIDUE AT 180 DEGREES

CELSIUS, IN MILLIGRAMS

PER LITER

1970 1975 1980 1985 1990 1995 2000

0

60

20

0

SOLIDS, DISSOLVED, SUM OF

CONSTITUENTS, IN

MILLIGRAMS PER LITER

0

200

50

100

150

SAMPLE DEPTH BELOW

SURFACE, IN FEET

4
Hydrologic and Water-Quality Data 115

Figure 37. Temporal variations in periodic measurements and concentrations at Horsetooth Reservoir, near Spring 

Canyon Dam, sampled near the bottom.—Continued

EXPLANATION

LOWESS CURVE

 ALL DATA

LOWESS CURVE

 SEASONAL

UNCENSORED

CENSORED

0

15

5

10

CALCIUM, DISSOLVED,

IN MILLIGRAMS PER LITER

0

1.5

0.5

1.0

POTASSIUM, DISSOLVED,

IN MILLIGRAMS PER LITER

0

4

1

2

3

SODIUM, DISSOLVED,

IN MILLIGRAMS PER LITER

0

50

10

20

30

40

HARDNESS, TOTAL, IN

MILLIGRAMS PER LITER

AS CACO3 0

0.4

0.1

0.2

0.3

SODIUM ADSORPTION RATIO

1970 1975 1980 1985 1990 1995 2000

0

20

5

10

15

SODIUM, IN PERCENT

1970 1975 1980 1985 1990 1995 2000

0

3

1

2

MAGNESIUM, DISSOLVED,

IN MILLIGRAMS PER LITER
116 Water Quality and Trend Analysis of Colorado–Big Thompson System Reservoirs and Related Conveyances

0

0.4

0.1

0.2

0.3

FLUORIDE, DISSOLVED,

IN MILLIGRAMS PER LITER

0

40

10

20

30

ACID-NEUTRALIZING CAPACITY,

IN MILLGRAMS PER LITER

1970 1975 1980 1985 1990 1995 2000

0

6

2

4

SULFATE, DISSOLVED,

IN MILLIGRAMS PER LITER

0

2

1

CHLORIDE, DISSOLVED,

IN MILLIGRAMS PER LITER

1970 1975 1980 1985 1990 1995 2000

0

8

2

4

6

SILICA, DISSOLVED,

IN MILLIGRAMS PER LITER

EXPLANATION

LOWESS CURVE

 ALL DATA

LOWESS CURVE

 SEASONAL

UNCENSORED

CENSORED

Figure 37. Temporal variations in periodic measurements and concentrations at Horsetooth Reservoir, near Spring 

Canyon Dam, sampled near the bottom.—Continued
Hydrologic and Water-Quality Data 117

EXPLANATION

LOWESS CURVE

 ALL DATA

LOWESS CURVE

 SEASONAL

UNCENSORED

CENSORED

0

0.3

0.1

0.2

NITROGEN, NITRITE PLUS 

NITRATE, DISSOLVED, IN

MILLIGRAMS PER LITER

0

0.10

0.02

0.04

0.06

0.08

PHOSPHORUS, DISSOLVED,

IN MILLIGRAMS PER LITER

0

0.8

0.2

0.4

0.6

NITROGEN, AMMONIA PLUS 

ORGANIC, TOTAL, IN

MILLIGRAMS PER LITER

0

0.20

0.05

0.10

0.15

NITROGEN, AMMONIA, 

DISSOLVED, IN MILLIGRAMS

PER LITER

1970 1975 1980 1985 1990 1995 2000

0

0.08

0.02

0.04

0.06

PHOSPHORUS, ORTHO, 

DISSOLVED, IN MILLIGRAMS

PER LITER

1970 1975 1980 1985 1990 1995 2000

0

0.20

0.05

0.10

0.15

PHOSPHORUS, TOTAL,

IN MILLIGRAMS PER LITER

0

0.08

0.02

0.04

0.06

NITROGEN, NITRITE, 

DISSOLVED, IN

MILLIGRAMS PER LITER

Figure 37. Temporal variations in periodic measurements and concentrations at Horsetooth Reservoir, near Spring 

Canyon Dam, sampled near the bottom.—Continued
118 Water Quality and Trend Analysis of Colorado–Big Thompson System Reservoirs and Related Conveyances

0

0 0

150

50

100

IRON, DISSOLVED,

IN MICROGRAMS PER LITER

0

150

50

100

MANGANESE, TOTAL,

IN MICROGRAMS PER LITER

1970 1975 1980 1985 1990 1995 2000

0

50

10

20

30

40

ZINC, DISSOLVED,

IN MICROGRAMS PER LITER

0

800

200

400

600

MANGANESE, DISSOLVED,

IN MICROGRAMS PER LITER

1970 1975 1980 1985 1990 1995 2000

0

80

20

40

60

STRONTIUM, DISSOLVED,

IN MICROGRAMS PER LITER

40

10

20

30

BARIUM, DISSOLVED,

IN MICROGRAMS PER LITER

EXPLANATION

LOWESS CURVE

 ALL DATA

LOWESS CURVE

 SEASONAL

UNCENSORED

CENSORED

Figure 37. Temporal variations in periodic measurements and concentrations at Horsetooth Reservoir, near Spring 

Canyon Dam, sampled near the bottom.—Continued
Hydrologic and Water-Quality Data 119

Table 4. Summary statistics for site 1, Lake Granby, near spillway, near surface.

------------------------------------------------------------------------------------------------------------------------------------

STATISTICAL SUMMARY OF SELECTED WATER QUALITY DATA COLLECTED FROM AUG 1973 TO NOV 2000

PERCENT OF SAMPLES IN WHICH VALUES

DESCRIPTIVE STATISTICS WERE LESS THAN OR EQUAL TO THOSE SHOWN

------------------------------------- -------------------------------------- ------------------------------------------------------

SAMPLE (MEDIAN)

WATER-QUALITY CONSTITUENT SIZE MAXIMUM MINIMUM MEAN 95 % 75 % 50 % 25 % 5 %

------------------------------------- -------------------------------------- ------------------------------------------------------

70300 RESIDUE DIS 180C MG/L 57 56.000 16.000 37.158 50.100 41.500 37.000 33.500 20.60070301 DISSOLVED SOLIDS MG/L 59 43.800 27.800 34.620 40.800 37.000 35.000 31.700 28.90000077 TRANSPARENCY (IN (INCHES) 67 236.000 67.000 129.881 217.400 158.000 118.000 94.000 69.60000300 OXYGEN DISSOLVED (MG/L) 79 10.400 5.000 7.794 9.300 8.600 7.500 7.200 6.50000400 PH, WH, FIELD (STANDARD UNIT 75 9.100 5.700 7.931 8.920 8.300 7.900 7.600 7.08000403 PH, WH, LABORATO (STANDARD UNIT 59 8.700 6.800 7.664 8.400 7.900 7.600 7.500 7.10090095 SPECIFIC CONDUCT MICROSIEMENS/C 59 73.000 46.000 56.915 64.000 62.000 57.000 53.000 47.00000095 SPECIFIC CONDUCT US/CM @ 25C 77 68.000 41.000 54.805 67.100 61.000 53.000 48.500 43.00000010 WATER TEMPERATUR (DEGREES C) 78 19.500 0.000 13.887 19.005 17.125 15.150 11.175 5.50000900 HARDNESS TOTAL (MG/L AS CAO3) 60 28.000 18.000 22.617 27.095 24.600 22.200 20.825 18.13000915 CALCIUM DISSOLVE (MG/L AS CA) 60 8.700 5.500 6.961 8.490 7.600 6.870 6.430 5.62100925 MAGNESIUM DISSOL (MG/L AS MG) 60 1.600 0.972 1.255 1.500 1.400 1.200 1.150 1.00500935 POTASSIUM DISSOL (MG/L AS K) 60 1.000 0.550 0.681 0.800 0.700 0.700 0.600 0.57100931 SODIUM ADSORPTIO (RATIO) 60 0.300 0.159 0.198 0.238 0.214 0.199 0.176 0.16200930 SODIUM DISSOLVED (MG/L AS NA) 60 2.900 1.600 2.160 2.700 2.475 2.200 1.800 1.60500932 SODIUM, PERCENT PERCENT 60 24.000 14.600 16.660 18.100 17.475 16.450 15.725 14.80590410 ANC, TIT. 4.5, L MG/L AS CACO3 59 31.000 20.000 24.814 29.000 27.000 24.000 23.000 21.00000940 CHLORIDE DISSOLV (MG/L AS CL) 60 1.100 0.100 0.447 0.800 0.500 0.400 0.325 0.10500950 FLUORIDE DISSOLV (MG/L AS F) 60 0.300 -- 0.111* *0.200 *0.100 *0.100 *0.100 *0.06400955 SILICA DISSOLVED (MG/L AS SIO2) 60 7.200 2.300 5.047 6.690 5.700 4.850 4.400 3.90500945 SULFATE DISSOLVE (MG/L AS SO4) 60 5.000 1.700 2.945 4.095 3.300 2.950 2.425 2.00000608 NITROGEN AMMONIA (MG/L AS N) 59 0.050 -- 0.012* *0.030 *0.020 *0.010 *0.005 *0.00300625 NITROGEN AMM+ORG (MG/L AS N) 78 3.900 -- 0.344* *0.758 *0.300 *0.200 *0.156 *0.08100610 NITROGEN AMMONIA (MG/L AS N) 24 0.060 -- 0.023* *0.060 *0.040 *0.015 *0.007 *0.00300631 NO2 + NO3 DISSOL (MG/L AS N) 60 0.089 -- 0.015* *0.072 *0.016 *0.006 *0.002 *0.00100630 NO2 + NO3 TOTAL (MG/L AS N) 30 0.100 -- 0.100* *0.100 *0.100 *0.100 *0.100 *0.10000613 NITROGEN,NITRITE MG/L AS N 59 -- -- -- -- -- -- -- --00666 PHOSPHORUS DISS. (MG/L AS P) 57 0.053 -- 0.005* *0.020 *0.006 *0.003 *0.001 *0.00100671 PHOSPHORUS ORTHO (MG/L AS P) 60 0.020 -- 0.002* *0.011 *0.002 *0.001 *0.000 *0.00000665 PHOSPHORUS TOTAL (MG/L AS P) 78 0.170 -- 0.013* *0.040 *0.020 *0.009 *0.005 *0.00231625 COLIFORM FECAL 0 COLS./100 ML 58 2.000 -- 0.765* *1.000 *0.909 *0.731 *0.607 *0.46070953 CHL-A PHY CHROMA UG/L 60 13.000 0.400 2.947 8.360 3.600 2.050 1.300 0.90070954 CHLOROPHYLL-B, P UG/L 58 0.300 -- 0.084* *0.300 *0.100 *0.056 *0.031 *0.01301005 BARIUM DISSOLVED (UG/L AS BA) 59 9.000 2.000 7.932 9.000 8.100 8.000 7.800 7.00001010 BERYLLIUM DISSOL (UG/L AS BE) 59 -- -- -- -- -- -- -- --01020 BORON DISSOLVED (UG/L AS B) 59 40.000 -- 4.569* *15.000 *5.586 *2.682 *1.345 *0.50401025 CADMIUM DISSOLVE (UG/L AS CD) 69 2.000 -- 0.516* *2.000 *0.649 *0.380 *0.214 *0.09901030 CHROMIUM DISSOLV (UG/L AS CR) 59 -- -- -- -- -- -- -- --01035 COBALT DISSOLVED (UG/L AS CO) 59 -- -- -- -- -- -- -- --01040 COPPER DISSOLVED (UG/L AS CU) 69 14.022 -- 2.418* *8.718 *3.013 *1.608 *0.759 *0.29501046 IRON DISSOLVED (UG/L AS FE) 60 2100.000 -- 51.669* *59.000 *30.000 *10.000 *4.245 *1.16501049 LEAD DISSOLVED (UG/L AS PB) 75 13.700 -- 2.597* *10.000 *3.307 *1.734 *0.919 *0.44701130 LITHIUM DISSOLVE (UG/L AS LI) 59 5.000 -- 1.468* *4.000 *1.902 *1.212 *0.800 *0.42401056 MANGANESE DISSOL (UG/L AS MN) 60 30.000 -- 2.120* *9.950 *2.000 *0.878 *0.325 *0.08801060 MOLYBDENUM DISSO (UG/L AS MO) 59 -- -- -- -- -- -- -- --01065 NICKEL DISSOLVED (UG/L AS NI) 73 10.000 -- 1.196* *4.987 *1.300 *0.581 *0.257 *0.08701075 SILVER DISSOLVED (UG/L AS AG) 58 -- -- -- -- -- -- -- --01080 STRONTIUM DISSOL (UG/L AS SR) 59 57.000 32.900 42.766 52.000 47.000 42.000 38.000 33.50001085 VANADIUM DISSOLV (UG/L AS V) 59 -- -- -- -- -- -- -- --01090 ZINC DISSOLVED (UG/L AS ZN) 71 40.000 -- 5.077* *12.400 *8.000 *3.000 *1.834 *0.819-------------------------------------------------------------------------------------------------------------------------------------* - VALUE IS ESTIMATED BY USING A LOG-PROBABILITY REGRESSION TO PREDICT

THE VALUES OF DATA BELOW THE DETECTION LIMIT (Helsel and Cohn, 1988)
120 Water Quality and Trend Analysis of Colorado–Big Thompson System Reservoirs and Related Conveyances Table 5. Summary statistics for site 1, Lake Granby, near spillway, near bottom. ------------------------------------------------------------------------------------------------------------------------------------ STATISTICAL SUMMARY OF SELECTED WATER QUALITY DATA COLLECTED FROM AUG 1973 TO NOV 2000 PERCENT OF SAMPLES IN WHICH VALUES DESCRIPTIVE STATISTICS WERE LESS THAN OR EQUAL TO THOSE SHOWN ------------------------------------- -------------------------------------- ------------------------------------------------------ SAMPLE (MEDIAN) WATER-QUALITY CONSTITUENT SIZE MAXIMUM MINIMUM MEAN 95 % 75 % 50 % 25 % 5 % ------------------------------------- -------------------------------------- ------------------------------------------------------ 70300 RESIDUE DIS 180C MG/L 57 60.000 24.000 40.088 58.000 44.000 40.000 36.000 26.900 70301 DISSOLVED SOLIDS MG/L 58 50.500 29.800 37.459 44.335 40.875 37.850 33.550 31.975 00300 OXYGEN DISSOLVED (MG/L) 72 8.600 0.000 4.571 7.875 6.175 4.800 3.050 1.230 00400 PH, WH, FIELD (STANDARD UNIT 67 8.300 6.500 7.166 7.760 7.300 7.100 7.000 6.700 00403 PH, WH, LABORATO (STANDARD UNIT 58 8.000 6.600 7.166 7.710 7.325 7.100 6.975 6.795 90095 SPECIFIC CONDUCT MICROSIEMENS/C 58 80.000 50.000 60.328 70.050 63.000 61.000 55.750 53.000 00095 SPECIFIC CONDUCT US/CM @ 25C 69 80.000 43.000 56.942 70.500 65.000 55.000 49.500 44.500 00010 WATER TEMPERATUR (DEGREES C) 73 18.000 4.000 6.658 8.790 7.400 6.500 5.550 4.410 00900 HARDNESS TOTAL (MG/L AS CAO3) 59 30.600 20.300 23.937 28.200 25.500 23.900 21.700 20.700 00915 CALCIUM DISSOLVE (MG/L AS CA) 59 9.600 6.290 7.376 8.800 7.800 7.300 6.700 6.330 00925 MAGNESIUM DISSOL (MG/L AS MG) 59 1.600 1.090 1.323 1.500 1.400 1.300 1.200 1.160 00935 POTASSIUM DISSOL (MG/L AS K) 59 1.400 0.560 0.730 0.900 0.800 0.700 0.660 0.580 00931 SODIUM ADSORPTIO (RATIO) 59 0.400 0.162 0.205 0.252 0.217 0.202 0.183 0.165 00930 SODIUM DISSOLVED (MG/L AS NA) 59 4.100 1.700 2.298 2.900 2.600 2.300 1.900 1.800 00932 SODIUM, PERCENT PERCENT 59 26.000 14.300 16.695 18.500 17.400 16.500 15.900 14.600 90410 ANC, TIT. 4.5, L MG/L AS CACO3 58 35.000 19.000 25.672 29.100 27.000 26.000 23.000 22.000 00940 CHLORIDE DISSOLV (MG/L AS CL) 59 1.000 0.100 0.444 0.800 0.500 0.400 0.300 0.200 00950 FLUORIDE DISSOLV (MG/L AS F) 59 0.300 -- 0.108* *0.200 *0.100 *0.100 *0.100 *0.061 00955 SILICA DISSOLVED (MG/L AS SIO2) 59 8.600 4.800 6.305 8.100 7.200 5.900 5.500 5.000 00945 SULFATE DISSOLVE (MG/L AS SO4) 59 4.000 2.000 2.963 3.700 3.300 3.000 2.600 2.300 00608 NITROGEN AMMONIA (MG/L AS N) 58 0.070 -- 0.017* *0.060 *0.030 *0.010 *0.006 *0.003 00625 NITROGEN AMM+ORG (MG/L AS N) 70 1.300 -- 0.232* *0.745 *0.221 *0.179 *0.135 *0.064 00610 NITROGEN AMMONIA (MG/L AS N) 18 0.040 -- 0.019* *0.040 *0.030 *0.020 *0.009 *0.004 00631 NO2 + NO3 DISSOL (MG/L AS N) 59 0.200 -- 0.091* *0.180 *0.110 *0.087 *0.059 *0.032 00630 NO2 + NO3 TOTAL (MG/L AS N) 23 0.200 -- 0.107* *0.200 *0.100 *0.100 *0.072 *0.051 00613 NITROGEN,NITRITE MG/L AS N 58 -- -- -- -- -- -- -- -- 00666 PHOSPHORUS DISS. (MG/L AS P) 56 0.030 -- 0.008* *0.028 *0.010 *0.005 *0.003 *0.001 00671 PHOSPHORUS ORTHO (MG/L AS P) 59 0.030 -- 0.006* *0.020 *0.008 *0.004 *0.002 *0.001 00665 PHOSPHORUS TOTAL (MG/L AS P) 70 0.290 -- 0.019* *0.064 *0.020 *0.010 *0.005 *0.002 31625 COLIFORM FECAL 0 COLS./100 ML 2 -- -- -- -- -- -- -- -- 70954 CHLOROPHYLL-B, P UG/L 1 -- -- -- -- -- -- -- -- 01005 BARIUM DISSOLVED (UG/L AS BA) 58 11.000 2.000 7.545 9.000 8.000 7.900 7.000 4.950 01010 BERYLLIUM DISSOL (UG/L AS BE) 58 -- -- -- -- -- -- -- -- 01020 BORON DISSOLVED (UG/L AS B) 58 10.000 -- 4.679* *10.000 *5.761 *3.997 *2.894 *1.790 01025 CADMIUM DISSOLVE (UG/L AS CD) 69 5.000 -- 0.358* *1.000 *0.376 *0.159 *0.064 *0.018 01030 CHROMIUM DISSOLV (UG/L AS CR) 58 -- -- -- -- -- -- -- -- 01035 COBALT DISSOLVED (UG/L AS CO) 58 -- -- -- -- -- -- -- -- 01040 COPPER DISSOLVED (UG/L AS CU) 68 12.832 -- 2.858* *8.746 *3.570 *2.000 *1.171 *0.525 01046 IRON DISSOLVED (UG/L AS FE) 59 2500.000 -- 60.812* *50.000 *20.000 *20.000 *10.000 *3.139 01049 LEAD DISSOLVED (UG/L AS PB) 68 10.000 -- 2.008* *10.000 *2.194 *1.153 *0.545 *0.226 01130 LITHIUM DISSOLVE (UG/L AS LI) 58 -- -- -- -- -- -- -- -- 01056 MANGANESE DISSOL (UG/L AS MN) 57 560.000 -- 28.046* *131.000 *19.500 *6.000 *1.000 *0.141 01060 MOLYBDENUM DISSO (UG/L AS MO) 58 -- -- -- -- -- -- -- -- 01065 NICKEL DISSOLVED (UG/L AS NI) 68 -- -- -- -- -- -- -- -- 01075 SILVER DISSOLVED (UG/L AS AG) 58 -- -- -- -- -- -- -- -- 01080 STRONTIUM DISSOL (UG/L AS SR) 58 59.000 37.900 45.545 55.050 49.000 46.500 40.950 37.995 01085 VANADIUM DISSOLV (UG/L AS V) 58 -- -- -- -- -- -- -- -- 01090 ZINC DISSOLVED (UG/L AS ZN) 69 40.000 -- 6.627* *19.000 *10.000 *4.000 *2.340 *1.102 ------------------------------------------------------------------------------------------------------------------------------------

* - VALUE IS ESTIMATED BY USING A LOG-PROBABILITY REGRESSION TO PREDICT

THE VALUES OF DATA BELOW THE DETECTION LIMIT (Helsel and Cohn, 1988)
Hydrologic and Water-Quality Data 121

Table 6. Summary statistics for site 2, Lake Granby (Rainbow Bay), sampled near surface.

------------------------------------------------------------------------------------------------------------------------------------

STATISTICAL SUMMARY OF SELECTED WATER QUALITY DATA COLLECTED FROM MAY 1989 TO NOV 2000

PERCENT OF SAMPLES IN WHICH VALUES

DESCRIPTIVE STATISTICS WERE LESS THAN OR EQUAL TO THOSE SHOWN

------------------------------------- -------------------------------------- ------------------------------------------------------

SAMPLE (MEDIAN)

WATER-QUALITY CONSTITUENT SIZE MAXIMUM MINIMUM MEAN 95 % 75 % 50 % 25 % 5 %

------------------------------------- -------------------------------------- ------------------------------------------------------

