.”

Assessment of Impacts of Glen Canyon Dam Operations

on Water Quality Resources in Lake Powell

and the Colorado River in Grand Canyon

Susan J. Hueftle

and

William S. Vernieu

DRAFT

March 6, 1998

Grand Canyon Monitoring and Research Center

121 E. Birch Avenue, Suite 307

Flagstaff, AZ 8600l-4652

DRAFT Page i of 62
.

I

Acknowledgments

The Grand Canyon Monitoring and Research Center funded this assessment. Monitoring

programs which provided data for the assessment include the Bureau of Reclamation, Glen Canyon

Environmental Studies, the U.S. Geological Survey, and the Grand Canyon Monitoring and

Research Center. We would like to extend specific thanks to the following agencies for their

support: Upper Colorado Region of the USBR, Glen Canyon National Recreational Area, and

Western Area Power Administration. Thanks to Dave Garrett for supporting the Lake Powell

program and this assessment in the face of fiscal and critical scrutiny.

We appreciate the contribution of the three external reviewing limnologists, Drs. Alex J. Home,

Geoffrey Schladow, and David Merritt; and the three intragovernmental reviewing limnologists,

James LaBounty, Jerry Miller and Richard Marzolf for their efforts in evaluating and guiding this

assessment.

Big thanks to the odd assortment of data crunchers of the hysterical