70300 RESIDUE DIS 180C MG/L 58 71.000 14.000 38.569 54.100 44.000 38.000 34.000 14.95070301 DISSOLVED SOLIDS MG/L 57 54.600 28.100 36.440 47.570 37.800 36.200 32.050 28.89000077 TRANSPARENCY (IN (INCHES) 56 206.000 9.500 109.812 187.650 145.250 102.500 75.500 42.25000300 OXYGEN DISSOLVED (MG/L) 59 9.400 4.900 7.519 8.900 8.200 7.400 7.000 6.50000400 PH, WH, FIELD (STANDARD UNIT 60 9.000 7.100 7.982 8.890 8.375 7.950 7.700 7.30000403 PH, WH, LABORATO (STANDARD UNIT 59 8.500 7.000 7.649 8.300 7.800 7.600 7.500 7.20090095 SPECIFIC CONDUCT MICROSIEMENS/C 59 87.000 47.000 59.508 79.000 63.000 59.000 53.000 48.00000095 SPECIFIC CONDUCT US/CM @ 25C 60 87.000 41.000 56.367 75.850 61.000 55.000 49.250 44.00000010 WATER TEMPERATUR (DEGREES C) 60 19.700 4.800 14.158 19.300 17.475 14.550 11.125 7.32000900 HARDNESS TOTAL (MG/L AS CAO3) 59 35.800 18.200 23.732 31.600 25.400 23.200 21.200 19.30000915 CALCIUM DISSOLVE (MG/L AS CA) 59 11.000 5.660 7.327 10.000 7.900 7.100 6.540 5.88000925 MAGNESIUM DISSOL (MG/L AS MG) 59 2.000 0.981 1.305 1.700 1.400 1.300 1.180 1.09000935 POTASSIUM DISSOL (MG/L AS K) 59 1.000 0.550 0.687 0.800 0.700 0.700 0.600 0.58000931 SODIUM ADSORPTIO (RATIO) 59 0.284 0.160 0.201 0.265 0.215 0.203 0.175 0.16500930 SODIUM DISSOLVED (MG/L AS NA) 59 3.900 1.600 2.263 3.300 2.500 2.300 1.900 1.60000932 SODIUM, PERCENT PERCENT 59 19.900 14.400 16.595 18.800 17.400 16.500 15.800 14.80090410 ANC, TIT. 4.5, L MG/L AS CACO3 59 38.000 20.000 25.881 33.000 28.000 25.000 23.000 21.00000940 CHLORIDE DISSOLV (MG/L AS CL) 58 1.300 0.100 0.438 1.005 0.500 0.400 0.300 0.10000950 FLUORIDE DISSOLV (MG/L AS F) 58 0.300 -- 0.112* *0.200 *0.100 *0.100 *0.100 *0.06500955 SILICA DISSOLVED (MG/L AS SIO2) 59 9.600 2.400 5.551 8.900 6.300 5.200 4.600 4.10000945 SULFATE DISSOLVE (MG/L AS SO4) 58 4.500 1.700 3.012 4.020 3.325 3.000 2.500 1.99500608 NITROGEN AMMONIA (MG/L AS N) 59 0.075 -- 0.014* *0.050 *0.020 *0.009 *0.004 *0.00200625 NITROGEN AMM+ORG (MG/L AS N) 59 0.600 -- 0.215* *0.300 *0.250 *0.200 *0.170 *0.12700610 NITROGEN AMMONIA (MG/L AS N) 18 0.040 -- 0.016* *0.040 *0.022 *0.010 *0.007 *0.00300631 NO2 + NO3 DISSOL (MG/L AS N) 59 0.070 -- 0.018* *0.068 *0.020 *0.011 *0.006 *0.00200613 NITROGEN,NITRITE MG/L AS N 59 -- -- -- -- -- -- -- --00666 PHOSPHORUS DISS. (MG/L AS P) 56 0.118 -- 0.008* *0.022 *0.008 *0.004 *0.002 *0.00100671 PHOSPHORUS ORTHO (MG/L AS P) 59 0.020 -- 0.003* *0.012 *0.002 *0.001 *0.000 *0.00000665 PHOSPHORUS TOTAL (MG/L AS P) 60 0.045 -- 0.014* *0.040 *0.020 *0.010 *0.007 *0.00400680 CARBON ORGANIC T (MG/L AS C) 11 9.000 3.300 4.318 9.000 4.300 3.600 3.500 3.30031625 COLIFORM FECAL 0 COLS./100 ML 53 70.000 -- 2.077* *5.300 *1.000 *0.301 *0.091 *0.01670953 CHL-A PHY CHROMA UG/L 59 12.000 0.500 2.822 7.900 3.700 2.000 1.200 0.50070954 CHLOROPHYLL-B, P UG/L 58 0.300 -- 0.080* *0.300 *0.100 *0.053 *0.028 *0.01101005 BARIUM DISSOLVED (UG/L AS BA) 59 11.000 2.000 8.110 10.000 9.000 8.000 7.500 7.00001010 BERYLLIUM DISSOL (UG/L AS BE) 59 -- -- -- -- -- -- -- --01020 BORON DISSOLVED (UG/L AS B) 59 20.000 -- 4.549* *10.000 *5.958 *3.672 *2.405 *1.32401025 CADMIUM DISSOLVE (UG/L AS CD) 59 -- -- -- -- -- -- -- --01030 CHROMIUM DISSOLV (UG/L AS CR) 59 -- -- -- -- -- -- -- --01035 COBALT DISSOLVED (UG/L AS CO) 59 -- -- -- -- -- -- -- --01040 COPPER DISSOLVED (UG/L AS CU) 59 -- -- -- -- -- -- -- --01046 IRON DISSOLVED (UG/L AS FE) 59 130.000 -- 22.002* *70.000 *30.000 *10.000 *5.379 *1.89501049 LEAD DISSOLVED (UG/L AS PB) 58 20.000 -- 3.599* *10.060 *4.509 *2.546 *1.444 *0.62801130 LITHIUM DISSOLVE (UG/L AS LI) 59 4.000 -- 1.690* *3.208 *2.158 *1.559 *1.101 *0.74301056 MANGANESE DISSOL (UG/L AS MN) 59 23.000 -- 2.531* *14.000 *3.000 *1.000 *0.400 *0.10701060 MOLYBDENUM DISSO (UG/L AS MO) 59 -- -- -- -- -- -- -- --01065 NICKEL DISSOLVED (UG/L AS NI) 59 -- -- -- -- -- -- -- --01075 SILVER DISSOLVED (UG/L AS AG) 59 -- -- -- -- -- -- -- --01080 STRONTIUM DISSOL (UG/L AS SR) 59 73.000 33.200 45.022 65.000 48.000 44.000 38.100 33.90001085 VANADIUM DISSOLV (UG/L AS V) 59 -- -- -- -- -- -- -- --01090 ZINC DISSOLVED (UG/L AS ZN) 58 106.000 -- 5.562* *14.100 *5.556 *2.442 *1.093 *0.384------------------------------------------------------------------------------------------------------------------------------------* - VALUE IS ESTIMATED BY USING A LOG-PROBABILITY REGRESSION TO PREDICT

THE VALUES OF DATA BELOW THE DETECTION LIMIT (Helsel and Cohn, 1988)
122 Water Quality and Trend Analysis of Colorado–Big Thompson System Reservoirs and Related Conveyances Table 7. Summary statistics for site 2, Lake Granby (Rainbow Bay), sampled near bottom. ------------------------------------------------------------------------------------------------------------------------------------ STATISTICAL SUMMARY OF SELECTED WATER QUALITY DATA COLLECTED FROM MAY 1989 TO SEPT 2000 PERCENT OF SAMPLES IN WHICH VALUES DESCRIPTIVE STATISTICS WERE LESS THAN OR EQUAL TO THOSE SHOWN ------------------------------------- -------------------------------------- ------------------------------------------------------ SAMPLE (MEDIAN) WATER-QUALITY CONSTITUENT SIZE MAXIMUM MINIMUM MEAN 95 % 75 % 50 % 25 % 5 % ------------------------------------- -------------------------------------- ------------------------------------------------------ 70300 RESIDUE DIS 180C MG/L 57 67.000 18.000 39.789 60.000 46.000 39.000 34.000 25.700 70301 DISSOLVED SOLIDS MG/L 56 57.400 30.000 37.329 52.790 39.050 36.550 33.100 31.155 00300 OXYGEN DISSOLVED (MG/L) 59 9.000 1.100 5.349 8.800 7.300 5.300 3.300 1.500 00400 PH, WH, FIELD (STANDARD UNIT 60 8.100 6.800 7.400 8.000 7.600 7.400 7.125 6.900 00403 PH, WH, LABORATO (STANDARD UNIT 58 8.200 6.700 7.252 7.715 7.500 7.200 7.000 6.800 90095 SPECIFIC CONDUCT MICROSIEMENS/C 58 92.000 51.000 60.690 83.100 63.250 60.000 55.000 52.950 00095 SPECIFIC CONDUCT US/CM @ 25C 60 90.000 44.000 58.017 86.850 63.000 55.500 50.000 44.100 00010 WATER TEMPERATUR (DEGREES C) 60 15.500 4.700 9.453 14.685 11.775 8.450 7.600 5.800 00900 HARDNESS TOTAL (MG/L AS CAO3) 58 35.800 18.600 24.181 34.640 25.800 23.450 21.675 20.625 00915 CALCIUM DISSOLVE (MG/L AS CA) 58 11.000 5.750 7.460 11.000 8.000 7.300 6.682 6.285 00925 MAGNESIUM DISSOL (MG/L AS MG) 58 2.000 1.010 1.334 1.710 1.400 1.300 1.200 1.096 00935 POTASSIUM DISSOL (MG/L AS K) 58 0.900 0.500 0.688 0.800 0.800 0.700 0.608 0.579 00931 SODIUM ADSORPTIO (RATIO) 58 0.299 0.160 0.204 0.278 0.215 0.203 0.182 0.168 00930 SODIUM DISSOLVED (MG/L AS NA) 58 4.100 1.700 2.317 3.515 2.500 2.300 1.900 1.700 00932 SODIUM, PERCENT PERCENT 58 21.000 14.400 16.657 20.105 17.200 16.500 15.875 14.885 90410 ANC, TIT. 4.5, L MG/L AS CACO3 58 38.000 20.000 26.103 37.000 28.000 25.500 23.000 22.950 00940 CHLORIDE DISSOLV (MG/L AS CL) 57 1.300 0.100 0.453 0.930 0.500 0.400 0.400 0.190 00950 FLUORIDE DISSOLV (MG/L AS F) 57 0.300 -- 0.106* *0.200 *0.100 *0.100 *0.100 *0.063 00955 SILICA DISSOLVED (MG/L AS SIO2) 58 11.000 3.300 6.086 10.050 6.800 5.650 5.075 4.300 00945 SULFATE DISSOLVE (MG/L AS SO4) 57 4.500 1.900 3.000 4.210 3.300 3.000 2.600 2.190 00608 NITROGEN AMMONIA (MG/L AS N) 59 0.070 -- 0.018* *0.050 *0.020 *0.013 *0.008 *0.004 00625 NITROGEN AMM+ORG (MG/L AS N) 59 0.600 -- 0.215* *0.400 *0.300 *0.195 *0.156 *0.108 00610 NITROGEN AMMONIA (MG/L AS N) 18 0.070 -- 0.022* *0.070 *0.030 *0.020 *0.010 *0.005 00631 NO2 + NO3 DISSOL (MG/L AS N) 59 0.110 -- 0.036* *0.100 *0.053 *0.025 *0.014 *0.006 00613 NITROGEN,NITRITE MG/L AS N 59 0.030 -- 0.002* *0.010 *0.001 *0.000 *0.000 *0.000 00666 PHOSPHORUS DISS. (MG/L AS P) 56 0.501 -- 0.017* *0.030 *0.011 *0.005 *0.003 *0.001 00671 PHOSPHORUS ORTHO (MG/L AS P) 59 0.020 -- 0.004* *0.017 *0.005 *0.002 *0.001 *0.000 00665 PHOSPHORUS TOTAL (MG/L AS P) 59 0.040 -- 0.014* *0.030 *0.020 *0.010 *0.008 *0.004 00680 CARBON ORGANIC T (MG/L AS C) 11 9.300 3.200 4.018 9.300 3.800 3.500 3.300 3.200 31625 COLIFORM FECAL 0 COLS./100 ML 1 1.000 -- -- -- -- -- -- -- 01005 BARIUM DISSOLVED (UG/L AS BA) 58 14.000 7.000 8.510 11.000 9.000 8.000 7.975 7.295 01010 BERYLLIUM DISSOL (UG/L AS BE) 58 -- -- -- -- -- -- -- -- 01020 BORON DISSOLVED (UG/L AS B) 59 10.000 -- 4.358* *10.000 *5.448 *4.000 *2.785 *1.811 01025 CADMIUM DISSOLVE (UG/L AS CD) 58 2.000 -- 0.416* *1.007 *0.522 *0.323 *0.192 *0.096 01030 CHROMIUM DISSOLV (UG/L AS CR) 58 -- -- -- -- -- -- -- -- 01035 COBALT DISSOLVED (UG/L AS CO) 58 -- -- -- -- -- -- -- -- 01040 COPPER DISSOLVED (UG/L AS CU) 58 -- -- -- -- -- -- -- -- 01046 IRON DISSOLVED (UG/L AS FE) 57 80.000 -- 22.992* *71.000 *30.000 *20.000 *8.020 *3.245 01049 LEAD DISSOLVED (UG/L AS PB) 58 -- -- -- -- -- -- -- -- 01130 LITHIUM DISSOLVE (UG/L AS LI) 58 -- -- -- -- -- -- -- -- 01056 MANGANESE DISSOL (UG/L AS MN) 58 67.000 -- 7.927* *30.850 *8.000 *4.300 *1.108 *0.403 01060 MOLYBDENUM DISSO (UG/L AS MO) 58 -- -- -- -- -- -- -- -- 01065 NICKEL DISSOLVED (UG/L AS NI) 58 -- -- -- -- -- -- -- -- 01075 SILVER DISSOLVED (UG/L AS AG) 58 -- -- -- -- -- -- -- -- 01080 STRONTIUM DISSOL (UG/L AS SR) 58 77.000 35.100 46.384 71.200 49.000 45.000 40.675 37.715 01085 VANADIUM DISSOLV (UG/L AS V) 58 -- -- -- -- -- -- -- -- 01090 ZINC DISSOLVED (UG/L AS ZN) 58 12.000 -- 4.181* *9.050 *5.843 *3.279 *2.172 *1.224 ------------------------------------------------------------------------------------------------------------------------------------ * - VALUE IS ESTIMATED BY USING A LOG-PROBABILITY REGRESSION TO PREDICT

THE VALUES OF DATA BELOW THE DETECTION LIMIT (Helsel and Cohn, 1988)
Hydrologic and Water-Quality Data 123

Table 8. Summary statistics for site 3, Granby Pump Canal.

------------------------------------------------------------------------------------------------------------------------------------

STATISTICAL SUMMARY OF SELECTED WATER QUALITY DATA COLLECTED FROM NOV 1970 TO NOV 2000

PERCENT OF SAMPLES IN WHICH VALUES

DESCRIPTIVE STATISTICS WERE LESS THAN OR EQUAL TO THOSE SHOWN

------------------------------------- -------------------------------------- ------------------------------------------------------

SAMPLE (MEDIAN)

WATER-QUALITY CONSTITUENT SIZE MAXIMUM MINIMUM MEAN 95 % 75 % 50 % 25 % 5 %

------------------------------------- -------------------------------------- ------------------------------------------------------

70300 RESIDUE DIS 180C MG/L 55 66.000 26.000 40.691 54.400 44.000 40.000 36.000 29.60070301 DISSOLVED SOLIDS MG/L 55 42.500 30.100 36.747 41.780 39.400 37.700 33.200 31.40000300 OXYGEN DISSOLVED (MG/L) 170 12.000 2.000 7.014 9.600 8.425 7.550 5.600 3.31000400 PH, WH, FIELD (STANDARD UNIT 171 8.900 5.300 7.270 8.240 7.600 7.300 6.900 6.40000403 PH, WH, LABORATO (STANDARD UNIT 56 8.800 6.600 7.489 8.030 7.775 7.500 7.200 6.78590095 SPECIFIC CONDUCT MICROSIEMENS/C 56 70.000 49.000 60.554 70.000 64.000 62.000 56.000 52.70000095 SPECIFIC CONDUCT US/CM @ 25C 179 105.000 30.000 58.827 70.000 63.000 60.000 53.000 47.00000020 AIR TEMPERATURE DEGREES C 8 22.100 -6.000 4.525 22.100 12.400 3.250 -4.500 -6.00000010 WATER TEMPERATUR (DEGREES C) 180 17.500 1.000 5.426 12.490 7.500 4.100 3.000 2.00000900 HARDNESS TOTAL (MG/L AS CAO3) 56 28.000 19.300 23.682 27.330 25.300 23.900 21.700 20.11000915 CALCIUM DISSOLVE (MG/L AS CA) 56 8.700 5.760 7.305 8.515 7.800 7.400 6.632 6.16000925 MAGNESIUM DISSOL (MG/L AS MG) 56 1.500 1.120 1.306 1.500 1.400 1.300 1.200 1.15500935 POTASSIUM DISSOL (MG/L AS K) 56 0.900 0.560 0.726 0.900 0.800 0.700 0.665 0.59700931 SODIUM ADSORPTIO (RATIO) 56 0.236 0.154 0.199 0.229 0.216 0.202 0.178 0.16900930 SODIUM DISSOLVED (MG/L AS NA) 56 2.800 1.600 2.229 2.700 2.500 2.300 1.900 1.70000932 SODIUM, PERCENT PERCENT 56 18.200 14.500 16.452 17.915 17.100 16.500 15.625 15.05590410 ANC, TIT. 4.5, L MG/L AS CACO3 56 30.000 21.000 25.964 30.000 28.000 26.000 24.000 22.00000940 CHLORIDE DISSOLV (MG/L AS CL) 56 1.800 0.100 0.507 1.020 0.575 0.400 0.400 0.18500950 FLUORIDE DISSOLV (MG/L AS F) 57 3.000 -- 0.162* *0.210 *0.100 *0.100 *0.100 *0.04400955 SILICA DISSOLVED (MG/L AS SIO2) 56 7.700 3.700 5.636 7.400 6.400 5.400 5.025 3.90000945 SULFATE DISSOLVE (MG/L AS SO4) 56 5.400 1.900 3.068 4.150 3.375 3.000 2.700 2.28500608 NITROGEN AMMONIA (MG/L AS N) 55 0.080 -- 0.013* *0.039 *0.020 *0.010 *0.004 *0.00200625 NITROGEN AMM+ORG (MG/L AS N) 139 1.200 -- 0.358* *0.890 *0.500 *0.300 *0.180 *0.09900610 NITROGEN AMMONIA (MG/L AS N) 21 0.070 -- 0.019* *0.066 *0.020 *0.020 *0.010 *0.00500631 NO2 + NO3 DISSOL (MG/L AS N) 55 0.180 -- 0.078* *0.156 *0.100 *0.074 *0.048 *0.03300630 NO2 + NO3 TOTAL (MG/L AS N) 87 0.300 -- 0.092* *0.200 *0.100 *0.100 *0.065 *0.04500613 NITROGEN,NITRITE MG/L AS N 55 0.020 -- 0.002* *0.020 *0.002 *0.001 *0.000 *0.00000615 NITROGEN,NITRITE MG/L AS N 14 -- -- -- -- -- -- -- --00666 PHOSPHORUS DISS. (MG/L AS P) 55 0.040 -- 0.010* *0.020 *0.010 *0.008 *0.005 *0.00300671 PHOSPHORUS ORTHO (MG/L AS P) 55 0.032 -- 0.007* *0.016 *0.010 *0.005 *0.004 *0.00270507 PHOS ORTHO TOT A MG/L AS P 14 0.020 -- 0.011* *0.020 *0.020 *0.009 *0.006 *0.00400665 PHOSPHORUS TOTAL (MG/L AS P) 139 0.350 -- 0.022* *0.053 *0.030 *0.018 *0.010 *0.00431501 TOT COLI,MENDO M COLS./100 ML 104 110.000 -- 10.110* *43.800 *13.000 *3.000 *0.818 *0.14231625 COLIFORM FECAL 0 COLS./100 ML 72 10.000 -- 0.483* *3.000 *0.279 *0.068 *0.017 *0.00231616 FECAL COLI,MFC M COLS./100 ML 38 1.000 -- 1.000* *1.000 *1.000 *1.000 *1.000 *1.00001005 BARIUM DISSOLVED (UG/L AS BA) 56 9.000 2.000 7.298 8.030 8.000 7.350 7.000 5.02001010 BERYLLIUM DISSOL (UG/L AS BE) 56 -- -- -- -- -- -- -- --01025 CADMIUM DISSOLVE (UG/L AS CD) 99 5.000 -- 0.390* *1.000 *0.433 *0.197 *0.090 *0.02701030 CHROMIUM DISSOLV (UG/L AS CR) 55 -- -- -- -- -- -- -- --01035 COBALT DISSOLVED (UG/L AS CO) 56 -- -- -- -- -- -- -- --01040 COPPER DISSOLVED (UG/L AS CU) 100 11.000 -- 2.743* *6.716 *3.939 *2.000 *1.311 *0.69601046 IRON DISSOLVED (UG/L AS FE) 56 80.000 -- 23.657* *80.000 *40.000 *20.000 *7.787 *3.10401049 LEAD DISSOLVED (UG/L AS PB) 100 37.000 -- 2.074* *10.000 *2.000 *0.842 *0.318 *0.08901130 LITHIUM DISSOLVE (UG/L AS LI) 55 11.000 -- 1.951* *4.480 *2.461 *1.457 *0.933 *0.45901056 MANGANESE DISSOL (UG/L AS MN) 54 81.000 -- 10.307* *71.250 *7.250 *2.350 *1.000 *0.16601060 MOLYBDENUM DISSO (UG/L AS MO) 56 -- -- -- -- -- -- -- --01065 NICKEL DISSOLVED (UG/L AS NI) 99 15.000 -- 1.932* *6.000 *2.599 *1.134 *0.649 *0.27701075 SILVER DISSOLVED (UG/L AS AG) 55 2.000 -- 0.700* *2.000 *0.884 *0.568 *0.364 *0.18901080 STRONTIUM DISSOL (UG/L AS SR) 56 51.000 29.000 44.086 50.150 48.000 45.000 40.000 34.46501085 VANADIUM DISSOLV (UG/L AS V) 56 -- -- -- -- -- -- -- --01090 ZINC DISSOLVED (UG/L AS ZN) 98 30.000 -- 7.703* *20.000 *10.000 *5.000 *2.988 *1.452------------------------------------------------------------------------------------------------------------------------------------* - VALUE IS ESTIMATED BY USING A LOG-PROBABILITY REGRESSION TO PREDICT

THE VALUES OF DATA BELOW THE DETECTION LIMIT (Helsel and Cohn, 1988)
124 Water Quality and Trend Analysis of Colorado–Big Thompson System Reservoirs and Related Conveyances Table 9. Summary statistics for site 4, Shadow Mountain Lake, sampled at the surface. ------------------------------------------------------------------------------------------------------------------------------------ STATISTICAL SUMMARY OF SELECTED WATER QUALITY DATA COLLECTED FROM AUG 1973 TO SEPT 2000 PERCENT OF SAMPLES IN WHICH VALUES DESCRIPTIVE STATISTICS WERE LESS THAN OR EQUAL TO THOSE SHOWN ------------------------------------- -------------------------------------- ------------------------------------------------------ SAMPLE (MEDIAN) WATER-QUALITY CONSTITUENT SIZE MAXIMUM MINIMUM MEAN 95 % 75 % 50 % 25 % 5 % ------------------------------------- -------------------------------------- ------------------------------------------------------ 70300 RESIDUE DIS 180C MG/L 58 54.000 8.000 36.190 50.100 41.250 37.500 32.750 19.800 70301 DISSOLVED SOLIDS MG/L 54 41.300 19.600 33.009 40.275 37.200 33.650 30.400 22.375 00077 TRANSPARENCY (IN (INCHES) 64 157.000 42.000 86.984 129.250 97.500 83.500 70.000 56.750 00300 OXYGEN DISSOLVED (MG/L) 65 9.900 3.500 7.340 9.070 8.150 7.400 6.750 4.630 00400 PH, WH, FIELD (STANDARD UNIT 64 9.500 5.600 7.625 9.150 7.800 7.600 7.300 6.725 00403 PH, WH, LABORATO (STANDARD UNIT 58 9.200 6.800 7.469 8.815 7.650 7.400 7.100 6.800 90095 SPECIFIC CONDUCT MICROSIEMENS/C 58 67.000 30.000 52.534 62.150 59.250 55.000 48.500 33.900 00095 SPECIFIC CONDUCT US/CM @ 25C 65 66.000 27.000 50.631 64.400 57.000 51.000 46.000 31.600 00900 HARDNESS TOTAL (MG/L AS CAO3) 59 27.400 11.900 20.751 25.600 23.300 21.900 18.800 13.200 00915 CALCIUM DISSOLVE (MG/L AS CA) 59 8.300 3.600 6.294 8.000 7.000 6.510 5.700 4.000 00925 MAGNESIUM DISSOL (MG/L AS MG) 59 1.600 0.710 1.212 1.420 1.340 1.280 1.100 0.770 00935 POTASSIUM DISSOL (MG/L AS K) 59 1.100 0.400 0.696 1.000 0.800 0.700 0.600 0.500 00931 SODIUM ADSORPTIO (RATIO) 59 0.300 0.139 0.183 0.233 0.199 0.175 0.163 0.144 00930 SODIUM DISSOLVED (MG/L AS NA) 59 3.100 1.100 1.919 2.600 2.200 1.900 1.700 1.200 00932 SODIUM, PERCENT PERCENT 59 22.000 13.500 16.181 19.500 16.900 16.000 15.100 14.100 90410 ANC, TIT. 4.5, L MG/L AS CACO3 58 30.000 12.000 22.241 28.050 25.000 23.500 20.000 13.950 00940 CHLORIDE DISSOLV (MG/L AS CL) 58 1.000 -- 0.385* *0.715 *0.500 *0.400 *0.300 *0.140 00950 FLUORIDE DISSOLV (MG/L AS F) 58 0.300 0.100 0.140 0.200 0.200 0.100 0.100 0.100 00955 SILICA DISSOLVED (MG/L AS SIO2) 60 7.600 3.400 5.752 7.395 6.575 5.700 4.900 4.405 00945 SULFATE DISSOLVE (MG/L AS SO4) 58 4.000 1.800 2.903 3.805 3.200 2.900 2.600 2.100 00608 NITROGEN AMMONIA (MG/L AS N) 59 0.089 -- 0.015* *0.050 *0.020 *0.010 *0.004 *0.002 00625 NITROGEN AMM+ORG (MG/L AS N) 65 3.700 -- 0.308* *0.570 *0.300 *0.210 *0.171 *0.095 00610 NITROGEN AMMONIA (MG/L AS N) 24 0.060 -- 0.017* *0.055 *0.020 *0.010 *0.006 *0.003 00631 NO2 + NO3 DISSOL (MG/L AS N) 60 0.130 -- 0.035* *0.090 *0.052 *0.027 *0.018 *0.010 00630 NO2 + NO3 TOTAL (MG/L AS N) 18 0.100 -- 0.100* *0.100 *0.100 *0.100 *0.100 *0.100 00613 NITROGEN,NITRITE MG/L AS N 59 -- -- -- -- -- -- -- -- 00615 NITROGEN,NITRITE MG/L AS N 9 -- -- -- -- -- -- -- -- 00605 NITROGEN ORGANIC (MG/L AS N) 30 3.700 0.105 0.401 2.100 0.373 0.261 0.182 0.130 00600 NITROGEN TOTAL (MG/L AS N) 18 3.700 0.250 0.509 3.700 0.385 0.300 0.268 0.250 00666 PHOSPHORUS DISS. (MG/L AS P) 57 0.020 -- 0.005* *0.011 *0.006 *0.004 *0.003 *0.002 00671 PHOSPHORUS ORTHO (MG/L AS P) 60 0.014 -- 0.001* *0.010 *0.001 *0.000 *0.000 *0.000 70507 PHOS ORTHO TOT A MG/L AS P 9 -- -- -- -- -- -- -- -- 00665 PHOSPHORUS TOTAL (MG/L AS P) 65 0.048 -- 0.014* *0.030 *0.020 *0.010 *0.007 *0.004 31625 COLIFORM FECAL 0 COLS./100 ML 52 17.000 -- 1.207* *8.050 *1.000 *0.298 *0.092 *0.017 70953 CHL-A PHY CHROMA UG/L 57 23.000 0.200 3.942 14.200 4.950 2.800 1.350 0.390 70954 CHLOROPHYLL-B, P UG/L 58 0.500 -- 0.117* *0.405 *0.200 *0.072 *0.037 *0.013 01005 BARIUM DISSOLVED (UG/L AS BA) 59 9.000 2.000 6.541 8.000 7.700 6.800 5.700 4.000 01010 BERYLLIUM DISSOL (UG/L AS BE) 59 -- -- -- -- -- -- -- -- 01020 BORON DISSOLVED (UG/L AS B) 59 20.000 -- 3.734* *10.000 *4.529 *2.836 *1.667 *0.846 01025 CADMIUM DISSOLVE (UG/L AS CD) 59 -- -- -- -- -- -- -- -- 01030 CHROMIUM DISSOLV (UG/L AS CR) 59 -- -- -- -- -- -- -- -- 01035 COBALT DISSOLVED (UG/L AS CO) 59 -- -- -- -- -- -- -- -- 01040 COPPER DISSOLVED (UG/L AS CU) 58 -- -- -- -- -- -- -- -- 01046 IRON DISSOLVED (UG/L AS FE) 60 1000.000 0.000 78.000 160.000 87.500 50.000 30.000 10.500 01049 LEAD DISSOLVED (UG/L AS PB) 64 10.000 -- 3.681* *7.484 *4.706 *3.229 *2.258 *1.439 01130 LITHIUM DISSOLVE (UG/L AS LI) 59 -- -- -- -- -- -- -- -- 01056 MANGANESE DISSOL (UG/L AS MN) 60 87.000 -- 9.194* *46.950 *6.875 *3.000 *1.657 *0.302 71890 MERCURY DISSOLVE UG/L AS HG 6 -- -- -- -- -- -- -- -- 01060 MOLYBDENUM DISSO (UG/L AS MO) 59 -- -- -- -- -- -- -- -- 01065 NICKEL DISSOLVED (UG/L AS NI) 62 -- -- -- -- -- -- -- -- 01075 SILVER DISSOLVED (UG/L AS AG) 59 -- -- -- -- -- -- -- -- 01080 STRONTIUM DISSOL (UG/L AS SR) 59 49.000 20.000 36.290 47.000 42.000 38.400 31.000 21.000

01085 VANADIUM DISSOLV (UG/L AS V) 59 -- -- -- -- -- -- -- --01090 ZINC DISSOLVED (UG/L AS ZN) 59 13.000 -- 4.114* *10.000 *6.000 *3.000 *1.896 *0.983------------------------------------------------------------------------------------------------------------------------------------* - VALUE IS ESTIMATED BY USING A LOG-PROBABILITY REGRESSION TO PREDICT

THE VALUES OF DATA BELOW THE DETECTION LIMIT (Helsel and Cohn, 1988)
Hydrologic and Water-Quality Data 125

Table 10. Summary statistics for site 4, Shadow Mountain Lake, sampled near bottom.

------------------------------------------------------------------------------------------------------------------------------------

STATISTICAL SUMMARY OF SELECTED WATER QUALITY DATA COLLECTED FROM AUG 1973 TO SEPT 2000

PERCENT OF SAMPLES IN WHICH VALUES

DESCRIPTIVE STATISTICS WERE LESS THAN OR EQUAL TO THOSE SHOWN

------------------------------------- -------------------------------------- ------------------------------------------------------

SAMPLE (MEDIAN)

WATER-QUALITY CONSTITUENT SIZE MAXIMUM MINIMUM MEAN 95 % 75 % 50 % 25 % 5 %

------------------------------------- -------------------------------------- ------------------------------------------------------

70300 RESIDUE DIS 180C MG/L 57 60.000 24.000 38.211 54.200 42.000 38.000 32.500 26.00070301 DISSOLVED SOLIDS MG/L 54 41.800 23.100 34.239 40.100 38.050 34.750 31.850 23.85000300 OXYGEN DISSOLVED (MG/L) 58 8.800 0.700 5.455 8.010 6.725 5.600 4.200 1.94000400 PH, WH, FIELD (STANDARD UNIT 57 8.600 6.800 7.318 7.850 7.400 7.300 7.100 6.99000403 PH, WH, LABORATO (STANDARD UNIT 58 8.100 6.500 7.203 7.705 7.400 7.200 7.000 6.69590095 SPECIFIC CONDUCT MICROSIEMENS/C 58 67.000 35.000 54.224 65.000 61.000 55.500 50.000 37.85000095 SPECIFIC CONDUCT US/CM @ 25C 58 76.000 30.000 51.448 67.050 57.500 52.500 46.750 32.95000900 HARDNESS TOTAL (MG/L AS CAO3) 59 27.400 13.700 21.522 26.100 24.100 22.000 19.600 14.10000915 CALCIUM DISSOLVE (MG/L AS CA) 59 8.300 4.100 6.537 8.100 7.400 6.800 5.900 4.17000925 MAGNESIUM DISSOL (MG/L AS MG) 59 1.600 0.840 1.251 1.500 1.370 1.300 1.200 0.88000935 POTASSIUM DISSOL (MG/L AS K) 59 1.500 0.500 0.711 0.900 0.800 0.700 0.600 0.50000931 SODIUM ADSORPTIO (RATIO) 59 0.300 0.127 0.185 0.233 0.203 0.185 0.164 0.14100930 SODIUM DISSOLVED (MG/L AS NA) 59 2.800 1.100 1.968 2.700 2.200 2.000 1.700 1.20000932 SODIUM, PERCENT PERCENT 59 21.000 13.200 15.995 18.400 16.900 15.900 14.900 13.60090410 ANC, TIT. 4.5, L MG/L AS CACO3 58 29.000 14.000 22.741 28.050 26.000 24.000 20.750 14.00000940 CHLORIDE DISSOLV (MG/L AS CL) 58 1.000 0.100 0.386 0.905 0.400 0.400 0.300 0.10000950 FLUORIDE DISSOLV (MG/L AS F) 58 0.300 0.100 0.138 0.200 0.200 0.100 0.100 0.10000955 SILICA DISSOLVED (MG/L AS SIO2) 59 8.000 4.800 6.227 7.600 7.000 6.200 5.500 4.90000945 SULFATE DISSOLVE (MG/L AS SO4) 58 4.000 2.200 2.940 3.610 3.300 2.900 2.600 2.20000608 NITROGEN AMMONIA (MG/L AS N) 58 0.150 -- 0.021* *0.090 *0.021 *0.010 *0.004 *0.00200625 NITROGEN AMM+ORG (MG/L AS N) 58 0.700 -- 0.250* *0.505 *0.300 *0.200 *0.170 *0.11400610 NITROGEN AMMONIA (MG/L AS N) 18 0.140 -- 0.043* *0.140 *0.065 *0.030 *0.020 *0.00700631 NO2 + NO3 DISSOL (MG/L AS N) 59 0.130 -- 0.039* *0.093 *0.060 *0.028 *0.017 *0.00800630 NO2 + NO3 TOTAL (MG/L AS N) 12 0.100 -- 0.100* *0.100 *0.100 *0.100 *0.100 *0.10000613 NITROGEN,NITRITE MG/L AS N 58 -- -- -- -- -- -- -- --00615 NITROGEN,NITRITE MG/L AS N 9 -- -- -- -- -- -- -- --00605 NITROGEN ORGANIC (MG/L AS N) 26 0.600 0.103 0.273 0.590 0.307 0.270 0.180 0.11900600 NITROGEN TOTAL (MG/L AS N) 11 0.461 0.200 0.346 0.461 0.452 0.364 0.280 0.20000666 PHOSPHORUS DISS. (MG/L AS P) 56 0.050 -- 0.007* *0.022 *0.008 *0.004 *0.002 *0.00100671 PHOSPHORUS ORTHO (MG/L AS P) 59 0.040 -- 0.003* *0.020 *0.002 *0.000 *0.000 *0.00070507 PHOS ORTHO TOT A MG/L AS P 9 -- -- -- -- -- -- -- --00665 PHOSPHORUS TOTAL (MG/L AS P) 58 0.060 -- 0.017* *0.041 *0.020 *0.013 *0.008 *0.00401005 BARIUM DISSOLVED (UG/L AS BA) 58 10.000 2.000 7.138 9.000 8.000 7.000 6.700 5.47501010 BERYLLIUM DISSOL (UG/L AS BE) 58 1.170 -- 0.221* *0.800 *0.274 *0.157 *0.090 *0.04001020 BORON DISSOLVED (UG/L AS B) 58 -- -- -- -- -- -- -- --01025 CADMIUM DISSOLVE (UG/L AS CD) 58 3.000 -- 0.513* *2.000 *0.624 *0.310 *0.151 *0.05701030 CHROMIUM DISSOLV (UG/L AS CR) 58 -- -- -- -- -- -- -- --01035 COBALT DISSOLVED (UG/L AS CO) 58 -- -- -- -- -- -- -- --01040 COPPER DISSOLVED (UG/L AS CU) 58 -- -- -- -- -- -- -- --01046 IRON DISSOLVED (UG/L AS FE) 59 260.000 0.000 72.373 210.000 110.000 50.000 20.000 10.00001049 LEAD DISSOLVED (UG/L AS PB) 58 -- -- -- -- -- -- -- --01130 LITHIUM DISSOLVE (UG/L AS LI) 58 5.000 -- 1.548* *3.596 *1.989 *1.298 *0.890 *0.48601056 MANGANESE DISSOL (UG/L AS MN) 59 610.000 -- 56.163* *340.000 *57.000 *10.100 *3.000 *0.89801060 MOLYBDENUM DISSO (UG/L AS MO) 58 -- -- -- -- -- -- -- --01065 NICKEL DISSOLVED (UG/L AS NI) 58 -- -- -- -- -- -- -- --01075 SILVER DISSOLVED (UG/L AS AG) 57 -- -- -- -- -- -- -- --01077 SILVER TOTAL (UG/L AS AG) 2 -- -- -- -- -- -- -- --01080 STRONTIUM DISSOL (UG/L AS SR) 58 52.000 20.000 38.105 47.100 44.250 39.150 33.000 22.00001085 VANADIUM DISSOLV (UG/L AS V) 58 -- -- -- -- -- -- -- --01090 ZINC DISSOLVED (UG/L AS ZN) 58 29.000 -- 5.199* *15.150 *7.000 *3.218 *1.734 *0.739------------------------------------------------------------------------------------------------------------------------------------* - VALUE IS ESTIMATED BY USING A LOG-PROBABILITY REGRESSION TO PREDICT

THE VALUES OF DATA BELOW THE DETECTION LIMIT (Helsel and Cohn, 1988)
126 Water Quality and Trend Analysis of Colorado–Big Thompson System Reservoirs and Related Conveyances Table 11. Summary statistics for site 5, Alva B. Adams Tunnel east portal. ------------------------------------------------------------------------------------------------------------------------------------ STATISTICAL SUMMARY OF SELECTED WATER QUALITY DATA COLLECTED FROM SEPT 1970 TO DEC 2000 PERCENTAGE OF SAMPLES IN WHICH VALUES DESCRIPTIVE STATISTICS WERE LESS THAN OR EQUAL TO THOSE SHOWN ------------------------------------- -------------------------------------- ------------------------------------------------------ SAMPLE (MEDIAN) WATER-QUALITY CONSTITUENT SIZE MAXIMUM MINIMUM MEAN 95 % 75 % 50 % 25 % 5 % ------------------------------------- -------------------------------------- ------------------------------------------------------ 00061 DISCHARGE, INST. CFS 228 619.000 0.400 402.434 555.000 532.500 459.000 360.250 9.100 70300 RESIDUE DIS 180C MG/L 69 60.000 4.000 33.333 49.000 40.000 34.000 27.500 13.000 70301 DISSOLVED SOLIDS MG/L 186 50.000 10.000 30.263 40.000 35.000 30.000 28.750 20.000 00300 OXYGEN DISSOLVED (MG/L) 276 13.400 5.700 8.320 9.600 8.795 8.300 7.800 7.285 00301 OXYGEN DIS. PERC % OF SATURATIO 136 130.000 8.000 91.776 114.000 102.000 92.000 86.400 75.950 00400 PH, WH, FIELD (STANDARD UNIT 270 8.700 6.200 7.472 8.400 7.800 7.500 7.100 6.600 00403 PH, WH, LABORATO (STANDARD UNIT 161 9.300 6.800 7.619 8.400 7.850 7.600 7.375 7.100 90095 SPECIFIC CONDUCT MICROSIEMENS/C 157 89.000 17.000 48.089 68.000 57.000 50.000 39.500 23.000 00095 SPECIFIC CONDUCT US/CM @ 25C 279 90.000 -- 47.088* *66.000 *55.000 *48.000 *38.077 *23.000 00010 WATER TEMPERATUR (DEGREES C) 279 23.800 1.000 7.280 16.500 12.000 5.000 3.000 1.900 00900 HARDNESS TOTAL (MG/L AS CAO3) 207 30.000 5.000 18.227 26.000 22.000 20.000 15.000 8.000 00915 CALCIUM DISSOLVE (MG/L AS CA) 207 9.200 1.800 5.552 7.960 6.700 5.900 4.400 2.500 00925 MAGNESIUM DISSOL (MG/L AS MG) 207 1.800 0.100 1.054 1.500 1.300 1.100 0.860 0.500 00935 POTASSIUM DISSOL (MG/L AS K) 203 2.700 0.200 0.671 0.980 0.800 0.700 0.500 0.300 00931 SODIUM ADSORPTIO (RATIO) 207 0.500 0.130 0.201 0.300 0.200 0.200 0.180 0.150 00930 SODIUM DISSOLVED (MG/L AS NA) 207 5.300 0.800 1.949 2.960 2.200 1.900 1.600 1.000 00932 SODIUM, PERCENT PERCENT 203 39.000 13.000 18.995 28.600 20.000 18.000 17.000 15.000 90410 ANC, TIT. 4.5, L MG/L AS CACO3 156 36.000 6.000 19.726 29.000 24.000 21.000 16.000 9.000 00421 ALKALINITY,DIS,F (MG/L AS CACO3 3 21.000 13.000 -- -- -- -- -- -- 00410 ANC, FET, FIELD (MG/L AS CACO3 45 27.000 4.000 17.778 26.400 21.000 19.000 14.500 6.300 00940 CHLORIDE DISSOLV (MG/L AS CL) 205 2.700 0.100 0.451 0.900 0.500 0.400 0.300 0.200 00950 FLUORIDE DISSOLV (MG/L AS F) 203 0.500 -- 0.132* *0.200 *0.200 *0.100 *0.100 *0.064 00955 SILICA DISSOLVED (MG/L AS SIO2) 203 14.000 2.300 4.532 6.380 5.000 4.300 3.700 3.000 00945 SULFATE DISSOLVE (MG/L AS SO4) 203 10.000 0.300 3.924 6.860 5.000 4.000 2.500 1.300 00608 NITROGEN AMMONIA (MG/L AS N) 73 0.040 -- 0.014* *0.040 *0.020 *0.010 *0.006 *0.003 00625 NITROGEN AMM+ORG (MG/L AS N) 189 5.400 -- 0.400* *0.885 *0.500 *0.300 *0.200 *0.083 00610 NITROGEN AMMONIA (MG/L AS N) 25 0.060 -- 0.020* *0.057 *0.030 *0.020 *0.010 *0.005 00631 NO2 + NO3 DISSOL (MG/L AS N) 205 0.210 -- 0.061* *0.140 *0.084 *0.053 *0.030 *0.015 00613 NITROGEN,NITRITE MG/L AS N 68 0.030 -- 0.004* *0.020 *0.004 *0.002 *0.001 *0.000 00666 PHOSPHORUS DISS. (MG/L AS P) 183 0.200 -- 0.014* *0.030 *0.020 *0.010 *0.005 *0.002 00671 PHOSPHORUS ORTHO (MG/L AS P) 96 0.100 -- 0.007* *0.030 *0.010 *0.003 *0.001 *0.000 00665 PHOSPHORUS TOTAL (MG/L AS P) 192 0.100 -- 0.018* *0.050 *0.020 *0.010 *0.008 *0.004 01000 ARSENIC DISSOLVE (UG/L AS AS) 3 -- -- -- -- -- -- -- -- 01005 BARIUM DISSOLVED (UG/L AS BA) 67 8.000 3.000 5.672 8.000 7.000 6.000 5.000 3.000 01010 BERYLLIUM DISSOL (UG/L AS BE) 67 -- -- -- -- -- -- -- -- 01020 BORON DISSOLVED (UG/L AS B) 67 30.000 -- 3.791* *16.000 *4.379 *2.084 *0.981 *0.377 01025 CADMIUM DISSOLVE (UG/L AS CD) 109 3.000 -- 0.498* *2.000 *0.614 *0.340 *0.187 *0.083 01030 CHROMIUM DISSOLV (UG/L AS CR) 67 -- -- -- -- -- -- -- -- 01035 COBALT DISSOLVED (UG/L AS CO) 67 -- -- -- -- -- -- -- -- 01040 COPPER DISSOLVED (UG/L AS CU) 110 20.000 -- 2.039* *7.000 *2.352 *1.120 *0.784 *0.294 01046 IRON DISSOLVED (UG/L AS FE) 207 360.000 7.000 47.425 116.000 58.000 33.000 20.000 12.400 01049 LEAD DISSOLVED (UG/L AS PB) 111 19.000 -- 1.799* *6.728 *2.000 *0.847 *0.363 *0.127 01130 LITHIUM DISSOLVE (UG/L AS LI) 67 -- -- -- -- -- -- -- -- 01056 MANGANESE DISSOL (UG/L AS MN) 207 20.000 -- 2.487* *6.640 *3.000 *2.000 *1.000 *0.474 01060 MOLYBDENUM DISSO (UG/L AS MO) 67 -- -- -- -- -- -- -- -- 01065 NICKEL DISSOLVED (UG/L AS NI) 109 12.000 -- 1.211* *4.459 *1.278 *0.638 *0.294 *0.100 01075 SILVER DISSOLVED (UG/L AS AG) 69 4.000 -- 0.647* *2.000 *0.790 *0.453 *0.257 *0.120 01080 STRONTIUM DISSOL (UG/L AS SR) 67 60.000 10.000 33.552 49.600 41.000 35.000 26.000 12.800 01085 VANADIUM DISSOLV (UG/L AS V) 67 -- -- -- -- -- -- -- -- 01090 ZINC DISSOLVED (UG/L AS ZN) 108 35.000 -- 6.430* *18.550 *9.000 *4.398 *2.549 *1.146

------------------------------------------------------------------------------------------------------------------------------------* - VALUE IS ESTIMATED BY USING A LOG-PROBABILITY REGRESSION TO PREDICT

THE VALUES OF DATA BELOW THE DETECTION LIMIT (Helsel and Cohn, 1988)
Hydrologic and Water-Quality Data 127

Table 12. Summary statistics for site 6, Lake Estes, sampled at surface.

------------------------------------------------------------------------------------------------------------------------------------

STATISTICAL SUMMARY OF SELECTED WATER QUALITY DATA COLLECTED FROM MAY 1998 TO SEPT 2000

PERCENT OF SAMPLES IN WHICH VALUES

DESCRIPTIVE STATISTICS WERE LESS THAN OR EQUAL TO THOSE SHOWN

------------------------------------- -------------------------------------- ------------------------------------------------------

SAMPLE (MEDIAN)

WATER-QUALITY CONSTITUENT SIZE MAXIMUM MINIMUM MEAN 95 % 75 % 50 % 25 % 5 %

------------------------------------- -------------------------------------- ------------------------------------------------------

00077 TRANSPARENCY (IN (INCHES) 8 119.000 60.000 81.750 119.000 92.250 79.000 67.500 60.00000300 OXYGEN DISSOLVED (MG/L) 8 9.300 6.400 7.838 9.300 8.925 7.700 6.725 6.40000400 PH, WH, FIELD (STANDARD UNIT 8 7.800 7.200 7.550 7.800 7.675 7.550 7.500 7.20000403 PH, WH, LABORATO (STANDARD UNIT 8 8.100 7.200 7.475 8.100 7.650 7.400 7.225 7.20090095 SPECIFIC CONDUCT MICROSIEMENS/C 8 56.000 26.000 41.500 56.000 49.000 44.500 31.250 26.00000095 SPECIFIC CONDUCT US/CM @ 25C 8 46.000 16.000 33.500 46.000 43.750 36.000 22.250 16.00000010 WATER TEMPERATUR (DEGREES C) 8 17.800 6.600 12.650 17.800 16.625 12.900 8.825 6.60000900 HARDNESS TOTAL (MG/L AS CAO3) 8 19.400 9.010 15.089 19.400 18.650 16.150 11.400 9.01000915 CALCIUM DISSOLVE (MG/L AS CA) 8 5.720 2.640 4.474 5.720 5.552 4.700 3.420 2.64000925 MAGNESIUM DISSOL (MG/L AS MG) 8 1.330 0.582 0.950 1.330 1.135 1.011 0.698 0.58200935 POTASSIUM DISSOL (MG/L AS K) 8 0.720 0.350 0.504 0.720 0.585 0.490 0.405 0.35000931 SODIUM ADSORPTIO (RATIO) 8 0.259 0.156 0.200 0.259 0.242 0.183 0.167 0.15600930 SODIUM DISSOLVED (MG/L AS NA) 8 2.600 1.100 1.787 2.600 2.250 1.800 1.250 1.10000932 SODIUM, PERCENT PERCENT 8 22.700 16.300 19.913 22.700 22.075 20.300 17.800 16.30090410 ANC, TIT. 4.5, L MG/L AS CACO3 8 21.000 10.000 16.125 21.000 19.750 16.500 12.500 10.00000940 CHLORIDE DISSOLV (MG/L AS CL) 8 1.900 -- 0.830* *1.900 *1.575 *0.450 *0.400 *0.13600950 FLUORIDE DISSOLV (MG/L AS F) 8 0.200 -- 0.114* *0.200 *0.175 *0.100 *0.072 *0.05100955 SILICA DISSOLVED (MG/L AS SIO2) 8 7.000 4.000 5.038 7.000 6.100 4.650 4.250 4.00000945 SULFATE DISSOLVE (MG/L AS SO4) 8 3.600 1.400 2.175 3.600 2.575 2.050 1.650 1.40070300 RESIDUE DIS 180C MG/L 8 42.000 22.000 31.875 42.000 41.000 30.500 23.500 22.00070301 DISSOLVED SOLIDS MG/L 7 30.700 17.200 24.700 30.700 29.800 25.800 20.400 17.20000608 NITROGEN AMMONIA (MG/L AS N) 8 0.052 -- 0.019* *0.052 *0.032 *0.012 *0.003 *0.00200625 NITROGEN AMM+ORG (MG/L AS N) 8 0.310 0.160 0.240 0.310 0.257 0.245 0.222 0.16000631 NO2 + NO3 DISSOL (MG/L AS N) 8 0.091 -- 0.035* *0.091 *0.063 *0.022 *0.008 *0.00600613 NITROGEN,NITRITE MG/L AS N 8 -- -- -- -- -- -- -- --00666 PHOSPHORUS DISS. (MG/L AS P) 8 -- -- -- -- -- -- -- --00671 PHOSPHORUS ORTHO (MG/L AS P) 8 -- -- -- -- -- -- -- --00665 PHOSPHORUS TOTAL (MG/L AS P) 8 -- -- -- -- -- -- -- --31625 COLIFORM FECAL 0 COLS./100 ML 8 84.000 -- 22.584* *84.000 *37.500 *13.500 *1.626 *0.50470953 CHL-A PHY CHROMA UG/L 8 13.200 0.900 5.900 13.200 10.675 3.200 3.000 0.90070954 CHLOROPHYLL-B, P UG/L 8 -- -- -- -- -- -- -- --01005 BARIUM DISSOLVED (UG/L AS BA) 8 6.200 4.100 5.162 6.200 6.025 5.350 4.200 4.10001010 BERYLLIUM DISSOL (UG/L AS BE) 8 -- -- -- -- -- -- -- --01020 BORON DISSOLVED (UG/L AS B) 8 -- -- -- -- -- -- -- --01025 CADMIUM DISSOLVE (UG/L AS CD) 8 -- -- -- -- -- -- -- --01030 CHROMIUM DISSOLV (UG/L AS CR) 8 -- -- -- -- -- -- -- --01035 COBALT DISSOLVED (UG/L AS CO) 8 -- -- -- -- -- -- -- --01040 COPPER DISSOLVED (UG/L AS CU) 8 -- -- -- -- -- -- -- --01046 IRON DISSOLVED (UG/L AS FE) 8 90.000 30.000 53.750 90.000 70.000 50.000 32.500 30.00001049 LEAD DISSOLVED (UG/L AS PB) 8 -- -- -- -- -- -- -- --01130 LITHIUM DISSOLVE (UG/L AS LI) 8 -- -- -- -- -- -- -- --01056 MANGANESE DISSOL (UG/L AS MN) 8 -- -- -- -- -- -- -- --01060 MOLYBDENUM DISSO (UG/L AS MO) 8 -- -- -- -- -- -- -- --01065 NICKEL DISSOLVED (UG/L AS NI) 8 -- -- -- -- -- -- -- --01075 SILVER DISSOLVED (UG/L AS AG) 8 -- -- -- -- -- -- -- --01080 STRONTIUM DISSOL (UG/L AS SR) 7 34.600 14.500 23.686 34.600 29.300 23.400 17.600 14.50001085 VANADIUM DISSOLV (UG/L AS V) 8 -- -- -- -- -- -- -- --01090 ZINC DISSOLVED (UG/L AS ZN) 8 -- -- -- -- -- -- -- --------------------------------------------------------------------------------------------------------------------------------------* - VALUE IS ESTIMATED BY USING A LOG-PROBABILITY REGRESSION TO PREDICT

THE VALUES OF DATA BELOW THE DETECTION LIMIT (Helsel and Cohn, 1988)
128 Water Quality and Trend Analysis of Colorado–Big Thompson System Reservoirs and Related Conveyances Table 13. Summary statistics for site 6, Lake Estes, sampled near bottom. ------------------------------------------------------------------------------------------------------------------------------------ STATISTICAL SUMMARY OF SELECTED WATER QUALITY DATA COLLECTED FROM MAY 1998 TO SEPT 2000 PERCENT OF SAMPLES IN WHICH VALUES DESCRIPTIVE STATISTICS WERE LESS THAN OR EQUAL TO THOSE SHOWN ------------------------------------- -------------------------------------- ------------------------------------------------------ SAMPLE (MEDIAN) WATER-QUALITY CONSTITUENT SIZE MAXIMUM MINIMUM MEAN 95 % 75 % 50 % 25 % 5 % ------------------------------------- -------------------------------------- ------------------------------------------------------ 00300 OXYGEN DISSOLVED (MG/L) 8 38.400 2.700 10.088 38.400 8.500 7.000 3.700 2.700 00400 PH, WH, FIELD (STANDARD UNIT 8 8.100 6.800 7.375 8.100 7.475 7.350 7.225 6.800 00403 PH, WH, LABORATO (STANDARD UNIT 8 7.600 7.000 7.312 7.600 7.500 7.300 7.075 7.000 90095 SPECIFIC CONDUCT MICROSIEMENS/C 8 54.000 24.000 40.125 54.000 48.000 44.500 28.500 24.000 00095 SPECIFIC CONDUCT US/CM @ 25C 8 44.000 18.000 34.500 44.000 42.750 38.500 25.000 18.000 00010 WATER TEMPERATUR (DEGREES C) 8 15.000 6.400 10.387 15.000 14.300 9.950 6.900 6.400 00900 HARDNESS TOTAL (MG/L AS CAO3) 8 18.900 8.140 14.381 18.900 18.275 16.000 9.932 8.140 00915 CALCIUM DISSOLVE (MG/L AS CA) 8 5.700 2.370 4.244 5.700 5.378 4.650 2.898 2.370 00925 MAGNESIUM DISSOL (MG/L AS MG) 8 1.220 0.538 0.911 1.220 1.125 1.003 0.632 0.538 00935 POTASSIUM DISSOL (MG/L AS K) 8 0.620 0.340 0.502 0.620 0.587 0.495 0.440 0.340 00931 SODIUM ADSORPTIO (RATIO) 8 0.251 0.157 0.200 0.251 0.241 0.183 0.170 0.157 00930 SODIUM DISSOLVED (MG/L AS NA) 8 2.400 1.000 1.750 2.400 2.275 1.800 1.225 1.000 00932 SODIUM, PERCENT PERCENT 8 23.800 16.800 20.375 23.800 21.600 20.850 18.175 16.800 90410 ANC, TIT. 4.5, L MG/L AS CACO3 8 21.000 8.000 15.125 21.000 19.750 15.500 11.000 8.000 00940 CHLORIDE DISSOLV (MG/L AS CL) 8 2.200 0.300 0.938 2.200 1.675 0.500 0.425 0.300 00950 FLUORIDE DISSOLV (MG/L AS F) 8 0.200 -- 0.104* *0.200 *0.100 *0.100 *0.078 *0.061 00955 SILICA DISSOLVED (MG/L AS SIO2) 8 6.600 4.100 5.037 6.600 6.175 4.750 4.200 4.100 00945 SULFATE DISSOLVE (MG/L AS SO4) 8 3.300 1.300 2.075 3.300 2.575 2.100 1.425 1.300 70300 RESIDUE DIS 180C MG/L 8 52.000 21.000 32.500 52.000 39.500 32.500 22.000 21.000 70301 DISSOLVED SOLIDS MG/L 8 34.100 15.600 25.162 34.100 30.150 27.150 18.575 15.600 00608 NITROGEN AMMONIA (MG/L AS N) 8 0.074 -- 0.045* *0.074 *0.066 *0.042 *0.026 *0.019 00625 NITROGEN AMM+ORG (MG/L AS N) 8 0.300 0.240 0.264 0.300 0.280 0.260 0.242 0.240 00631 NO2 + NO3 DISSOL (MG/L AS N) 8 0.081 0.014 0.057 0.081 0.072 0.067 0.036 0.014 00613 NITROGEN,NITRITE MG/L AS N 8 -- -- -- -- -- -- -- -- 00666 PHOSPHORUS DISS. (MG/L AS P) 8 -- -- -- -- -- -- -- -- 00671 PHOSPHORUS ORTHO (MG/L AS P) 8 -- -- -- -- -- -- -- -- 00665 PHOSPHORUS TOTAL (MG/L AS P) 8 -- -- -- -- -- -- -- -- 01005 BARIUM DISSOLVED (UG/L AS BA) 8 6.600 4.000 5.350 6.600 6.175 5.500 4.375 4.000 01010 BERYLLIUM DISSOL (UG/L AS BE) 8 -- -- -- -- -- -- -- -- 01020 BORON DISSOLVED (UG/L AS B) 8 -- -- -- -- -- -- -- -- 01025 CADMIUM DISSOLVE (UG/L AS CD) 8 -- -- -- -- -- -- -- -- 01030 CHROMIUM DISSOLV (UG/L AS CR) 8 -- -- -- -- -- -- -- -- 01035 COBALT DISSOLVED (UG/L AS CO) 8 -- -- -- -- -- -- -- -- 01040 COPPER DISSOLVED (UG/L AS CU) 8 -- -- -- -- -- -- -- -- 01046 IRON DISSOLVED (UG/L AS FE) 8 150.000 40.000 92.500 150.000 122.500 90.000 70.000 40.000 01049 LEAD DISSOLVED (UG/L AS PB) 8 -- -- -- -- -- -- -- -- 01130 LITHIUM DISSOLVE (UG/L AS LI) 8 -- -- -- -- -- -- -- -- 01056 MANGANESE DISSOL (UG/L AS MN) 8 9.800 -- 5.854* *9.800 *8.800 *6.000 *2.900 *1.829 01060 MOLYBDENUM DISSO (UG/L AS MO) 8 -- -- -- -- -- -- -- -- 01065 NICKEL DISSOLVED (UG/L AS NI) 8 -- -- -- -- -- -- -- -- 01075 SILVER DISSOLVED (UG/L AS AG) 8 -- -- -- -- -- -- -- -- 01080 STRONTIUM DISSOL (UG/L AS SR) 8 34.300 12.800 23.000 34.300 28.075 24.750 15.475 12.800 01085 VANADIUM DISSOLV (UG/L AS V) 8 -- -- -- -- -- -- -- -- 01090 ZINC DISSOLVED (UG/L AS ZN) 8 -- -- -- -- -- -- -- -- ------------------------------------------------------------------------------------------------------------------------------------ * - VALUE IS ESTIMATED BY USING A LOG-PROBABILITY REGRESSION TO PREDICT THE VALUES OF DATA BELOW THE DETECTION LIMIT (Helsel and Cohn, 1988)
Hydrologic and Water-Quality Data 129

Table 14. Summary statistics for site 7, Olympus Tunnel.

------------------------------------------------------------------------------------------------------------------------------------

STATISTICAL SUMMARY OF SELECTED WATER QUALITY DATA COLLECTED FROM SEPT 1970 TO DEC 2000

PERCENT OF SAMPLES IN WHICH VALUES

DESCRIPTIVE STATISTICS WERE LESS THAN OR EQUAL TO THOSE SHOWN

------------------------------------- -------------------------------------- ------------------------------------------------------

SAMPLE (MEDIAN)

WATER-QUALITY CONSTITUENT SIZE MAXIMUM MINIMUM MEAN 95 % 75 % 50 % 25 % 5 %

------------------------------------- -------------------------------------- ------------------------------------------------------

70300 RESIDUE DIS 180C MG/L 95 67.000 10.000 33.547 48.400 41.000 35.000 25.000 16.00070301 DISSOLVED SOLIDS MG/L 172 57.000 13.500 30.098 43.105 35.300 31.000 24.350 16.13000300 OXYGEN DISSOLVED (MG/L) 240 14.800 6.400 8.925 11.195 9.800 8.800 8.000 7.20000400 PH, WH, FIELD (STANDARD UNIT 236 8.800 6.300 7.446 8.200 7.700 7.400 7.200 6.80000403 PH, WH, LABORATO (STANDARD UNIT 123 8.500 6.900 7.590 8.200 7.800 7.600 7.400 7.10090095 SPECIFIC CONDUCT MICROSIEMENS/C 121 76.000 19.000 46.017 66.000 56.000 47.000 36.500 22.10000095 SPECIFIC CONDUCT US/CM @ 25C 243 95.000 -- 47.294* *70.000 *58.000 *50.000 *36.565 *22.43000010 WATER TEMPERATUR (DEGREES C) 242 21.000 0.000 7.332 16.000 12.000 6.250 2.000 1.00000900 HARDNESS TOTAL (MG/L AS CAO3) 184 39.000 5.000 17.769 25.600 22.000 19.000 13.625 7.45000915 CALCIUM DISSOLVE (MG/L AS CA) 184 9.600 1.500 5.331 7.875 6.600 5.700 4.000 2.30000925 MAGNESIUM DISSOL (MG/L AS MG) 184 3.700 0.100 1.074 1.600 1.300 1.100 0.850 0.41700935 POTASSIUM DISSOL (MG/L AS K) 180 1.800 0.100 0.677 1.000 0.800 0.700 0.582 0.30000931 SODIUM ADSORPTIO (RATIO) 184 0.400 0.153 0.207 0.300 0.206 0.200 0.197 0.16700930 SODIUM DISSOLVED (MG/L AS NA) 184 4.700 0.900 2.006 2.900 2.300 2.000 1.600 1.12500932 SODIUM, PERCENT PERCENT 179 34.000 13.000 19.658 27.000 21.000 19.000 17.500 15.00090410 ANC, TIT. 4.5, L MG/L AS CACO3 121 31.000 6.000 18.207 28.900 23.000 18.000 13.500 8.10000410 ANC, FET, FIELD (MG/L AS CACO3 61 30.000 4.000 18.230 26.900 23.000 20.000 13.500 6.00000940 CHLORIDE DISSOLV (MG/L AS CL) 184 4.100 0.100 0.726 1.675 0.800 0.600 0.500 0.30000950 FLUORIDE DISSOLV (MG/L AS F) 179 0.400 -- 0.141* *0.200 *0.200 *0.100 *0.100 *0.07100955 SILICA DISSOLVED (MG/L AS SIO2) 181 10.000 2.400 4.580 6.600 5.200 4.500 3.800 3.10000945 SULFATE DISSOLVE (MG/L AS SO4) 184 14.000 -- 4.113* *7.650 *5.175 *4.100 *2.700 *1.32500608 NITROGEN AMMONIA (MG/L AS N) 39 0.080 -- 0.026* *0.066 *0.035 *0.020 *0.011 *0.00600625 NITROGEN AMM+ORG (MG/L AS N) 40 0.600 -- 0.263* *0.495 *0.300 *0.240 *0.185 *0.12800610 NITROGEN AMMONIA (MG/L AS N) 156 0.230 -- 0.048* *0.150 *0.070 *0.030 *0.010 *0.00500631 NO2 + NO3 DISSOL (MG/L AS N) 186 2.300 -- 0.089* *0.176 *0.100 *0.063 *0.037 *0.01600613 NITROGEN,NITRITE MG/L AS N 179 0.220 -- 0.008* *0.015 *0.009 *0.006 *0.004 *0.00200666 PHOSPHORUS DISS. (MG/L AS P) 149 0.090 -- 0.015* *0.040 *0.020 *0.010 *0.006 *0.00300671 PHOSPHORUS ORTHO (MG/L AS P) 82 0.070 -- 0.008* *0.038 *0.010 *0.004 *0.002 *0.00100665 PHOSPHORUS TOTAL (MG/L AS P) 184 5.000 -- 0.057* *0.060 *0.030 *0.020 *0.018 *0.00631501 TOT COLI,MENDO M COLS./100 ML 149 13000.000 0.000 370.711 1500.000 300.000 53.000 12.000 1.00031625 COLIFORM FECAL 0 COLS./100 ML 78 1200.000 -- 78.076* *523.000 *60.000 *14.000 *1.500 *0.17831616 FECAL COLI,MFC M COLS./100 ML 70 210.000 -- 10.613* *47.850 *9.000 *1.000 *0.288 *0.03101000 ARSENIC DISSOLVE (UG/L AS AS) 3 -- -- -- -- -- -- -- --01005 BARIUM DISSOLVED (UG/L AS BA) 34 8.000 3.000 5.409 8.000 6.000 5.600 4.375 3.00001010 BERYLLIUM DISSOL (UG/L AS BE) 34 -- -- -- -- -- -- -- --01020 BORON DISSOLVED (UG/L AS B) 34 -- -- -- -- -- -- -- --01025 CADMIUM DISSOLVE (UG/L AS CD) 34 -- -- -- -- -- -- -- --01030 CHROMIUM DISSOLV (UG/L AS CR) 34 -- -- -- -- -- -- -- --01035 COBALT DISSOLVED (UG/L AS CO) 34 -- -- -- -- -- -- -- --01040 COPPER DISSOLVED (UG/L AS CU) 37 -- -- -- -- -- -- -- --01046 IRON DISSOLVED (UG/L AS FE) 184 280.000 10.000 82.717 180.000 100.000 70.000 50.000 20.00001049 LEAD DISSOLVED (UG/L AS PB) 37 -- -- -- -- -- -- -- --01130 LITHIUM DISSOLVE (UG/L AS LI) 34 -- -- -- -- -- -- -- --01056 MANGANESE DISSOL (UG/L AS MN) 184 39.000 -- 5.569* *20.000 *6.152 *3.626 *2.000 *0.73201055 MANGANESE TOTAL (UG/L AS MN) 12 27.000 -- 13.245* *27.000 *19.000 *11.000 *8.066 *5.62971890 MERCURY DISSOLVE UG/L AS HG 3 -- -- -- -- -- -- -- --01060 MOLYBDENUM DISSO (UG/L AS MO) 34 -- -- -- -- -- -- -- --01065 NICKEL DISSOLVED (UG/L AS NI) 37 -- -- -- -- -- -- -- --01075 SILVER DISSOLVED (UG/L AS AG) 37 2.000 -- 0.698* *2.000 *0.912 *0.547 *0.323 *0.16901080 STRONTIUM DISSOL (UG/L AS SR) 34 51.000 10.000 28.915 48.000 37.750 28.500 19.550 10.75001085 VANADIUM DISSOLV (UG/L AS V) 34 -- -- -- -- -- -- -- --01090 ZINC DISSOLVED (UG/L AS ZN) 34 18.000 -- 5.623* *15.000 *8.500 *3.409 *1.880 *0.755------------------------------------------------------------------------------------------------------------------------------------* - VALUE IS ESTIMATED BY USING A LOG-PROBABILITY REGRESSION TO PREDICT

THE VALUES OF DATA BELOW THE DETECTION LIMIT (Helsel and Cohn, 1988)
130 Water Quality and Trend Analysis of Colorado–Big Thompson System Reservoirs and Related Conveyances Table 15. Summary statistics for site 8, Carter Lake, sampled near surface. ------------------------------------------------------------------------------------------------------------------------------------ STATISTICAL SUMMARY OF SELECTED WATER QUALITY DATA COLLECTED FROM FEB 1970 TO OCT 2000 PERCENT OF SAMPLES IN WHICH VALUES DESCRIPTIVE STATISTICS WERE LESS THAN OR EQUAL TO THOSE SHOWN ------------------------------------- -------------------------------------- ------------------------------------------------------ SAMPLE (MEDIAN) WATER-QUALITY CONSTITUENT SIZE MAXIMUM MINIMUM MEAN 95 % 75 % 50 % 25 % 5 % ------------------------------------- -------------------------------------- ------------------------------------------------------ 70300 RESIDUE DIS 180C MG/L 145 116.000 31.000 49.966 69.700 54.000 48.000 44.000 37.000 70301 DISSOLVED SOLIDS MG/L 34 50.000 34.600 40.379 48.125 42.600 39.500 37.475 34.750 00077 TRANSPARENCY (IN (INCHES) 116 258.000 8.000 100.941 162.300 123.500 95.000 79.000 49.170 00300 OXYGEN DISSOLVED (MG/L) 143 11.800 6.000 8.347 11.200 9.400 7.800 7.300 6.720 00400 PH, WH, FIELD (STANDARD UNIT 149 8.900 5.800 7.691 8.400 8.100 7.800 7.400 6.650 00403 PH, WH, LABORATO (STANDARD UNIT 54 8.400 7.100 7.852 8.225 8.100 7.900 7.675 7.300 90095 SPECIFIC CONDUCT MICROSIEMENS/C 54 90.000 60.000 74.889 86.750 80.000 74.500 70.750 62.750 00095 SPECIFIC CONDUCT US/CM @ 25C 156 145.000 56.000 83.577 102.000 90.000 82.000 75.000 64.000 00010 WATER TEMPERATUR (DEGREES C) 157 24.000 2.000 14.378 22.500 20.000 15.200 10.250 3.000 00900 HARDNESS TOTAL (MG/L AS CAO3) 37 41.000 25.100 30.803 36.680 33.300 30.100 27.950 25.190 00915 CALCIUM DISSOLVE (MG/L AS CA) 37 14.000 8.090 10.110 12.200 11.000 9.800 9.050 8.108 00925 MAGNESIUM DISSOL (MG/L AS MG) 37 1.800 1.180 1.333 1.530 1.400 1.300 1.265 1.189 00935 POTASSIUM DISSOL (MG/L AS K) 37 0.900 0.510 0.693 0.810 0.800 0.700 0.615 0.573 00931 SODIUM ADSORPTIO (RATIO) 37 0.220 0.151 0.176 0.202 0.183 0.175 0.169 0.154 00930 SODIUM DISSOLVED (MG/L AS NA) 37 2.800 1.900 2.224 2.620 2.400 2.200 2.100 1.900 00932 SODIUM, PERCENT PERCENT 37 16.100 11.000 13.330 14.750 13.800 13.300 12.950 11.180 90410 ANC, TIT. 4.5, L MG/L AS CACO3 36 39.000 26.000 32.472 39.000 34.750 32.000 30.250 26.850 00940 CHLORIDE DISSOLV (MG/L AS CL) 35 1.500 0.100 0.634 1.180 0.800 0.700 0.400 0.180 00950 FLUORIDE DISSOLV (MG/L AS F) 35 0.300 -- 0.121* *0.220 *0.100 *0.100 *0.100 *0.059 00955 SILICA DISSOLVED (MG/L AS SIO2) 37 4.300 0.500 2.659 4.120 3.400 2.500 2.050 0.950 00945 SULFATE DISSOLVE (MG/L AS SO4) 35 4.700 1.000 2.880 4.540 3.200 2.800 2.600 1.480 00608 NITROGEN AMMONIA (MG/L AS N) 66 0.270 -- 0.033* *0.156 *0.033 *0.011 *0.005 *0.002 00625 NITROGEN AMM+ORG (MG/L AS N) 35 0.500 -- 0.219* *0.420 *0.300 *0.200 *0.158 *0.109 00610 NITROGEN AMMONIA (MG/L AS N) 87 0.400 -- 0.036* *0.132 *0.040 *0.020 *0.008 *0.002 00618 NITROGEN NITRATE (MG/L AS N) 93 2.300 0.000 0.071 0.203 0.075 0.030 0.010 0.000 00631 NO2 + NO3 DISSOL (MG/L AS N) 138 2.300 -- 0.062* *0.161 *0.060 *0.030 *0.012 *0.005 00613 NITROGEN,NITRITE MG/L AS N 146 0.040 -- 0.005* *0.016 *0.006 *0.003 *0.002 *0.001 00666 PHOSPHORUS DISS. (MG/L AS P) 33 -- -- -- -- -- -- -- -- 00671 PHOSPHORUS ORTHO (MG/L AS P) 36 0.023 -- 0.005* *0.020 *0.005 *0.003 *0.001 *0.000 00665 PHOSPHORUS TOTAL (MG/L AS P) 135 2.000 -- 0.139* *1.300 *0.070 *0.020 *0.007 *0.001 00680 CARBON ORGANIC T (MG/L AS C) 9 5.000 3.400 3.878 5.000 4.100 3.800 3.450 3.400 31625 COLIFORM FECAL 0 COLS./100 ML 78 6.000 -- 0.634* *2.050 *1.000 *0.387 *0.197 *0.075 70953 CHL-A PHY CHROMA UG/L 35 4.200 0.300 1.411 3.160 1.900 1.200 0.800 0.380 70954 CHLOROPHYLL-B, P UG/L 35 0.200 -- 0.053* *0.200 *0.066 *0.035 *0.018 *0.007 01005 BARIUM DISSOLVED (UG/L AS BA) 36 32.000 13.000 22.892 30.300 25.750 23.150 18.850 15.550 01010 BERYLLIUM DISSOL (UG/L AS BE) 36 -- -- -- -- -- -- -- -- 01020 BORON DISSOLVED (UG/L AS B) 36 20.000 -- 6.106* *11.500 *7.275 *5.425 *4.000 *2.773 01025 CADMIUM DISSOLVE (UG/L AS CD) 36 2.000 -- 0.497* *2.000 *0.639 *0.351 *0.196 *0.088 01030 CHROMIUM DISSOLV (UG/L AS CR) 36 -- -- -- -- -- -- -- -- 01035 COBALT DISSOLVED (UG/L AS CO) 36 -- -- -- -- -- -- -- -- 01040 COPPER DISSOLVED (UG/L AS CU) 36 -- -- -- -- -- -- -- -- 01046 IRON DISSOLVED (UG/L AS FE) 37 -- -- -- -- -- -- -- -- 01049 LEAD DISSOLVED (UG/L AS PB) 36 -- -- -- -- -- -- -- -- 01130 LITHIUM DISSOLVE (UG/L AS LI) 36 -- -- -- -- -- -- -- -- 01056 MANGANESE DISSOL (UG/L AS MN) 37 10.000 -- 1.032* *5.500 *0.952 *0.467 *0.242 *0.072 01055 MANGANESE TOTAL (UG/L AS MN) 12 13.000 -- 3.720* *13.000 *4.730 *2.702 *1.922 *1.294 01060 MOLYBDENUM DISSO (UG/L AS MO) 36 -- -- -- -- -- -- -- -- 01065 NICKEL DISSOLVED (UG/L AS NI) 36 -- -- -- -- -- -- -- -- 01075 SILVER DISSOLVED (UG/L AS AG) 36 -- -- -- -- -- -- -- -- 01080 STRONTIUM DISSOL (UG/L AS SR) 36 64.000 31.700 41.422 52.100 44.750 41.000 37.475 32.975 01085 VANADIUM DISSOLV (UG/L AS V) 36 -- -- -- -- -- -- -- -- 01090 ZINC DISSOLVED (UG/L AS ZN) 36 33.000 -- 5.952* *21.950 *7.750 *3.185 *1.710 *0.731

------------------------------------------------------------------------------------------------------------------------------------* - VALUE IS ESTIMATED BY USING A LOG-PROBABILITY REGRESSION TO PREDICT

THE VALUES OF DATA BELOW THE DETECTION LIMIT (Helsel and Cohn, 1988)
Hydrologic and Water-Quality Data 131

Table 16. Summary statistics for site 8, Carter Lake, sampled near bottom.

------------------------------------------------------------------------------------------------------------------------------------

STATISTICAL SUMMARY OF SELECTED WATER QUALITY DATA COLLECTED FROM FEB 1970 TO OCT 2000

PERCENT OF SAMPLES IN WHICH VALUES

DESCRIPTIVE STATISTICS WERE LESS THAN OR EQUAL TO THOSE SHOWN

------------------------------------- -------------------------------------- ------------------------------------------------------

SAMPLE (MEDIAN)

WATER-QUALITY CONSTITUENT SIZE MAXIMUM MINIMUM MEAN 95 % 75 % 50 % 25 % 5 %

------------------------------------- -------------------------------------- ------------------------------------------------------

00003 SAMPLING DEPTH ( FEET 147 160.000 25.000 105.143 146.000 120.000 100.000 90.000 60.00070300 RESIDUE DIS 180C MG/L 113 94.000 18.000 48.230 70.300 53.000 47.000 41.000 33.70070301 DISSOLVED SOLIDS MG/L 34 50.000 29.900 38.697 45.425 41.200 38.300 35.900 33.20000300 OXYGEN DISSOLVED (MG/L) 133 12.700 2.400 7.530 10.760 9.150 7.800 6.000 3.71000301 OXYGEN DIS. PERC % OF SATURATIO 10 96.000 41.200 71.680 96.000 83.725 76.800 56.900 41.20000400 PH, WH, FIELD (STANDARD UNIT 117 8.700 5.600 7.315 8.000 7.600 7.300 7.000 6.59000403 PH, WH, LABORATO (STANDARD UNIT 42 8.000 6.900 7.417 7.900 7.600 7.450 7.200 6.90090095 SPECIFIC CONDUCT MICROSIEMENS/C 42 81.000 52.000 67.452 75.850 72.000 67.500 63.000 59.00000095 SPECIFIC CONDUCT US/CM @ 25C 125 149.000 48.000 76.872 92.700 85.000 78.000 68.000 55.30000010 WATER TEMPERATUR (DEGREES C) 151 15.500 2.000 6.599 12.140 7.500 6.000 5.000 3.00000900 HARDNESS TOTAL (MG/L AS CAO3) 37 38.000 20.700 28.014 33.770 30.700 27.600 24.950 23.04000915 CALCIUM DISSOLVE (MG/L AS CA) 37 13.000 6.570 9.058 11.200 10.000 8.800 8.015 7.36200925 MAGNESIUM DISSOL (MG/L AS MG) 37 1.500 1.040 1.297 1.437 1.400 1.300 1.200 1.12100935 POTASSIUM DISSOL (MG/L AS K) 37 0.900 0.570 0.662 0.900 0.700 0.630 0.600 0.57000931 SODIUM ADSORPTIO (RATIO) 37 0.204 0.152 0.178 0.200 0.187 0.178 0.171 0.15500930 SODIUM DISSOLVED (MG/L AS NA) 37 2.600 1.600 2.154 2.510 2.300 2.200 2.000 1.78000932 SODIUM, PERCENT PERCENT 37 15.300 11.000 14.041 15.210 14.650 14.300 13.500 11.81090410 ANC, TIT. 4.5, L MG/L AS CACO3 36 35.000 22.000 29.417 34.150 32.000 30.000 27.000 24.55000940 CHLORIDE DISSOLV (MG/L AS CL) 35 1.300 0.100 0.617 1.140 0.800 0.600 0.400 0.10000950 FLUORIDE DISSOLV (MG/L AS F) 35 0.200 -- 0.121* *0.200 *0.200 *0.100 *0.100 *0.06200955 SILICA DISSOLVED (MG/L AS SIO2) 37 4.900 2.100 3.673 4.900 4.450 3.800 2.850 2.28000945 SULFATE DISSOLVE (MG/L AS SO4) 34 4.700 1.600 2.891 4.625 3.100 2.900 2.575 1.82500608 NITROGEN AMMONIA (MG/L AS N) 65 0.210 -- 0.034* *0.103 *0.050 *0.020 *0.010 *0.00500625 NITROGEN AMM+ORG (MG/L AS N) 35 0.600 -- 0.230* *0.440 *0.246 *0.200 *0.174 *0.12200610 NITROGEN AMMONIA (MG/L AS N) 60 0.270 -- 0.031* *0.090 *0.040 *0.020 *0.010 *0.00400618 NITROGEN NITRATE (MG/L AS N) 73 0.240 0.000 0.060 0.213 0.080 0.040 0.020 0.00000631 NO2 + NO3 DISSOL (MG/L AS N) 107 0.979 -- 0.071* *0.212 *0.082 *0.045 *0.029 *0.01000613 NITROGEN,NITRITE MG/L AS N 115 0.030 -- 0.005* *0.011 *0.006 *0.004 *0.002 *0.00100666 PHOSPHORUS DISS. (MG/L AS P) 33 0.012 -- 0.004* *0.011 *0.005 *0.003 *0.002 *0.00100671 PHOSPHORUS ORTHO (MG/L AS P) 36 0.011 -- 0.004* *0.010 *0.004 *0.003 *0.002 *0.00100665 PHOSPHORUS TOTAL (MG/L AS P) 107 1.500 -- 0.061* *0.306 *0.030 *0.020 *0.006 *0.00200680 CARBON ORGANIC T (MG/L AS C) 9 4.100 3.200 3.556 4.100 3.850 3.400 3.350 3.20031625 COLIFORM FECAL 0 COLS./100 ML 15 -- -- -- -- -- -- -- --01005 BARIUM DISSOLVED (UG/L AS BA) 36 26.000 12.000 18.000 24.300 19.000 17.450 16.075 13.44501010 BERYLLIUM DISSOL (UG/L AS BE) 36 -- -- -- -- -- -- -- --01020 BORON DISSOLVED (UG/L AS B) 36 10.000 -- 4.987* *8.640 *6.078 *4.618 *3.606 *2.59801025 CADMIUM DISSOLVE (UG/L AS CD) 36 2.000 -- 0.293* *1.227 *0.344 *0.145 *0.065 *0.02101030 CHROMIUM DISSOLV (UG/L AS CR) 36 -- -- -- -- -- -- -- --01035 COBALT DISSOLVED (UG/L AS CO) 36 -- -- -- -- -- -- -- --01040 COPPER DISSOLVED (UG/L AS CU) 36 -- -- -- -- -- -- -- --01046 IRON DISSOLVED (UG/L AS FE) 37 20.000 -- 7.732* *20.000 *10.000 *6.691 *4.641 *2.98601049 LEAD DISSOLVED (UG/L AS PB) 36 -- -- -- -- -- -- -- --01130 LITHIUM DISSOLVE (UG/L AS LI) 36 4.000 -- 1.615* *3.564 *2.033 *1.391 *0.979 *0.56101056 MANGANESE DISSOL (UG/L AS MN) 37 35.000 -- 4.753* *32.300 *5.050 *2.000 *0.964 *0.24201055 MANGANESE TOTAL (UG/L AS MN) 12 33.000 -- 9.522* *33.000 *11.750 *7.520 *3.668 *3.00001060 MOLYBDENUM DISSO (UG/L AS MO) 36 -- -- -- -- -- -- -- --01065 NICKEL DISSOLVED (UG/L AS NI) 36 -- -- -- -- -- -- -- --01075 SILVER DISSOLVED (UG/L AS AG) 36 -- -- -- -- -- -- -- --01080 STRONTIUM DISSOL (UG/L AS SR) 36 47.000 29.600 39.283 47.000 43.750 39.500 35.100 31.30001085 VANADIUM DISSOLV (UG/L AS V) 36 -- -- -- -- -- -- -- --01090 ZINC DISSOLVED (UG/L AS ZN) 36 23.000 -- 5.884* *17.900 *7.798 *4.000 *2.037 *0.931------------------------------------------------------------------------------------------------------------------------------------* - VALUE IS ESTIMATED BY USING A LOG-PROBABILITY REGRESSION TO PREDICT

THE VALUES OF DATA BELOW THE DETECTION LIMIT (Helsel and Cohn, 1988)
132 Water Quality and Trend Analysis of Colorado–Big Thompson System Reservoirs and Related Conveyances Table 17. Summary statistics for site 9, Horsetooth Reservoir (Soldier Canyon Dam), sampled near surface. ------------------------------------------------------------------------------------------------------------------------------------ STATISTICAL SUMMARY OF SELECTED WATER QUALITY DATA COLLECTED FROM OCT 1969 TO AUG 1999 PERCENT OF SAMPLES IN WHICH VALUES DESCRIPTIVE STATISTICS WERE LESS THAN OR EQUAL TO THOSE SHOWN ------------------------------------- -------------------------------------- ------------------------------------------------------ SAMPLE (MEDIAN) WATER-QUALITY CONSTITUENT SIZE MAXIMUM MINIMUM MEAN 95 % 75 % 50 % 25 % 5 % ------------------------------------- -------------------------------------- ------------------------------------------------------ 70300 RESIDUE DIS 180C MG/L 137 78.000 16.000 50.416 72.100 56.500 50.000 44.000 34.000 70301 DISSOLVED SOLIDS MG/L 46 53.000 28.400 39.174 49.470 42.525 39.000 35.300 29.275 00077 TRANSPARENCY (IN (INCHES) 103 177.000 8.000 79.871 142.600 98.000 76.000 57.000 38.000 00300 OXYGEN DISSOLVED (MG/L) 142 12.200 0.000 8.208 10.700 9.125 7.800 7.200 6.500 00400 PH, WH, FIELD (STANDARD UNIT 144 8.800 6.000 7.473 8.300 7.800 7.600 7.100 6.400 00403 PH, WH, LABORATO (STANDARD UNIT 55 8.400 7.100 7.764 8.220 7.900 7.800 7.600 7.280 90095 SPECIFIC CONDUCT MICROSIEMENS/C 55 163.000 49.000 70.836 81.200 76.000 70.000 64.000 51.800 00095 SPECIFIC CONDUCT US/CM @ 25C 154 125.000 48.000 75.994 105.000 80.000 75.500 70.000 55.000 00010 WATER TEMPERATUR (DEGREES C) 154 25.000 2.000 14.769 22.500 20.700 15.500 10.000 3.500 00900 HARDNESS TOTAL (MG/L AS CAO3) 50 35.000 19.400 28.474 33.340 31.000 28.700 26.800 20.760 00915 CALCIUM DISSOLVE (MG/L AS CA) 50 11.000 6.100 8.963 10.890 9.725 9.150 8.375 6.516 00925 MAGNESIUM DISSOL (MG/L AS MG) 50 1.900 1.000 1.472 1.800 1.600 1.500 1.337 1.073 00935 POTASSIUM DISSOL (MG/L AS K) 50 1.000 0.500 0.716 0.945 0.800 0.700 0.600 0.533 00931 SODIUM ADSORPTIO (RATIO) 50 0.500 0.166 0.199 0.217 0.200 0.197 0.180 0.170 00930 SODIUM DISSOLVED (MG/L AS NA) 50 5.800 1.800 2.456 3.190 2.600 2.400 2.175 1.855 00932 SODIUM, PERCENT PERCENT 50 30.000 12.400 15.354 17.135 16.000 15.150 14.075 13.110 90410 ANC, TIT. 4.5, L MG/L AS CACO3 49 40.000 7.000 29.286 36.500 33.000 29.000 27.000 20.500 00940 CHLORIDE DISSOLV (MG/L AS CL) 48 32.000 0.200 1.410 1.850 0.900 0.700 0.600 0.245 00950 FLUORIDE DISSOLV (MG/L AS F) 48 0.500 -- 0.136* *0.200 *0.200 *0.100 *0.100 *0.060 00955 SILICA DISSOLVED (MG/L AS SIO2) 50 4.900 0.700 2.778 4.645 3.825 2.750 1.900 0.800 00945 SULFATE DISSOLVE (MG/L AS SO4) 48 10.000 0.700 3.737 7.275 4.825 3.300 2.500 1.090 00608 NITROGEN AMMONIA (MG/L AS N) 87 0.190 -- 0.036* *0.128 *0.050 *0.020 *0.009 *0.004 00625 NITROGEN AMM+ORG (MG/L AS N) 49 1.300 -- 0.347* *0.890 *0.450 *0.280 *0.147 *0.067 00610 NITROGEN AMMONIA (MG/L AS N) 81 3.600 -- 0.079* *0.159 *0.055 *0.020 *0.006 *0.002 00618 NITROGEN NITRATE (MG/L AS N) 94 2.300 0.000 0.135 0.273 0.153 0.100 0.050 0.010 71851 NITR. NO3 AS NO3 MG/L AS NO3 103 10.000 0.000 0.566 1.200 0.660 0.400 0.200 0.044 00631 NO2 + NO3 DISSOL (MG/L AS N) 138 2.300 -- 0.111* *0.261 *0.132 *0.080 *0.031 *0.010 00613 NITROGEN,NITRITE MG/L AS N 147 0.060 -- 0.007* *0.020 *0.010 *0.005 *0.003 *0.001 00666 PHOSPHORUS DISS. (MG/L AS P) 47 0.140 -- 0.013* *0.060 *0.011 *0.005 *0.002 *0.000 00671 PHOSPHORUS ORTHO (MG/L AS P) 33 0.030 -- 0.005* *0.020 *0.007 *0.003 *0.001 *0.001 00665 PHOSPHORUS TOTAL (MG/L AS P) 126 2.300 -- 0.139* *0.747 *0.082 *0.020 *0.009 *0.001 31625 COLIFORM FECAL 0 COLS./100 ML 71 14.000 -- 0.889* *2.400 *1.000 *0.353 *0.148 *0.042 70953 CHL-A PHY CHROMA UG/L 31 6.500 0.200 2.242 5.780 2.700 1.900 1.100 0.380 70954 CHLOROPHYLL-B, P UG/L 31 0.400 -- 0.056* *0.280 *0.063 *0.025 *0.010 *0.003 01005 BARIUM DISSOLVED (UG/L AS BA) 32 28.400 14.800 18.444 25.540 19.750 18.000 16.725 15.385 01010 BERYLLIUM DISSOL (UG/L AS BE) 32 -- -- -- -- -- -- -- -- 01020 BORON DISSOLVED (UG/L AS B) 33 360.000 -- 13.657* *122.000 *4.581 *0.916 *0.255 *0.044 01025 CADMIUM DISSOLVE (UG/L AS CD) 32 -- -- -- -- -- -- -- -- 01030 CHROMIUM DISSOLV (UG/L AS CR) 32 -- -- -- -- -- -- -- -- 01035 COBALT DISSOLVED (UG/L AS CO) 32 -- -- -- -- -- -- -- -- 01040 COPPER DISSOLVED (UG/L AS CU) 32 -- -- -- -- -- -- -- -- 01046 IRON DISSOLVED (UG/L AS FE) 50 110.000 -- 25.214* *74.500 *40.000 *11.812 *7.367 *2.805 01049 LEAD DISSOLVED (UG/L AS PB) 32 -- -- -- -- -- -- -- -- 01130 LITHIUM DISSOLVE (UG/L AS LI) 32 -- -- -- -- -- -- -- -- 01056 MANGANESE DISSOL (UG/L AS MN) 50 20.000 -- 2.441* *7.900 *3.000 *1.852 *0.780 *0.303 01055 MANGANESE TOTAL (UG/L AS MN) 19 20.000 -- 5.305* *20.000 *6.704 *3.966 *2.025 *0.982 01060 MOLYBDENUM DISSO (UG/L AS MO) 32 -- -- -- -- -- -- -- -- 01065 NICKEL DISSOLVED (UG/L AS NI) 32 -- -- -- -- -- -- -- -- 01075 SILVER DISSOLVED (UG/L AS AG) 32 -- -- -- -- -- -- -- -- 01080 STRONTIUM DISSOL (UG/L AS SR) 32 53.000 26.500 38.469 51.050 43.000 38.500 32.275 27.475 01085 VANADIUM DISSOLV (UG/L AS V) 32 -- -- -- -- -- -- -- -- 01090 ZINC DISSOLVED (UG/L AS ZN) 32 16.000 -- 4.657* *14.050 *7.500 *3.000 *1.654 *0.672

------------------------------------------------------------------------------------------------------------------------------------* - VALUE IS ESTIMATED BY USING A LOG-PROBABILITY REGRESSION TO PREDICT

THE VALUES OF DATA BELOW THE DETECTION LIMIT (Helsel and Cohn, 1988)
Hydrologic and Water-Quality Data 133

Table 18. Summary statistics for site 9, Horsetooth Reservoir (Soldier Canyon Dam), sampled near bottom.

------------------------------------------------------------------------------------------------------------------------------------

STATISTICAL SUMMARY OF SELECTED WATER QUALITY DATA COLLECTED FROM SEPT 1969 TO AUG 1999

PERCENT OF SAMPLES IN WHICH VALUES

DESCRIPTIVE STATISTICS WERE LESS THAN OR EQUAL TO THOSE SHOWN

------------------------------------- -------------------------------------- ------------------------------------------------------

SAMPLE (MEDIAN)

WATER-QUALITY CONSTITUENT SIZE MAXIMUM MINIMUM MEAN 95 % 75 % 50 % 25 % 5 %

------------------------------------- -------------------------------------- ------------------------------------------------------

70300 RESIDUE DIS 180C MG/L 111 81.000 10.000 48.135 67.400 53.000 48.000 43.000 33.80070301 DISSOLVED SOLIDS MG/L 33 47.100 32.500 39.152 46.330 41.450 39.200 36.550 32.64000077 TRANSPARENCY (IN (INCHES) 1 48.000 -- -- -- -- -- -- --00300 OXYGEN DISSOLVED (MG/L) 125 12.600 0.000 7.227 10.700 9.150 7.500 5.900 1.60000400 PH, WH, FIELD (STANDARD UNIT 109 8.500 5.800 7.213 7.800 7.600 7.200 6.900 6.35000403 PH, WH, LABORATO (STANDARD UNIT 42 8.300 6.700 7.457 8.200 7.700 7.500 7.200 6.91590095 SPECIFIC CONDUCT MICROSIEMENS/C 42 80.000 57.000 69.405 79.700 74.000 70.000 64.750 59.00000095 SPECIFIC CONDUCT US/CM @ 25C 120 140.000 50.000 72.800 87.850 78.000 73.000 69.000 54.00000010 WATER TEMPERATUR (DEGREES C) 147 25.000 2.000 7.435 12.500 9.000 7.000 6.000 3.00000900 HARDNESS TOTAL (MG/L AS CAO3) 38 34.900 23.100 28.550 34.140 30.675 28.600 26.550 23.29000915 CALCIUM DISSOLVE (MG/L AS CA) 38 11.000 6.400 8.869 10.050 9.625 8.950 8.312 7.16900925 MAGNESIUM DISSOL (MG/L AS MG) 38 4.600 1.180 1.541 1.845 1.600 1.500 1.400 1.20900935 POTASSIUM DISSOL (MG/L AS K) 35 0.900 0.460 0.681 0.820 0.700 0.700 0.600 0.49200931 SODIUM ADSORPTIO (RATIO) 35 0.300 0.169 0.193 0.230 0.196 0.190 0.185 0.17000930 SODIUM DISSOLVED (MG/L AS NA) 35 3.400 2.000 2.349 2.840 2.500 2.300 2.200 2.00000932 SODIUM, PERCENT PERCENT 35 20.000 13.500 14.926 17.040 15.200 14.700 14.400 13.74090410 ANC, TIT. 4.5, L MG/L AS CACO3 36 37.000 25.000 29.611 35.300 32.000 29.000 27.250 25.00000940 CHLORIDE DISSOLV (MG/L AS CL) 33 1.300 0.200 0.706 1.300 0.900 0.700 0.450 0.20000950 FLUORIDE DISSOLV (MG/L AS F) 37 0.200 -- 0.117* *0.200 *0.100 *0.100 *0.100 *0.06400955 SILICA DISSOLVED (MG/L AS SIO2) 39 4.800 0.600 3.308 4.700 4.200 3.600 2.600 1.10000945 SULFATE DISSOLVE (MG/L AS SO4) 33 5.900 1.900 3.388 5.550 4.000 3.200 2.750 2.04000608 NITROGEN AMMONIA (MG/L AS N) 72 1.200 -- 0.055* *0.154 *0.050 *0.030 *0.010 *0.00400625 NITROGEN AMM+ORG (MG/L AS N) 38 1.000 -- 0.244* *0.810 *0.300 *0.194 *0.141 *0.07500610 NITROGEN AMMONIA (MG/L AS N) 57 0.160 -- 0.031* *0.083 *0.040 *0.030 *0.010 *0.00500618 NITROGEN NITRATE (MG/L AS N) 66 0.310 0.020 0.125 0.276 0.153 0.120 0.088 0.04071851 NITR. NO3 AS NO3 MG/L AS NO3 76 1.400 0.089 0.525 1.130 0.660 0.500 0.320 0.17200631 NO2 + NO3 DISSOL (MG/L AS N) 100 0.320 -- 0.117* *0.220 *0.155 *0.110 *0.063 *0.02800613 NITROGEN,NITRITE MG/L AS N 113 0.070 -- 0.004* *0.010 *0.004 *0.002 *0.001 *0.00000666 PHOSPHORUS DISS. (MG/L AS P) 36 0.030 -- 0.007* *0.022 *0.010 *0.005 *0.003 *0.00100671 PHOSPHORUS ORTHO (MG/L AS P) 33 0.030 -- 0.008* *0.030 *0.010 *0.004 *0.002 *0.00100665 PHOSPHORUS TOTAL (MG/L AS P) 98 1.500 -- 0.106* *0.943 *0.050 *0.030 *0.010 *0.00331625 COLIFORM FECAL 0 COLS./100 ML 11 -- -- -- -- -- -- -- --01005 BARIUM DISSOLVED (UG/L AS BA) 32 21.000 3.000 16.556 19.895 18.000 17.000 16.000 9.50001010 BERYLLIUM DISSOL (UG/L AS BE) 32 -- -- -- -- -- -- -- --01020 BORON DISSOLVED (UG/L AS B) 32 20.000 -- 6.136* *15.202 *7.985 *5.088 *3.697 *2.14301025 CADMIUM DISSOLVE (UG/L AS CD) 32 -- -- -- -- -- -- -- --01030 CHROMIUM DISSOLV (UG/L AS CR) 32 -- -- -- -- -- -- -- --01035 COBALT DISSOLVED (UG/L AS CO) 32 -- -- -- -- -- -- -- --01040 COPPER DISSOLVED (UG/L AS CU) 32 -- -- -- -- -- -- -- --01046 IRON DISSOLVED (UG/L AS FE) 39 80.000 -- 22.618* *70.000 *30.000 *20.000 *7.898 *3.71701049 LEAD DISSOLVED (UG/L AS PB) 32 -- -- -- -- -- -- -- --01130 LITHIUM DISSOLVE (UG/L AS LI) 32 -- -- -- -- -- -- -- --01056 MANGANESE DISSOL (UG/L AS MN) 39 210.000 -- 21.523* *140.000 *14.000 *2.000 *1.000 *0.10001055 MANGANESE TOTAL (UG/L AS MN) 14 38.000 -- 8.933* *38.000 *10.000 *6.379 *4.832 *2.69401060 MOLYBDENUM DISSO (UG/L AS MO) 32 -- -- -- -- -- -- -- --01065 NICKEL DISSOLVED (UG/L AS NI) 32 -- -- -- -- -- -- -- --01075 SILVER DISSOLVED (UG/L AS AG) 32 -- -- -- -- -- -- -- --01080 STRONTIUM DISSOL (UG/L AS SR) 32 50.000 31.600 40.409 48.700 45.000 40.500 35.700 31.60001085 VANADIUM DISSOLV (UG/L AS V) 32 -- -- -- -- -- -- -- --01090 ZINC DISSOLVED (UG/L AS ZN) 32 17.000 -- 5.181* *13.750 *6.750 *4.582 *2.471 *1.234------------------------------------------------------------------------------------------------------------------------------------* - VALUE IS ESTIMATED BY USING A LOG-PROBABILITY REGRESSION TO PREDICT

THE VALUES OF DATA BELOW THE DETECTION LIMIT (Helsel and Cohn, 1988)
134 Water Quality and Trend Analysis of Colorado–Big Thompson System Reservoirs and Related Conveyances Table 19. Summary statistics for site 10, Horsetooth Reservoir (Spring Canyon Dam), sampled at surface. ------------------------------------------------------------------------------------------------------------------------------------ STATISTICAL SUMMARY OF SELECTED WATER QUALITY DATA COLLECTED FROM MAY 1983 TO OCT 2000 PERCENT OF SAMPLES IN WHICH VALUES DESCRIPTIVE STATISTICS WERE LESS THAN OR EQUAL TO THOSE SHOWN ------------------------------------- -------------------------------------- ------------------------------------------------------ SAMPLE (MEDIAN) WATER-QUALITY CONSTITUENT SIZE MAXIMUM MINIMUM MEAN 95 % 75 % 50 % 25 % 5 % ------------------------------------- -------------------------------------- ------------------------------------------------------ 70300 RESIDUE DIS 180C MG/L 37 70.000 20.000 40.243 53.800 45.000 40.000 34.000 25.400 70301 DISSOLVED SOLIDS MG/L 35 63.000 27.800 37.791 50.120 39.900 37.700 34.700 28.680 00077 TRANSPARENCY (IN (INCHES) 40 144.000 30.000 82.525 128.700 107.500 81.000 60.000 36.150 00300 OXYGEN DISSOLVED (MG/L) 49 11.000 6.000 7.884 10.150 8.600 7.600 7.100 6.650 00400 PH, WH, FIELD (STANDARD UNIT 50 8.500 6.700 7.710 8.345 7.925 7.650 7.500 7.255 00403 PH, WH, LABORATO (STANDARD UNIT 45 8.200 7.200 7.689 8.170 7.900 7.700 7.500 7.200 90095 SPECIFIC CONDUCT MICROSIEMENS/C 45 115.000 51.000 68.378 83.700 73.500 68.000 63.000 52.200 00095 SPECIFIC CONDUCT US/CM @ 25C 50 108.000 49.000 66.440 85.250 71.000 66.000 59.000 49.550 00010 WATER TEMPERATUR (DEGREES C) 48 24.000 6.000 16.562 22.510 20.850 16.650 13.500 7.275 00900 HARDNESS TOTAL (MG/L AS CAO3) 40 44.600 19.100 27.812 31.800 29.675 28.150 25.700 20.270 00915 CALCIUM DISSOLVE (MG/L AS CA) 40 13.600 6.000 8.778 10.095 9.475 8.960 8.150 6.371 00925 MAGNESIUM DISSOL (MG/L AS MG) 40 2.580 1.000 1.420 1.885 1.500 1.400 1.300 1.043 00935 POTASSIUM DISSOL (MG/L AS K) 38 0.900 0.500 0.663 0.900 0.700 0.650 0.600 0.519 00931 SODIUM ADSORPTIO (RATIO) 38 0.239 0.165 0.193 0.214 0.203 0.192 0.183 0.170 00930 SODIUM DISSOLVED (MG/L AS NA) 38 3.100 1.800 2.326 2.910 2.500 2.400 2.100 1.895 00932 SODIUM, PERCENT PERCENT 38 17.100 12.300 15.200 17.005 16.125 15.250 14.375 13.440 90410 ANC, TIT. 4.5, L MG/L AS CACO3 40 40.000 21.000 29.025 38.750 31.000 29.000 27.000 22.050 00940 CHLORIDE DISSOLV (MG/L AS CL) 35 2.900 0.200 0.871 2.020 1.100 0.800 0.500 0.200 00950 FLUORIDE DISSOLV (MG/L AS F) 40 0.200 -- 0.128* *0.200 *0.200 *0.100 *0.100 *0.062 00955 SILICA DISSOLVED (MG/L AS SIO2) 42 5.800 0.800 2.862 4.985 3.825 2.750 1.750 1.030 00945 SULFATE DISSOLVE (MG/L AS SO4) 35 12.600 2.100 3.586 7.000 4.000 3.200 2.800 2.180 00608 NITROGEN AMMONIA (MG/L AS N) 42 0.090 -- 0.021* *0.083 *0.030 *0.013 *0.007 *0.003 00625 NITROGEN AMM+ORG (MG/L AS N) 42 1.000 -- 0.252* *0.600 *0.300 *0.200 *0.138 *0.078 00610 NITROGEN AMMONIA (MG/L AS N) 16 0.050 -- 0.016* *0.050 *0.020 *0.009 *0.004 *0.001 00631 NO2 + NO3 DISSOL (MG/L AS N) 41 0.160 -- 0.024* *0.110 *0.022 *0.008 *0.003 *0.001 00630 NO2 + NO3 TOTAL (MG/L AS N) 6 -- -- -- -- -- -- -- -- 00613 NITROGEN,NITRITE MG/L AS N 43 -- -- -- -- -- -- -- -- 00666 PHOSPHORUS DISS. (MG/L AS P) 40 0.020 -- 0.005* *0.010 *0.006 *0.004 *0.002 *0.001 00671 PHOSPHORUS ORTHO (MG/L AS P) 36 0.020 -- 0.004* *0.017 *0.005 *0.003 *0.002 *0.001 00665 PHOSPHORUS TOTAL (MG/L AS P) 40 0.050 -- 0.010* *0.025 *0.010 *0.008 *0.005 *0.003 00680 CARBON ORGANIC T (MG/L AS C) 9 5.400 2.900 4.078 5.400 4.800 4.000 3.350 2.900 31625 COLIFORM FECAL 0 COLS./100 ML 34 12.000 -- 1.879* *9.750 *2.000 *1.000 *0.357 *0.102 70953 CHL-A PHY CHROMA UG/L 35 14.000 0.200 2.657 8.160 3.100 2.100 1.500 0.440 70954 CHLOROPHYLL-B, P UG/L 35 0.200 -- 0.073* *0.200 *0.100 *0.060 *0.040 *0.021 01005 BARIUM DISSOLVED (UG/L AS BA) 36 30.400 14.000 19.011 29.125 20.950 18.400 16.175 14.000 01010 BERYLLIUM DISSOL (UG/L AS BE) 36 -- -- -- -- -- -- -- -- 01020 BORON DISSOLVED (UG/L AS B) 36 14.000 -- 6.290* *11.450 *7.959 *5.815 *4.365 *3.133 01025 CADMIUM DISSOLVE (UG/L AS CD) 36 5.000 -- 0.362* *2.450 *0.268 *0.072 *0.018 *0.003 01030 CHROMIUM DISSOLV (UG/L AS CR) 36 -- -- -- -- -- -- -- -- 01035 COBALT DISSOLVED (UG/L AS CO) 36 -- -- -- -- -- -- -- -- 01040 COPPER DISSOLVED (UG/L AS CU) 36 -- -- -- -- -- -- -- -- 01046 IRON DISSOLVED (UG/L AS FE) 42 80.000 -- 20.820* *70.000 *30.000 *10.000 *5.478 *2.057 01049 LEAD DISSOLVED (UG/L AS PB) 36 -- -- -- -- -- -- -- -- 01130 LITHIUM DISSOLVE (UG/L AS LI) 36 -- -- -- -- -- -- -- -- 01056 MANGANESE DISSOL (UG/L AS MN) 42 209.000 -- 8.089* *41.330 *3.000 *1.902 *0.445 *0.100 01055 MANGANESE TOTAL (UG/L AS MN) 18 208.000 -- 23.233* *208.000 *14.000 *7.346 *3.776 *1.179 01060 MOLYBDENUM DISSO (UG/L AS MO) 36 -- -- -- -- -- -- -- -- 01065 NICKEL DISSOLVED (UG/L AS NI) 36 -- -- -- -- -- -- -- -- 01075 SILVER DISSOLVED (UG/L AS AG) 36 -- -- -- -- -- -- -- -- 01080 STRONTIUM DISSOL (UG/L AS SR) 36 68.500 26.600 38.875 52.775 43.000 38.000 33.800 27.790 01085 VANADIUM DISSOLV (UG/L AS V) 36 -- -- -- -- -- -- -- -- 01090 ZINC DISSOLVED (UG/L AS ZN) 36 30.000 -- 5.381* *29.200 *5.000 *2.208 *0.897 *0.316

------------------------------------------------------------------------------------------------------------------------------------* - VALUE IS ESTIMATED BY USING A LOG-PROBABILITY REGRESSION TO PREDICT

THE VALUES OF DATA BELOW THE DETECTION LIMIT (Helsel and Cohn, 1988)
Hydrologic and Water-Quality Data 135

Table 20. Summary statistics for site 10, Horsetooth Reservoir (Spring Canyon Dam), sampled near bottom.

------------------------------------------------------------------------------------------------------------------------------------

STATISTICAL SUMMARY OF SELECTED WATER QUALITY DATA COLLECTED FROM MAY 1983 TO OCT 2000

PERCENT OF SAMPLES IN WHICH VALUES

DESCRIPTIVE STATISTICS WERE LESS THAN OR EQUAL TO THOSE SHOWN

------------------------------------- -------------------------------------- ------------------------------------------------------

SAMPLE (MEDIAN)

WATER-QUALITY CONSTITUENT SIZE MAXIMUM MINIMUM MEAN 95 % 75 % 50 % 25 % 5 %

------------------------------------- -------------------------------------- ------------------------------------------------------

70300 RESIDUE DIS 180C MG/L 38 66.000 17.000 42.237 59.350 47.250 42.000 38.000 27.45070301 DISSOLVED SOLIDS MG/L 34 58.000 30.700 40.118 50.275 41.800 40.200 37.900 32.35000300 OXYGEN DISSOLVED (MG/L) 49 10.800 0.000 5.178 9.850 7.950 5.200 2.500 0.10000400 PH, WH, FIELD (STANDARD UNIT 50 7.900 6.000 7.262 7.700 7.500 7.300 7.100 6.71000403 PH, WH, LABORATO (STANDARD UNIT 45 8.200 6.600 7.382 8.140 7.600 7.400 7.100 6.80090095 SPECIFIC CONDUCT MICROSIEMENS/C 45 103.000 53.000 71.733 82.700 76.500 72.000 67.500 54.50000095 SPECIFIC CONDUCT US/CM @ 25C 50 102.000 48.000 69.400 87.450 76.250 70.000 61.000 54.00000010 WATER TEMPERATUR (DEGREES C) 48 21.500 5.000 7.831 11.365 8.375 7.450 6.925 5.47000900 HARDNESS TOTAL (MG/L AS CAO3) 39 40.100 20.400 29.197 34.900 31.600 29.100 27.100 22.60000915 CALCIUM DISSOLVE (MG/L AS CA) 39 12.300 6.300 9.186 11.000 10.000 9.100 8.500 7.09000925 MAGNESIUM DISSOL (MG/L AS MG) 39 2.240 1.120 1.506 1.800 1.600 1.500 1.400 1.18000935 POTASSIUM DISSOL (MG/L AS K) 36 0.800 0.560 0.681 0.800 0.718 0.700 0.600 0.56800931 SODIUM ADSORPTIO (RATIO) 36 0.216 0.170 0.190 0.210 0.199 0.192 0.181 0.17000930 SODIUM DISSOLVED (MG/L AS NA) 36 2.800 1.900 2.353 2.715 2.500 2.300 2.200 1.90000932 SODIUM, PERCENT PERCENT 36 16.700 13.100 14.692 16.615 15.375 14.700 13.800 13.18590410 ANC, TIT. 4.5, L MG/L AS CACO3 39 42.000 23.000 29.974 35.000 32.000 30.000 28.000 24.00000940 CHLORIDE DISSOLV (MG/L AS CL) 34 1.700 0.200 0.762 1.625 1.000 0.700 0.475 0.20000950 FLUORIDE DISSOLV (MG/L AS F) 40 0.200 -- 0.129* *0.200 *0.200 *0.100 *0.100 *0.06600955 SILICA DISSOLVED (MG/L AS SIO2) 42 5.700 0.900 3.624 5.470 4.500 3.850 2.775 1.21500945 SULFATE DISSOLVE (MG/L AS SO4) 34 5.100 2.500 3.285 4.575 3.925 3.200 2.775 2.50000608 NITROGEN AMMONIA (MG/L AS N) 41 0.165 -- 0.035* *0.084 *0.050 *0.020 *0.010 *0.00600625 NITROGEN AMM+ORG (MG/L AS N) 42 0.700 -- 0.242* *0.500 *0.300 *0.200 *0.148 *0.09300610 NITROGEN AMMONIA (MG/L AS N) 17 0.060 -- 0.038* *0.060 *0.050 *0.040 *0.028 *0.02000631 NO2 + NO3 DISSOL (MG/L AS N) 40 0.230 -- 0.113* *0.216 *0.180 *0.102 *0.052 *0.02500630 NO2 + NO3 TOTAL (MG/L AS N) 6 -- -- -- -- -- -- -- --00613 NITROGEN,NITRITE MG/L AS N 42 0.060 -- 0.004* *0.010 *0.004 *0.002 *0.001 *0.00000666 PHOSPHORUS DISS. (MG/L AS P) 40 0.040 -- 0.013* *0.039 *0.020 *0.008 *0.004 *0.00200671 PHOSPHORUS ORTHO (MG/L AS P) 36 0.040 -- 0.011* *0.034 *0.019 *0.007 *0.004 *0.00100665 PHOSPHORUS TOTAL (MG/L AS P) 41 0.096 -- 0.022* *0.057 *0.030 *0.020 *0.008 *0.00300680 CARBON ORGANIC T (MG/L AS C) 9 5.200 3.200 3.956 5.200 4.600 3.700 3.350 3.20001005 BARIUM DISSOLVED (UG/L AS BA) 36 30.100 13.000 17.336 23.045 18.000 17.400 16.000 13.85001010 BERYLLIUM DISSOL (UG/L AS BE) 36 -- -- -- -- -- -- -- --01020 BORON DISSOLVED (UG/L AS B) 36 20.000 -- 6.469* *11.970 *8.000 *5.978 *4.130 *2.98401025 CADMIUM DISSOLVE (UG/L AS CD) 36 -- -- -- -- -- -- -- --01030 CHROMIUM DISSOLV (UG/L AS CR) 36 -- -- -- -- -- -- -- --01035 COBALT DISSOLVED (UG/L AS CO) 36 -- -- -- -- -- -- -- --01040 COPPER DISSOLVED (UG/L AS CU) 36 -- -- -- -- -- -- -- --01046 IRON DISSOLVED (UG/L AS FE) 42 140.000 0.000 25.548 78.500 30.000 20.000 10.000 0.45001049 LEAD DISSOLVED (UG/L AS PB) 36 -- -- -- -- -- -- -- --01130 LITHIUM DISSOLVE (UG/L AS LI) 36 -- -- -- -- -- -- -- --01056 MANGANESE DISSOL (UG/L AS MN) 41 685.000 -- 80.164* *590.000 *72.000 *7.000 *2.000 *0.17201055 MANGANESE TOTAL (UG/L AS MN) 18 684.000 -- 75.913* *684.000 *40.750 *19.000 *9.500 *2.21401060 MOLYBDENUM DISSO (UG/L AS MO) 36 -- -- -- -- -- -- -- --01065 NICKEL DISSOLVED (UG/L AS NI) 36 -- -- -- -- -- -- -- --01075 SILVER DISSOLVED (UG/L AS AG) 36 -- -- -- -- -- -- -- --01080 STRONTIUM DISSOL (UG/L AS SR) 36 59.300 27.300 41.342 52.245 45.750 40.500 37.000 30.78501085 VANADIUM DISSOLV (UG/L AS V) 36 -- -- -- -- -- -- -- --01090 ZINC DISSOLVED (UG/L AS ZN) 36 42.000 -- 5.786* *26.800 *7.000 *3.001 *1.457 *0.520------------------------------------------------------------------------------------------------------------------------------------* - VALUE IS ESTIMATED BY USING A LOG-PROBABILITY REGRESSION TO PREDICT

THE VALUES OF DATA BELOW THE DETECTION LIMIT (Helsel and Cohn, 1988)
136 Water Quality and Trend Analysis of Colorado–Big Thompson System Reservoirs and Related Conveyances Table 21. Trend analysis results for site 1, Granby Reservoir, near dam, surface samples. [0, no trend; (+), upward trend; (–), downward trend; significance: *, greater than 90 percent; **, greater than 95 percent; ***, greater than 99 percent; change is percent change in median since the first year of record; A, no trend computation because nondetects greater than 50 percent; B, no slope computation because nondetects greater than 20 percent; mg/L, milligrams per liter; μS/cm, microsiemens per centimeter at 25 degrees Celsius; °C, degree Celsius; in., inch; mL, milliliter; hplc, high-performance liquid chromatography; μg/L, micrograms per liter] Seasonal Kendall season-edited data Number Censored Percentage censored Period record (years)

p-value Slope Median Significance Change

Oxygen, dissolved (mg/L) 46 0 0 11 0.0383 –0.05001 7.5 (–)** –7.1pH, field (standard units) 46 0 0 11 0.1719 –0.02519 7.9 0 –3.4Specific conductance (μS/cm at 25°C) 46 0 0 11 0.0000 –1.768 54 (–)*** –30.1Water temperature (°C) 46 0 0 11 0.7760 –0.02014 15.2 0 –1.4Hardness, total (mg/L as CaCO3) 45 0 0 11 0.0000 –0.5543 22 (–)*** –24.1

Calcium, dissolved (mg/L) 45 0 0 11 0.0000 –0.1662 6.85 (–)*** –23.3Magnesium, dissolved (mg/L) 45 0 0 11 0.0000 –0.03141 1.2 (–)*** –24.9Potassium, dissolved (mg/L) 45 0 0 11 0.0000 –0.0125 0.7 (–)*** –17.7Sodium adsorption (ratio) 45 0 0 11 1.0000 0 0.2 0 0.0Sodium, dissolved (mg/L) 45 0 0 11 0.0000 –0.08776 2.2 (–)*** –35.4Sodium (percent) 45 0 0 11 0.0000 –0.2004 16 (–)*** –12.8Acid-neutralizing capacity (mg/L) 45 0 0 11 0.0000 –0.4997 24 (–)*** –20.4Chloride, dissolved (mg/L) 45 0 0 11 0.0554 0 0.4 0 0.0Fluoride, dissolved (mg/L) 45 9 20 11 0.0374 0 0.1 0 BSilica, dissolved (mg/L) 45 0 0 11 1.7247 0.04281 4.85 0 10.3Sulfate, dissolved (mg/L) 45 0 0 11 0.0000 –0.1189 2.95 (–)*** –35.7Nitrogen, ammonia plus organic (mg/L as N) 45 13 29 11 0.0024 0 0.3 0 BNitrogen, ammonia (mg/L as N) 45 32 71 11 A A A A ANitrite plus nitrate (mg/L as N) 45 45 100 11 A A A A ANitrogen, nitrite (mg/L as N) 45 43 96 11 A A A A APhosphorus, dissolved (mg/L as P) 39 34 87 11 A A A A APhophorus, ortho, dissolved (mg/L as P) 45 41 91 11 A A A A APhosphorus, total (mg/L as P) 41 25 61 11 A A A A AResidue, dissolved at 180°C (mg/L) 44 0 0 11 0.0296 –0.6707 37 (–)** –18.0Dissolved solids, sum of constituents (mg/L) 45 0 0 11 0.0000 0.6643 35 (+)*** 23.6Transparency, Secchi disk (in.) 44 0 0 11 0.0002 8.037 122 (+)*** 119.8Coliform, fecal (colonies per 100 mL) 42 30 71 11 A A A A AChlorophyll-a, hplc (μg/L) 46 0 0 11 0.0056 –0.2268 2.05 (–)*** –73.1Chlorophyll-b, hplc (μg/L) 45 33 73 11 A A A A ABarium, dissolved (μg/L) 45 0 0 11 0.2263 0 8 0 0.0Iron, dissolved (μg/L) 45 21 47 11 0.0000 B 26 (–)*** BManganese, dissolved (μg/L) 39 24 62 11 A A A A AStrontium, dissolved (μg/L) 45 0 0 11 0.0000 –1.219 42 (–)*** –27.2Zinc, dissolved (μg/L) 34 20 59 11 A A A A A
Hydrologic and Water-Quality Data 137

Table 22. Trend analysis results for site 1, Granby Reservoir, near dam, bottom samples.[0, no trend; (+), upward trend; (–), downward trend; significance: *, greater than 90 percent; **, greater than 95 percent; ***, greater than 99 percent; change is percent change in median since the first year of record; A, no trend computation because nondetects greater than 50 percent; B, no slope computation because nondetects greater than 20 percent; mg/L, milligrams per liter; μS/cm, microsiemens per centimeter at 25 degrees Celsius; °C, degree Celsius; μg/L, micrograms per liter]

Seasonal Kendall 

season-edited data Number Censored Percentage censored Period record (years) p-value Slope Median Significance Change

Oxygen, dissolved (mg/L) 44 0 0 11 0.0006 0.2182 4.8 (+)*** 68.8pH, field (standard units) 45 0 0 11 0.8224 0 7.1 0 0.0Specific conductance (μS/cm at 25°C) 45 0 0 11 0.0000 –2.15 55.5 (–)*** –34.6Water temperature (°C) 45 0 0 11 0.0016 –0.1245 6.5 (–)*** –18.9Hardness, total (mg/L as CaCO3) 45 0 0 11 0.0000 –0.5513 24 (–)*** –22.2

Calcium, dissolved (mg/L) 45 0 0 11 0.0000 –0.1789 7.3 (–)*** –23.5Magnesium, dissolved (mg/L) 45 0 0 11 0.0000 –0.0296 1.3 (–)*** –22.0Potassium, dissolved (mg/L) 45 0 0 11 0.0001 –0.01658 0.7 (–)*** –22.8Sodium adsorption (ratio) 45 0 0 11 0.4789 0 0.2 0 0.0Sodium, dissolved (mg/L) 45 0 0 11 0.0000 –0.09787 2.3 (–)*** –37.3Sodium (percent) 45 0 0 11 0.0000 –0.2008 17 (–)*** –12.1Acid-neutralizing capacity (mg/L) 45 0 0 11 0.0000 –0.6636 26 (–)*** –24.3Chloride, dissolved (mg/L) 45 0 0 11 0.5170 0 0.4 0 0.0Fluoride, dissolved (mg/L) 45 13 29 11 0.0730 B 0.1 0 BSilica, dissolved (mg/L) 45 0 0 11 0.1085 –0.08068 5.9 0 –13.9Sulfate, dissolved (mg/L) 45 0 0 11 0.0000 –0.08859 3 (–)*** –27.6Nitrogen, ammonia plus organic (mg/L as N) 46 24 52 11 A A A A ANitrogen, ammonia (mg/L as N) 45 29 64 11 A A A A ANitrite plus nitrate (mg/L as N) 45 25 56 11 A A A A ANitrogen, nitrite (mg/L as N) 45 42 93 11 A A A A APhosphorus, dissolved (mg/L as P) 39 28 72 11 A A A A APhophorus, ortho, dissolved (mg/L as P) 45 37 82 11 A A A A APhosphorus, total (mg/L as P) 42 15 36 11 0.4151 B 0.02 0 BResidue, dissolved at 180°C (mg/L) 44 0 0 11 0.3325 –0.377 40 0 –9.8Dissolved solids, sum of constituents (mg/L) 45 0 0 11 0.0000 –0.9844 38 (–)*** –24.7Barium, dissolved (μg/L) 45 0 0 11 0.1023 0 8 0 0.0Iron, dissolved (μg/L) 45 8 18 11 0.0002 B 19.5 (–)*** BManganese, dissolved (μg/L) 39 12 31 11 0.0001 B 10 (–)*** BStrontium, dissolved (μg/L) 45 0 0 11 0.0000 –1.218 46.5 (–)*** –24.9Zinc, dissolved (μg/L) 34 17 50 8 0.3221 B 8 0 B
138 Water Quality and Trend Analysis of Colorado–Big Thompson System Reservoirs and Related Conveyances Table 23. Trend analysis results for site 2, Granby Reservoir (Rainbow Bay), surface samples. [0, no trend; (+), upward trend; (–), downward trend; significance: *, greater than 90 percent; **, greater than 95 percent; ***, greater than 99 percent; change is percent change in median since the first year of record; A, no trend computation because nondetects greater than 50 percent; B, no slope computation because nondetects greater than 20 percent; mg/L, milligrams per liter; μS/cm, microsiemens per centimeter at 25 degrees Celsius; °C, degree Celsius; in., inch; mL, milliliter; hplc, high-performance liquid chromatography; μg/L, micrograms per liter] Seasonal Kendall season-edited data Number Censored Percentage censored Period record (years)

p-value Slope Median Significance Change

Oxygen, dissolved (mg/L) 45 0 0 11 0.2757 –0.02509 7.4 0 –3.7pH, field (standard units) 46 0 0 11 0.4714 –0.009972 7.8 0 –1.4Specific conductance (μS/cm at 25°C) 46 0 0 11 0.0000 –1.838 54.75 (–)*** –30.7Water temperature (°C) 46 0 0 11 0.7486 0.03633 13.45 0 3.0Hardness, total (mg/L as CaCO3) 45 0 0 11 0.0000 –0.6377 23 (–)*** –26.1

Calcium, dissolved (mg/L) 45 0 0 11 0.0000 –0.1905 7.1 (–)*** –25.4Magnesium, dissolved (mg/L) 45 0 0 11 0.0000 –0.02791 1.3 (–)*** –20.9Potassium, dissolved (mg/L) 45 0 0 11 0.0000 –0.01419 0.7 (–)*** –19.9Sodium adsorption (ratio) 45 0 0 11 0.0586 0 0.2 0 0.0Sodium, dissolved (mg/L) 45 0 0 11 0.0000 –0.09753 2.3 (–)*** –37.2Sodium (percent) 45 0 0 11 0.0000 –0.2013 16 (–)*** –12.9Acid-neutralizing capacity (mg/L) 45 0 0 11 0.0000 –0.6262 26 (–)*** –23.1Chloride, dissolved (mg/L) 44 0 0 11 0.1148 0 0.4 0 0.0Fluoride, dissolved (mg/L) 44 9 20 11 0.4901 B 0.1 0 BSilica, dissolved (mg/L) 45 0 0 11 0.7990 0.01451 5 0 3.2Sulfate, dissolved (mg/L) 44 0 0 11 0.0000 –0.1371 2.9 (–)*** –40.5Nitrogen, ammonia plus organic (mg/L as N) 46 16 35 11 0.0044 0 0.2 0 BNitrogen, ammonia (mg/L as N) 45 30 67 11 A A A A ANitrite plus nitrate (mg/L as N) 39 32 82 9 A A A A ANitrogen, nitrite (mg/L as N) 45 42 93 11 A A A A APhosphorus, dissolved (mg/L as P) 35 28 80 9 A A A A APhophorus, ortho, dissolved (mg/L as P) 45 37 82 11 A A A A APhosphorus, total (mg/L as P) 41 18 44 11 0.1100 B 0.02 0 BResidue, dissolved at 180°C (mg/L) 44 0 0 11 0.1052 –0.6271 38.5 0 –16.3Dissolved solids, sum of constituents (mg/L) 44 0 0 11 0.0000 –0.8526 36 (–)*** –22.8Transparency, Secchi disk (in.) 43 0 0 11 0.0009 7.043 113 (+)*** 109.5Coliform, fecal (colonies per 100 mL) 42 25 60 11 A A A A AChlorophyll-a, hplc (μg/L) 45 0 0 11 0.0059 –0.2487 1.8 (–)*** –83.1Chlorophyll-b, hplc (μg/L) 44 34 77 11 A A A A ABarium, dissolved (μg/L) 45 0 0 11 0.0036 0 8 0 0.0Iron, dissolved (μg/L) 35 2 6 9 0.0000 B 20 (–)*** BManganese, dissolved (μg/L) 34 15 44 8 0.0010 B 4 (–)*** BStrontium, dissolved (μg/L) 45 0 0 11 0.0000 –1.19 44 (–)*** –25.6Zinc, dissolved (μg/L) 34 18 53 8 A A A A A
Hydrologic and Water-Quality Data 139

Table 24. Trend analysis results for site 2, Granby Reservoir (Rainbow Bay), bottom samples.[0, no trend; (+), upward trend; (–), downward trend; significance: *, greater than 90 percent; **, greater than 95 percent; ***, greater than 99 percent; change is percent change in median since the first year of record; A, no trend computation because nondetects greater than 50 percent; B, no slope computation because nondetects greater than 20 percent; mg/L, milligrams per liter; μS/cm, microsiemens per centimeter at 25 degrees Celsius; °C, degree Celsius; μg/L, micrograms per liter]

Seasonal Kendall 

season-edited data Number Censored Percentage censored Period record (years) p-value Slope Median Significance Change

Oxygen, dissolved (mg/L) 45 0 0 11 0.0878 –0.1285 5.1 (–)* –24.1pH, field (standard units) 46 0 0 11 0.0433 –0.03346 7.4 (–)** –4.8Specific conductance (μS/cm at 25°C) 46 0 0 11 0.0000 –2.02 54.5 (–)*** –33.4Water temperature (°C) 46 0 0 11 0.0000 –0.6136 8.35 (–)*** –56.1Hardness, total (mg/L as CaCO3) 45 0 0 11 0.0000 –0.5703 23 (–)*** –23.7

Calcium, dissolved (mg/L) 45 0 0 11 0.0000 –0.1956 7 (–)*** –26.3Magnesium, dissolved (mg/L) 45 0 0 11 0.0000 –0.03258 1.3 (–)*** –24.0Potassium, dissolved (mg/L) 45 0 0 11 0.0009 –0.01109 0.7 (–)*** –15.9Sodium adsorption (ratio) 45 0 0 11 0.1164 0 0.2 0 0.0Sodium, dissolved (mg/L) 45 0 0 11 0.0000 –0.08457 2.3 (–)*** –33.1Sodium (percent) 45 0 0 11 0.0001 –0.1677 16 (–)*** –10.8Acid-neutralizing capacity (mg/L) 45 0 0 11 0.0000 –0.5325 25 (–)*** –20.8Chloride, dissolved (mg/L) 44 0 0 11 0.1621 0 0.4 0 0.0Fluoride, dissolved (mg/L) 44 12 27 11 0.0884 0 0.100 0 BSilica, dissolved (mg/L) 45 0 0 11 0.7429 –0.01608 5.6 0 –3.1Sulfate, dissolved (mg/L) 44 0 0 11 0.0000 –0.09972 2.900 (–)*** –31.4Nitrogen, ammonia plus organic (mg/L as N) 45 18 40 11 0.0067 0 0.2 0 BNitrogen, ammonia (mg/L as N) 45 26 58 11 A A A A ANitrite plus nitrate (mg/L as N) 40 28 70 11 A A A A ANitrogen, nitrite (mg/L as N) 45 43 96 11 A A A A APhosphorus, dissolved (mg/L as P) 35 27 77 9 A A A A APhophorus, ortho, dissolved (mg/L as P) 45 38 84 11 A A A A APhosphorus, total (mg/L as P) 40 13 33 11 0.6043 B 0.02 0 BResidue, dissolved at 80°C (mg/L) 44 0 0 11 0.0897 –0.7479 39 (–)* –18.9Dissolved solids, sum of constituents (mg/L) 44 0 0 11 0.0000 –0.7537 36 (–)*** –20.5Barium, dissolved (μg/L) 45 0 0 11 0.0007 0 8 0 0.0Iron, dissolved (μg/L) 45 3 7 11 0.0002 B 15 (–)*** BManganese, dissolved (μg/L) 45 12 27 11 0.0076 B 7 (–)*** BStrontium, dissolved (μg/L) 45 1 2 11 0.0000 –1.151 44 (–)*** –24.9Zinc, dissolved (μg/L) 46 26 57 11 A A A A A
140 Water Quality and Trend Analysis of Colorado–Big Thompson System Reservoirs and Related Conveyances Table 25. Trend analysis results for site 3, Granby Pump Canal, samples. [0, no trend; (+), upward trend; (–), downward trend; significance: *, greater than 90 percent; **, greater than 95 percent; ***, greater than 99 percent; change is percent change in median since the first year of record; A, no trend computation because nondetects greater than 50 percent; B, no slope computation because nondetects greater than 20 percent; mg/L, milligrams per liter; μS/cm, microsiemens per centimeter at 25 degrees Celsius; °C, degree Celsius; μg/L, micrograms per liter] Seasonal Kendall season-edited data Number Censored Percentage censored Period record (years)

p-value Slope Median Significance Change

Oxygen, dissolved (mg/L) 170 0 0 30 0.3610 0.0133 7.55 0 5.4pH, field (standard units) 173 0 0 30 0.0004 0.01736 7.3 (+)*** 7.4Specific conductance (μS/cm at 25°C) 179 0 0 30 0.0001 –0.2482 60 (–)*** –11.7Water temperature (°C) 180 0 0 30 0.0577 0.01998 4.1 (+)* 15.8Hardness, total (mg/L as CaCO3) 56 0 0 11 0.0000 –0.5282 24 (–)*** –21.4

Calcium, dissolved (mg/L) 56 0 0 11 0.0000 –0.176 7.4 (–)*** –22.9Magnesium, dissolved (mg/L) 56 0 0 11 0.0000 –0.02449 1.3 (–)*** –18.6Potassium, dissolved (mg/L) 56 0 0 11 0.0000 –0.01649 0.7 (–)*** –22.7Sodium adsorption (ratio) 56 0 0 11 1.0000 0 0.2 0 0.0Sodium, dissolved (mg/L) 56 0 0 11 0.0000 –0.08641 2.3 (–)*** –33.7Sodium (percent) 56 0 0 11 0.0000 –0.1822 17 (–)*** –11.1Acid-neutralizing capacity (mg/L) 56 0 0 11 0.0000 –0.6187 26 (–)*** –22.9Chloride, dissolved (mg/L) 56 1 2 11 0.0160 –0.01837 0.4 (–)** –39.6Fluoride, dissolved (mg/L) 56 10 18 11 0.1912 B 0.1 0 BSilica, dissolved (mg/L) 56 0 0 11 0.1479 0.06661 5.4 0 14.7Sulfate, dissolved (mg/L) 56 0 0 11 0.0000 –0.09868 3 (–)*** –30.2Nitrogen, ammonia plus organic (mg/L as N) 139 27 19 22 0.0000 –0.01652 0.38 (–)*** –63.8Nitrogen, ammonia (mg/L as N) 53 37 70 11 A A A A ANitrite plus nitrate (mg/L as N) 128 59 46 22 0.0010 B 0.1 (–)*** BNitrogen, nitrite (mg/L as N) 55 49 89 11 A A A A APhosphorus, dissolved (mg/L as P) 54 40 74 11 A A A A APhophorus, ortho, dissolved (mg/L as P) 54 40 74 11 A A A A APhosphorus, total (mg/L as P) 134 31 23 22 0.0000 B 0.02 (–)*** BResidue, dissolved at 180°C (mg/L) 55 0 0 11 0.0502 –0.4758 40 (–)* –12.2Dissolved solids, sum of constituents (mg/L) 55 0 0 11 0.0000 –0.745 38 (–)*** –19.3Barium, dissolved (μg/L) 56 1 2 11 0.4783 0 7 0 0.0Iron, dissolved (μg/L) 48 3 6 11 0.0032 B 23 (–)*** BManganese, dissolved (μg/L) 46 9 20 11 0.0000 B 4 (–)*** BStrontium, dissolved (μg/L) 56 0 0 11 0.0000 –1.137 45 (–)*** –24.1Zinc, dissolved (μg/L) 47 19 40 11 0.7031 B 5.5 0 B
Hydrologic and Water-Quality Data 141

Table 26. Trend analysis results for site 4, Shadow Mountain Reservoir, surface samples.[0, no trend; (+), upward trend; (–), downward trend; significance: *, greater than 90 percent; **, greater than 95 percent; ***, greater than 99 percent; change is percent change in median since the first year of record; A, no trend computation because nondetects greater than 50 percent; B, no slope computation because nondetects greater than 20 percent; mg/L, milligrams per liter; μS/cm, microsiemens per centimeter at 25 degrees Celsius; °C, degree Celsius; in., inch; mL, milliliter; hplc, high-performance liquid chromatography; μg/L, micrograms per liter]

Seasonal Kendall 

season-edited data Number Nondetects % nondetect

Period 

record 

(years)

p-value Slope Median Significance Change

Oxygen, dissolved (mg/L) 35 0 0 11 0.8687865534 0.01429 7.4 0 2.2pH, field (standard units) 35 0 0 11 0.6171454802 –0.009913 7.6 0 –1.4Specific conductance (μS/cm at 25°C) 36 0 0 11 0.119470507 –0.5274 49.6 0 –11.0Water temperature (°C) -- -- -- -- -- -- -- -- --Hardness, total (mg/L as CaCO3) 36 0 0 11 0.0898858202 –0.2003 22 (–)* –9.5

Calcium, dissolved (mg/L) 36 0 0 11 0.0469222566 –0.09798 6.5 (–)** –15.2Magnesium, dissolved (mg/L) 36 0 0 11 0.6445834148 0 1.3 0 0.0Potassium, dissolved (mg/L) 36 0 0 11 0.1453926983 0 0.7 0 0.0Sodium adsorption (ratio) 36 0 0 11 0.8533074921 0 0.2 0 0.0Sodium, dissolved (mg/L) 36 0 0 11 0.0005205513 –0.05838 1.9 (–)*** –28.5Sodium (percent) 36 0 0 11 0.0000174818 –0.2501 16 (–)*** –15.7Acid-neutralizing capacity (mg/L) 36 0 0 11 0.0690640193 –0.25 23.5 (–)* –11.0Chloride, dissolved (mg/L) 35 2 6 11 0.9999999996 B 0.4 0 BFluoride, dissolved (mg/L) 35 1 3 11 0.643221044 0 0.1 0 0.0Silica, dissolved (mg/L) 36 0 0 11 0.0315551014 0.0991 5.55 (+)** 22.0Sulfate, dissolved (mg/L) 35 0 0 11 0.0018395843 –0.05731 2.8 (–)*** –20.1Nitrogen, ammonia plus organic (mg/L as N) 36 5 14 11 0.0016781137 B 0.3 (–)*** BNitrogen, ammonia (mg/L as N) 36 23 64 11 A A A A ANitrite plus nitrate (mg/L as N) 30 21 70 11 A A A A ANitrogen, nitrite (mg/L as N) 36 33 92 11 A A A A APhosphorus, dissolved (mg/L as P) 27 23 85 11 A A A A APhophorus, ortho, dissolved (mg/L as P) 36 35 97 11 A A A A APhosphorus, total (mg/L as P) 29 11 38 11 0.046261307 0 0.02 0 BResidue, dissolved at 180°C (mg/L) 35 0 0 11 0.9670087749 0 38 0 0.0Dissolved solids, sum of constituents (mg/L) 32 0 0 11 0.1267610508 –0.25 34 0 –7.7Transparency, Secchi disk (in.) 36 1 3 11 0.3826629909 1.415 83 0 20.9Coliform, fecal (colonies per 100 mL) 33 20 61 11 A A A A AChlorophyll-a, hplc (μg/L) 34 1 3 11 0.1967038082 –0.1129 2.25 0 –42.4Chlorophyll-b, hplc (μg/L) 35 27 77 11 A A A A ABarium, dissolved (μg/L) 36 1 3 11 0.899098618 0 7 0 0.0Iron, dissolved (μg/L) 36 1 3 11 0.2332613662 –2.406 56 0 –37.6Manganese, dissolved (μg/L) 36 6 17 11 0.0091607158 B 4 (–)*** BStrontium, dissolved (μg/L) 36 1 3 11 0.038358664 –0.6355 38.685 (–)** –16.5Zinc, dissolved (μg/L) 27 11 41 11 0.0453922501 0 6 0 B
142 Water Quality and Trend Analysis of Colorado–Big Thompson System Reservoirs and Related Conveyances Table 27. Trend analysis results for site 4, Shadow Mountain Reservoir, bottom samples. [0, no trend; (+), upward trend; (–), downward trend; significance: *, greater than 90 percent; **, greater than 95 percent; ***, greater than 99 percent; change is percent change in median since the first year of record; A, no trend computation because nondetects greater than 50 percent; B, no slope computation because nondetects greater than 20 percent; mg/L, milligrams per liter; μS/cm, microsiemens per centimeter at 25 degrees Celsius; °C, degree Celsius; μg/L, micrograms per liter] Seasonal Kendall season-edited data Number Censored Percentage censored Period record (years)

p-value Slope Median Significance Change

Oxygen, dissolved (mg/L) 35 0 0 11 0.0044134641 0.2693 5.6 (+)*** 74.4pH, field (standard units) 35 0 0 11 0.254286403 0.01232 7.3 0 1.9Specific conductance (μS/cm at 25°C) 36 0 0 11 0.0003232226 –0.9775 52 (–)*** –18.6Water temperature (°C) -- -- -- -- -- -- -- --Hardness, total (mg/L as CaCO3) 36 0 0 11 0.0023115152 –0.3278 22 (–)*** –15.0

Calcium, dissolved (mg/L) 36 0 0 11 0.0009600972 –0.12 6.8 (–)*** –17.6Magnesium, dissolved (mg/L) 36 0 0 11 0.2914253772 0 1.3 0 0.0Potassium, dissolved (mg/L) 36 0 0 11 0.1815493972 0 0.7 0 0.0Sodium adsorption (ratio) 36 0 0 11 0.8629658527 0 0.2 0 0.0Sodium, dissolved (mg/L) 36 0 0 11 0.0000623096 –0.06004 1.9 (–)*** –29.2Sodium (percent) 36 0 0 11 0.0003709347 –0.2003 16 (–)*** –12.8Acid-neutralizing capacity (mg/L) 36 0 0 11 0.004847576 –0.2988 24 (–)*** –12.7Chloride, dissolved (mg/L) 35 1 3 11 0.1113847049 –0.00999 0.4 0 –23.9Fluoride, dissolved (mg/L) 35 2 6 11 0.9520599211 B 0.1 0 BSilica, dissolved (mg/L) 36 0 0 11 0.6332197237 –0.01821 6.2 0 –3.2Sulfate, dissolved (mg/L) 35 0 0 11 0.000936414 –0.07128 2.9 (–)*** –23.6Nitrogen, ammonia plus organic (mg/L as N) 36 6 17 11 0.0300825763 0 0.25 0 BNitrogen, ammonia (mg/L as N) 36 17 47 11 0.1713121915 B 0.02 0 BNitrite plus nitrate (mg/L as N) 30 20 67 11 A A A A ANitrogen, nitrite (mg/L as N) 36 34 94 11 A A A A APhosphorus, dissolved (mg/L as P) 27 22 81 11 A A A A APhophorus, ortho, dissolved (mg/L as P) 36 33 92 11 A A A A APhosphorus, total (mg/L as P) 36 9 25 11 0.1247838628 B 0.02 0 BResidue, dissolved at 180°C (mg/L) 35 0 0 11 0.0736147125 –0.4997 39 (–)* –13.1Dissolved solids, sum of constituents (mg/L) 32 0 0 11 0.028596739 –0.4938 34 (–)** –14.7Barium, dissolved (μg/L) 36 1 3 11 0.0465907953 0 7 0 0.0Iron, dissolved (μg/L) 36 1 3 11 0.1891830763 –1.917 77 0 –23.8Manganese, dissolved (μg/L) 36 3 8 11 0.0000252162 B 15 (–)*** BStrontium, dissolved (μg/L) 36 1 3 11 0.0002823951 –0.993 39 (–)*** –24.3Zinc, dissolved (μg/L) 27 15 56 11 A A A A A
Hydrologic and Water-Quality Data 143

Table 28. Trend analysis results for site 5, Adams Tunnel east portal.[0, no trend; (+), upward trend; (–), downward trend; significance: *, greater than 90 percent; **, greater than 95 percent; ***, greater than 99 percent; change is percent change in median since the first year of record; A, no trend computation because nondetects greater than 50 percent; B, no slope computation because nondetects greater than 5 percent; ft3/s, cubic feet per second; mg/L, milligrams per liter; μS/cm, 

microsiemens per centimeter at 25 degrees Celsius; °C, degree Celsius; μg/L, micrograms per liter]

Seasonal Kendall 

season-edited data Number Censored Percentage censored Period record (years) p-value Slope Median Significance Change

Instantaneous discharge (ft3/s) 175 0 0 30 0.2191 –0.997 455 0 –6.4

Oxygen, dissolved (mg/L) 176 0 0 30 0.3841 0 8.3 0 0.0pH, field (standard units) 176 0 0 30 0.0000 0.02726 7.5 (+)*** 11.6Specific conductance (μS/cm at 25°C) 165 0 0 30 0.0025 –0.267 50 (–)*** –14.8Water temperature (°C) 179 0 0 30 0.0022 0.02273 5 (+)*** 14.7Hardness, total (mg/L as CaCO3) 141 0 0 24 0.1679 –0.05411 19 0 –6.6

Calcium, dissolved (mg/L) 141 0 0 24 0.2766 –0.01659 5.9 0 –6.5Magnesium, dissolved (mg/L) 141 0 0 24 0.0078 –0.005393 1.1 (–)*** –11.1Potassium, dissolved (mg/L) 139 0 0 24 0.0015 –0.005548 0.7 (–)*** –17.3Sodium adsorption (ratio) 141 0 0 24 0.1300 0 0.2 0 0.0Sodium, dissolved (mg/L) 141 0 0 24 0.0006 –0.01545 1.9 (–)*** –17.7Sodium (percent) 139 0 0 24 0.0066 0 17 0 0.0Acid-neutralizing capacity (mg/L) 141 0 0 24 0.2348 0.04984 21 0 5.9Chloride, dissolved (mg/L) 139 1 1 24 0.0173 0 0.4 0 0.0Fluoride, dissolved (mg/L) 140 14 10 24 0.1741 B 0.1 0 BSilica, dissolved (mg/L) 139 0 0 24 0.0000 0.05004 4.4 (+)*** 31.8Sulfate, dissolved (mg/L) 138 0 0 24 0.0000 –0.1485 3.3 (–)*** –69.1Nitrogen, ammonia plus organic (mg/L as N) 133 32 24 22 0.0000 B 0.3 (–)*** BNitrogen, ammonia (mg/L as N) 67 42 63 11 A A A A ANitrite plus nitrate (mg/L as N) 55 24 44 9 0.0030 0 0.1 0 BNitrogen, nitrite (mg/L as N) 67 60 90 11 A A A A APhosphorus, dissolved (mg/L as P) 116 58 50 20 0.0020 0 0.02 0 BPhophorus, ortho, dissolved (mg/L as P) 67 51 76 11 A A A A APhosphorus, total (mg/L as P) 120 35 29 20 0.0057 0 0.02 0 BResidue, dissolved at 180°C (mg/L) 66 0 0 11 0.7760 0 34 0 0.0Dissolved solids, sum of constituents (mg/L) 131 0 0 24 0.3101 –0.04997 31 0 –3.8Barium, dissolved (μg/L) 67 0 0 11 0.9159 0 6 0 0.0Iron, dissolved (μg/L) 141 0 0 24 0.0000 –1.791 30 (–)*** –82.1Strontium, dissolved (μg/L) 67 0 0 11 0.0000 –1.392 35 (–)*** –35.3Zinc, dissolved (μg/L) 72 26 36 19 0.7935 B 8 0 B
144 Water Quality and Trend Analysis of Colorado–Big Thompson System Reservoirs and Related Conveyances Table 29. Trend analysis results for site 7, Olympus Tunnel, samples. [0, no trend; (+), upward trend; (–), downward trend; significance: *, greater than 90 percent; **, greater than 95 percent; ***, greater than 99 percent; change is percent change in median since the first year of record; A, no trend computation because nondetects greater than 50 percent; B, no slope computation because nondetects greater than 20 percent; mg/L, milligrams per liter; μS/cm, microsiemens per centimeter at 25 degrees Celsius; °C, degree Celsius; μg/L, micrograms per liter] Seasonal Kendall season-edited data Number Censored Percentage censored Period record (years)

p-value Slope Median Significance Change

Oxygen, dissolved (mg/L) 88 0 0 30 0.0402 0.02006 9.1 (+)** 6.8pH, field (standard units) 88 0 0 30 0.0530 0.01051 7.6 (+)* 4.2Specific conductance (μS/cm at 25°C) 81 0 0 30 0.0008 –0.3754 48 (–)*** –20.9Water temperature (°C) 89 0 0 30 0.3111 0 4.5 0 0.0Hardness, total (mg/L as CaCO3) 72 0 0 25 0.0215 –0.1313 17.5 (–)** –17.1

Calcium, dissolved (mg/L) 72 0 0 25 0.0271 –0.03847 5.15 (–)** –17.0Magnesium, dissolved (mg/L) 72 0 0 25 0.0100 –0.007212 1.1 (–)*** –15.1Potassium, dissolved (mg/L) 71 0 0 25 0.0154 0 0.6 0 0.0Sodium adsorption (ratio) 71 0 0 25 0.4704 0 0.2 0 0.0Sodium, dissolved (mg/L) 71 0 0 25 0.3087 –0.00499 2 0 –6.0Sodium (percent) 71 0 0 25 0.1399 0 19 0 0.0Acid-neutralizing capacity (mg/L) 72 0 0 25 0.9084 0 18 0 0.0Chloride, dissolved (mg/L) 72 0 0 25 0.4930 0 0.6 0 0.0Fluoride, dissolved (mg/L) 70 5 7 25 0.4827 B 0.1 0 BSilica, dissolved (mg/L) 72 0 0 25 0.0028 0.03356 4.6 (+)*** 20.2Sulfate, dissolved (mg/L) 71 0 0 25 0.0000 –0.1287 3.4 (–)*** –63.4Nitrogen, ammonia plus organic (mg/L as N) 34 10 29 11 0.3054 B 0.3 0 BNitrogen, ammonia (mg/L as N) 69 15 22 25 0.9754 B 0.04 0 BNitrite plus nitrate (mg/L as N) 74 27 36 25 0.4603 B 0.089 0 BNitrogen, nitrite (mg/L as N) 71 69 97 25 A A A A APhosphorus, dissolved (mg/L as P) 60 29 48 22 0.0000 B 0.01 (–)*** BPhophorus, ortho, dissolved (mg/L as P) 33 27 82 11 A A A A APhosphorus, total (mg/L as P) 68 10 15 25 0.0067 B 0.02 (–)*** BResidue, dissolved at 180°C (mg/L) 31 0 0 11 0.6943 0.1778 33 0 6.1Dissolved solids, sum of constituents (mg/L) 70 0 0 25 0.0137 –0.1671 30 (–)** –13.0Barium, dissolved (μg/L) 31 0 0 11 0.9577 0 6 0 0.0Iron, dissolved (μg/L) 72 0 0 25 0.0050 –1.497 62.5 (–)*** –45.7Manganese, dissolved (μg/L) 53 5 9 18 0.0413 B 4 (–)** BStrontium, dissolved (μg/L) 32 1 3 11 0.0478 –0.7347 28.5 (–)** –24.6Zinc, dissolved (μg/L) 25 10 40 11 0.0257 B 7 (–)** B
Hydrologic and Water-Quality Data 145

Table 30. Trend analysis results for site 8, Carter Reservoir, surface samples.[0, no trend; (+), upward trend; (–), downward trend; significance: *, greater than 90 percent; **, greater than 95 percent; ***, greater than 99 percent; change is percent change in median since the first year of record; A, no trend computation because nondetects greater than 50 percent; B, no slope computation because nondetects greater than 20 percent; mg/L, milligrams per liter; μS/cm, microsiemens per centimeter at 25 degrees Celsius; °C, degree Celsius; in., inch; mL, milliliter; hplc, high-performance liquid chromatography; μg/L, micrograms per liter]

Seasonal Kendall 

season-edited data Number Censored Percentage censored Period record (years) p-value Slope Median Significance Change

Oxygen, dissolved (mg/L) 85 0 0 30 0.1114 –0.008112 7.6 0 –3.2pH, field (standard units) 88 0 0 30 0.0121 0.01414 8 (+)** 5.5Specific conductance (μS/cm at 25°C) 88 0 0 30 0.0000 –0.9957 80 (–)*** –31.3Water temperature (°C) 88 0 0 30 0.9071 0 17 0 0.0Hardness, total (mg/L as CaCO3) 36 0 0 11 0.0002 –0.5723 30 (–)*** –18.8

Calcium, dissolved (mg/L) 36 0 0 11 0.0002 –0.1979 9.8 (–)*** –19.8Magnesium, dissolved (mg/L) 36 0 0 11 0.0001 –0.01417 1.3 (–)*** –11.3Potassium, dissolved (mg/L) 36 0 0 11 0.0370 0 0.7 0 0.0Sodium adsorption (ratio) 36 0 0 11 1.0000 0 0.2 0 0.0Sodium, dissolved (mg/L) 36 0 0 11 0.0000 –0.04227 2.2 (–)*** –19.0Sodium (percent) 36 0 0 11 0.4848 0 13 0 0.0Acid-neutralizing capacity (mg/L) 36 0 0 11 0.0009 –0.499 32 (–)*** –15.7Chloride, dissolved (mg/L) 34 0 0 11 0.0063 0.03313 0.7 (+)*** 72.7Fluoride, dissolved (mg/L) 34 5 15 11 0.3807 B 0.1 0 BSilica, dissolved (mg/L) 36 0 0 11 0.0005 0.1552 2.5 (+)*** 108.8Sulfate, dissolved (mg/L) 34 1 3 11 0.0004 –0.07395 2.9 (–)*** –24.3Nitrogen, ammonia plus organic (mg/L as N) 35 14 40 11 0.0466 0 0.2 0 BNitrogen, ammonia (mg/L as N) 74 31 42 30 0.0215 0 0.03 0 BNitrite plus nitrate (mg/L as N) 77 51 66 29 A A A A APhosphorus, dissolved (mg/L as P) 29 25 86 11 A A A A APhophorus, ortho, dissolved (mg/L as P) 35 31 89 11 A A A A APhosphorus, total (mg/L as P) 72 28 39 29 0.0000 B 0.03 (–)*** BResidue, dissolved at 180°C (mg/L) 79 0 0 30 0.0000 –0.4344 47 (–)*** –24.3Dissolved solids, sum of constituents (mg/L) 33 0 0 11 0.0025 –0.4891 39.5 (–)*** –12.7Transparency, Secchi disk (in.) 72 0 0 30 0.3452 0.4081 96 0 13.7Coliform, fecal (colonies per 100 mL) 56 38 68 24 A A A A AChlorophyll-a, hplc (μg/L) 35 0 0 11 0.1413 0.04964 1.2 0 60.5Barium, dissolved (μg/L) 36 0 0 11 0.0943 –0.2498 23 (–)* –11.2Iron, dissolved (μg/L) 27 8 30 11 0.0030 B 7 (–)*** BManganese, dissolved (μg/L) 27 19 70 11 A A A A AStrontium, dissolved (μg/L) 36 0 0 11 0.0000 –1.116 41 (–)*** –25.7Zinc, dissolved (μg/L) 27 11 41 11 0.6150 B 7.5 0 B
146 Water Quality and Trend Analysis of Colorado–Big Thompson System Reservoirs and Related Conveyances Table 31. Trend analysis results for site 8, Carter Reservoir, bottom samples. [0, no trend; (+), upward trend; (–), downward trend; significance: *, greater than 90 percent; **, greater than 95 percent; ***, greater than 99 percent; change is percent change in median since the first year of record; A, no trend computation because nondetects greater than 50 percent; B, no slope computation because nondetects greater than 20 percent; mg/L, milligrams per liter; μS/cm, microsiemens per centimeter at 25 degrees Celsius; °C, degree Celsius; μg/L, micrograms per liter] Seasonal Kendall season-edited data Number Censored Percentage censored Period record (years)

p-value Slope Median Significance Change

Oxygen, dissolved (mg/L) 79 0 0 30 <.00001 –0.08366 6.9 (–)*** –30.6pH, field (standard units) 74 0 0 30 0.4324 0 7.4 0 0.0Specific conductance (μS/cm at 25°C) 75 0 0 30 <.00001 –0.9488 71 (–)*** –33.2Water temperature (°C) 87 0 0 30 0.0385 0.02798 6.5 (+)** 13.8Hardness, total (mg/L as CaCO3) 36 0 0 11 <.00001 –0.5897 28 (–)*** –20.6

Calcium, dissolved (mg/L) 36 0 0 11 <.00001 –0.2046 8.8 (–)*** –22.4Magnesium, dissolved (mg/L) 36 0 0 11 <.00001 –0.0223 1.3 (–)*** –17.1Potassium, dissolved (mg/L) 36 0 0 11 0.0016 0 0.6 0 0.0Sodium adsorption (ratio) 36 0 0 11 1.0000 0 0.2 0 0.0Sodium, dissolved (mg/L) 36 0 0 11 0.0001 –0.04681 2.2 (–)*** –20.8Sodium (percent) 36 0 0 11 0.7898 0 14 0 0.0Acid-neutralizing capacity (mg/L) 36 0 0 11 <.00001 –0.6231 30 (–)*** –20.3Chloride, dissolved (mg/L) 34 0 0 11 0.0004 0.04391 0.6 (+)*** 143.5Fluoride, dissolved (mg/L) 34 4 12 11 0.8823 B 0.1 0 BSilica, dissolved (mg/L) 36 0 0 11 0.0003 0.1573 3.8 (+)*** 60.6Sulfate, dissolved (mg/L) 33 0 0 11 <.00001 –0.06811 2.9 *** –22.6Nitrogen, ammonia plus organic (mg/L as N) 35 18 51 11 A A A A ANitrogen, ammonia (mg/L as N) 64 16 25 30 0.6375 B 0.03 0 BNitrite plus nitrate (mg/L as N) 34 17 50 17 A A A A APhosphorus, dissolved (mg/L as P) 29 26 90 11 A A A A APhophorus, ortho, dissolved (mg/L as P) 35 32 91 11 A A A A APhosphorus, total (mg/L as P) 61 22 36 29 <.00001 B 0.02 (–)*** BResidue, dissolved at 180°C (mg/L) 64 0 0 30 <.00001 –0.4577 43 (–)*** –27.4Dissolved solids, sum of constituents (mg/L) 33 0 0 11 0.0003 –0.6581 38 (–)*** –17.3Barium, dissolved (μg/L) 36 0 0 11 0.0612 –0.3049 9 (–)* –31.0Iron, dissolved (μg/L) 27 4 15 8 0.0296 B 4 (–)** BManganese, dissolved (μg/L) 27 7 26 8 0.5328 B 9.95 0 BStrontium, dissolved (μg/L) 36 0 0 11 <.00001 –1.161 39.5 (–)*** –27.5Zinc, dissolved (μg/L) 27 10 37 8 0.3872 B 7 0 B
Hydrologic and Water-Quality Data 147

Table 32. Trend analysis results for site 9, Horsetooth Reservoir (Soldier Canyon Dam), surface samples.[0, no trend; (+), upward trend; (–), downward trend; significance: *, greater than 90 percent; **, greater than 95 percent; ***, greater than 99 percent; change is percent change in median since the first year of record; A, no trend computation because nondetects greater than 50 percent; B, no slope computation because nondetects greater than 20 percent; mg/L, milligrams per liter; μS/cm, microsiemens per centimeter at 25 degrees Celsius; °C, degree Celsius; in., inch; mL, milliliter; hplc, high-performance liquid chromatography; μg/L, micrograms per liter]

Seasonal Kendall 

season-edited data Number Censored Percentage censored Period record (years) p-value Slope Median Significance Change

Oxygen, dissolved (mg/L) 86 0 0 30 0.3364 –0.00384 7.8 0 –1.5pH, field (standard units) 86 0 0 30 0.0002 0.02104 7.6 (+)*** 8.7Specific conductance (μS/cm at 25°C) 88 0 0 31 0.0000 –0.6607 75 (–)*** –23.9Water temperature (°C) 88 0 0 31 0.8649 0 15.45 0 0.0Hardness, total (mg/L as CaCO3) 36 0 0 11 0.0100 –0.3969 28.5 (–)*** –14.1

Calcium, dissolved (mg/L) 36 0 0 11 0.0097 –0.1383 9.05 (–)*** –15.4Magnesium, dissolved (mg/L) 36 0 0 11 0.0515 –0.01102 1.5 (–)* –7.7Potassium, dissolved (mg/L) 36 0 0 11 0.0432 0 0.7 0 0.0Sodium adsorption (ratio) 36 0 0 11 1.0000 0 0.2 0 0.0Sodium, dissolved (mg/L) 36 0 0 11 0.0396 –0.01973 2.4 (–)** –8.6Sodium (percent) 36 0 0 11 0.2232 0 15 0 0.0Acid-neutralizing capacity (mg/L) 36 0 0 11 0.0025 –0.4926 29 (–)*** –17.0Chloride, dissolved (mg/L) 34 0 0 11 0.0001 0.0642 0.7 (+)*** 224.3Fluoride, dissolved (mg/L) 34 5 15 11 0.5380 B 0.1 0 BSilica, dissolved (mg/L) 36 0 0 11 0.0417 0.1041 2.75 (+)** 53.9Sulfate, dissolved (mg/L) 3.4 --Nitrogen, ammonia plus organic (mg/L as N) 36 14 39 11 0.0338 0 0.3 0 BNitrogen, ammonia (mg/L as N) 82 39 48 31 0.0348 0 0.04 0 BNitrite plus nitrate (mg/L as N) 81 57 70 30 A A A A ANitrogen, nitrite (mg/L as N) 77 57 74 31 A A A A APhosphorus, dissolved (mg/L as P) 30 28 93 11 A A A A APhophorus, ortho, dissolved (mg/L as P) 36 32 89 11 A A A A APhosphorus, total (mg/L as P) 67 29 43 29 0.0000 B 0.04 (–)*** BResidue, dissolved at 180°C (mg/L) 76 0 0 31 0.0000 –0.5625 50 (–)*** –29.6Dissolved solids, sum of constituents (mg/L) 34 0 0 11 0.0350 –0.4937 39 (–)** –12.9Transparency, Secchi disk (in.) 68 0 0 30 0.0065 1.066 75 (+)*** 54.7Coliform, fecal (colonies per 100 mL) 52 31 60 23 A A A A AChlorophyll-a, hplc (μg/L) 35 0 0 11 0.1716 –0.0784 1.9 0 –36.4Chlorophyll-b, hplc (μg/L) 35 28 80 11 A A A A ABarium, dissolved (μg/L) 36 0 0 11 0.7456 0 18.5 0 0.0Iron, dissolved (μg/L) 43 1 2 19 0.4434 –0.2706 13 0 –32.7Manganese, dissolved (μg/L) 42 15 36 19 0.4685 B 3 0 BStrontium, dissolved (μg/L) 36 0 0 11 0.0001 –1.229 37.5 (–)*** –30.1Zinc, dissolved (μg/L) 27 11 41 8 0.5916 B 5.5 0 B
148 Water Quality and Trend Analysis of Colorado–Big Thompson System Reservoirs and Related Conveyances Table 33. Trend analysis results for site 9, Horsetooth Reservoir (Soldier Canyon Dam), bottom samples. [0, no trend; (+), upward trend; (–), downward trend; significance: *, greater than 90 percent; **, greater than 95 percent; ***, greater than 99 percent; change is percent change in median since the first year of record; A, no trend computation because nondetects greater than 50 percent; B, no slope computation because nondetects greater than 20 percent; mg/L, milligrams per liter; μS/cm, microsiemens per centimeter at 25 degrees Celsius; °C, degree Celsius; μg/L, micrograms per liter] Seasonal Kendall season-edited data Number Censored Percentage censored Period record (years)

p-value Slope Median Significance Change

Oxygen, dissolved (mg/L) 78 0 0 30 0.0010 –0.07019 7.5 (–)*** –24.5pH, field (standard units) 49 0 0 17 0.6109 0 7.2 0 0.0Specific conductance (μS/cm at 25°C) 73 0 0 30 0.0000 –0.4997 73 (–)*** –18.6Water temperature (°C) 84 0 0 30 0.0806 0.0212 7.1 (+)* 9.4Hardness, total (mg/L as CaCO3) 36 0 0 11 0.0001 –0.5048 28.5 (–)*** –17.6

Calcium, dissolved (mg/L) 36 0 0 11 0.0001 –0.1742 8.95 (–)*** –19.2Magnesium, dissolved (mg/L) 36 0 0 11 0.0033 –0.01957 1.5 (–)*** –13.3Potassium, dissolved (mg/L) 36 0 0 11 0.0054 –0.009559 0.7 (–)*** –13.9Sodium adsorption (ratio) 36 0 0 11 1.0000 0 0.2 0 0.0Sodium, dissolved (mg/L) 36 0 0 11 0.0151 –0.02498 2.3 (–)** –11.2Sodium (percent) 36 0 0 11 0.0423 0 15 0 0.0Acid-neutralizing capacity (mg/L) 36 0 0 11 0.0001 –0.6692 29 (–)*** –22.3Chloride, dissolved (mg/L) 34 0 0 11 0.0000 0.07629 0.7 (+)*** 346.4Fluoride, dissolved (mg/L) 34 5 15 11 0.3881 B 0.1 0 BSilica, dissolved (mg/L) 36 0 0 11 0.0876 0.1125 3.65 (+)* 41.6Sulfate, dissolved (mg/L) 34 0 0 11 0.0019 –0.09884 3.2 0 –28.7Nitrogen, ammonia plus organic (mg/L as N) 36 19 53 11 A A A A ANitrogen, ammonia (mg/L as N) 45 20 44 17 0.3865 B 0.04 0 BNitrite plus nitrate (mg/L as N) 39 10 26 17 0.4192 B 0.12 0 BNitrogen, nitrite (mg/L as N) 43 41 95 17 A A A A APhosphorus, dissolved (mg/L as P) 36 25 69 17 A A A A APhophorus, ortho, dissolved (mg/L as P) 36 25 69 11 A A A A APhosphorus, total (mg/L as P) 37 12 32 17 0.2979 B 0.03 0 BResidue, dissolved at 180°C (mg/L) 64 0 0 31 0.0017 –0.2881 48 (–)*** –17.0Dissolved solids, sum of constituents (mg/L) 34 0 0 11 0.0006 –0.4333 38.5 (–)*** –11.6Barium, dissolved (μg/L) 36 0 0 11 0.0875 0.1098 17 (+)* 7.4Iron, dissolved (μg/L) 33 2 6 11 0.4980 B 15 0 BManganese, dissolved (μg/L) 36 19 53 11 A A A A AStrontium, dissolved (μg/L) 36 0 0 11 0.0000 –1.337 40 (–)*** –30.6Zinc, dissolved (μg/L) 27 11 41 11 0.8970 B 6 0 B
Hydrologic and Water-Quality Data 149

Table 34. Trend analysis results for site 10, Horsetooth Reservoir (Spring Canyon Dam), surface samples.[0, no trend; (+), upward trend; (–), downward trend; significance: *, greater than 90 percent; **, greater than 95 percent; ***, greater than 99 percent; change is percent change in median since the first year of record; A, no trend computation because nondetects greater than 50 percent; B, no slope computation because nondetects greater than 20 percent; mg/L, milligrams per liter; μS/cm, microsiemens per centimeter at 25 degrees Celsius; °C, degree Celsius; in., inch; mL, milliliter; hplc, high-performance liquid chromatography; μg/L, micrograms per liter]

Seasonal Kendall 

season-edited data Number Censored Percentage censored Period record (years) p-value Slope Median Significance Change

Oxygen, dissolved (mg/L) 48 0 0 11 1.0620 0 7.6 0 0.0pH, field (standard units) 50 0 0 11 1.8870 0.01251 7.65 0 1.8Specific conductance (μS/cm at 25°C) 50 0 0 11 0.0043 –0.7928 66 (–)*** –12.3Water temperature (°C) 48 0 0 11 0.0597 –0.158 16.65 (–)* –9.9Hardness, total (mg/L as CaCO3) 36 0 0 11 0.0161 –0.3353 28 (–)** –12.3

Calcium, dissolved (mg/L) 36 0 0 11 0.0234 –0.1126 8.95 (–)** –12.9Magnesium, dissolved (mg/L) 36 0 0 11 0.3846 0 1.4 0 0.0Potassium, dissolved (mg/L) 36 0 0 11 0.1143 0 0.6 0 0.0Sodium adsorption (ratio) 36 0 0 11 1.0000 0 0.2 0 0.0Sodium, dissolved (mg/L) 36 0 0 11 0.0243 –0.03251 2.4 (–)** –13.8Sodium (percent) 36 0 0 11 1.1007 0 15 0 0.0Acid-neutralizing capacity (mg/L) 36 0 0 11 0.0546 –0.3529 29 (–)* –12.5Chloride, dissolved (mg/L) 34 0 0 11 1.9996 0.06687 0.8 (+)*** 184.5Fluoride, dissolved (mg/L) 34 6 18 11 0.6458 B 0.1 0 BSilica, dissolved (mg/L) 36 0 0 11 1.9483 0.1376 2.75 (+)* 78.7Sulfate, dissolved (mg/L) 34 0 0 11 0.0688 –0.07948 3.2 (–)* –23.8Nitrogen, ammonia plus organic (mg/L as N) 36 16 44 11 0.0567 0 0.3 0 BNitrogen, ammonia (mg/L as N) 36 21 58 11 A A 0.03 A ANitrite plus nitrate (mg/L as N) 30 26 87 11 A A 0.07 A ANitrogen, nitrite (mg/L as N) 36 35 97 11 A A 0.03 A APhosphorus, dissolved (mg/L as P) 30 29 97 11 A A 0.01 A APhophorus, ortho, dissolved (mg/L as P) 36 34 94 11 A A 0.02 A APhosphorus, total (mg/L as P) 32 20 63 11 A A 0.01 A AResidue, dissolved at 180°C (mg/L) 33 0 0 11 1.3793 0.2486 40 0 7.1Dissolved solids, sum of constituents (mg/L) 34 0 0 11 0.1645 –0.2356 38 0 –6.6Transparency, Secchi disk (in.) 46 0 0 11 1.9991 0 79.5 0 0.0Coliform, fecal (colonies per 100 mL) 32 12 38 11 0.5629 B 2 0 BChlorophyll-a, hplc (μg/L) 35 0 0 11 0.2313 –0.07486 2.1 0 –32.3Chlorophyll-b, hplc (μg/L) 35 24 69 11 A A 0.1 A ABarium, dissolved (μg/L) 36 0 0 11 1.5075 0 18.5 0 0.0Iron, dissolved (μg/L) 36 16 44 11 1.2370 B 21.5 0 BManganese, dissolved (μg/L) 32 11 34 11 1.8638 B 3 0 BStrontium, dissolved (μg/L) 36 0 0 11 0.0000 –1.258 38 (–)*** –30.4Zinc, dissolved (μg/L) 27 20 74 8 A A 14 A A
150 Water Quality and Trend Analysis of Colorado–Big Thompson System Reservoirs and Related Conveyances Table 35. Trend analysis results for site 10, Horsetooth Reservoir (Spring Canyon Dam), bottom samples. [0, no trend; (+), upward trend; (–), downward trend; significance: *, greater than 90 percent; **, greater than 95 percent; ***, greater than 99 percent; change is percent change in median since the first year of record; A, no trend computation because nondetects greater than 50 percent; B, no slope computation because nondetects greater than 20 percent; mg/L, milligrams per liter; μS/cm, microsiemens per centimeter at 25 degrees Celsius; °C, degree Celsius; μg/L, micrograms per liter] Seasonal Kendall season-edited data Number Censored Percentage censored Period record (years)

p-value Slope Median Significance Change

Oxygen, dissolved (mg/L) 48 0 0 11 0.0709 –0.09875 5.2 (–)* –18.8pH, field (standard units) 50 0 0 11 0.1716 –0.00826 7.3 0 –1.2Specific conductance (μS/cm at 25°C) 50 0 0 11 0.0069 –0.8994 70 (–)*** –13.1Water temperature (°C) 48 0 0 11 0.0134 0.06656 7.45 (+)** 10.4Hardness, total (mg/L as CaCO3) 36 0 0 11 0.0002 –0.4982 29 (–)*** –17.1

Calcium, dissolved (mg/L) 36 0 0 11 0.0001 –0.1634 9.1 (–)*** –17.8Magnesium, dissolved (mg/L) 36 0 0 11 0.0070 –0.01736 1.5 (–)*** –11.9Potassium, dissolved (mg/L) 36 0 0 11 0.0239 0 0.7 0 0.0Sodium adsorption (ratio) 36 0 0 11 1.0000 0 0.2 0 0.0Sodium, dissolved (mg/L) 36 0 0 11 0.0759 –0.01957 2.3 (–)* –8.9Sodium (percent) 36 0 0 11 1.9892 0.133 15 (+)** 10.3Acid-neutralizing capacity (mg/L) 36 0 0 11 0.0020 –0.5249 30 (–)*** –17.4Chloride, dissolved (mg/L) 34 0 0 11 0.0000 0.09665 0.7 (+)*** 885.2Fluoride, dissolved (mg/L) 34 4 12 11 0.9048 B 0.1 0 BSilica, dissolved (mg/L) 36 0 0 11 0.0099 0.1971 3.85 (+)*** 81.3Sulfate, dissolved (mg/L) 34 0 0 11 0.0029 –0.09941 3.2 (–)*** –28.8Nitrogen, ammonia plus organic (mg/L as N) 36 20 56 11 A A A A ANitrogen, ammonia (mg/L as N) 36 14 39 11 0.1157 B 0.04 0 BNitrite plus nitrate (mg/L as N) 31 11 35 11 0.0518 B 0.155 (+)* BNitrogen, nitrite (mg/L as N) 36 34 94 11 A A A A APhosphorus, dissolved (mg/L as P) 32 21 66 11 A A A A APhophorus, ortho, dissolved (mg/L as P) 36 19 53 11 A A A A APhosphorus, total (mg/L as P) 34 11 32 11 1.0000 B 0.0295 0 BResidue, dissolved at 180°C (mg/L) 33 0 0 11 0.2788 0.4263 42 0 11.9Dissolved solids, sum of constituents (mg/L) 33 0 0 11 0.0568 –0.2659 40 (–)* –7.0Barium, dissolved (μg/L) 36 1 3 11 0.2815 0 17.5 0 0.0Iron, dissolved (μg/L) 35 3 9 11 0.7407 B 21 0 BManganese, dissolved (μg/L) 35 3 9 11 0.1998 B 13 0 BStrontium, dissolved (μg/L) 35 0 0 11 0.0001 –1.056 40.5 (–)*** –24.8Zinc, dissolved (μg/L) 25 13 52 11 A A A A A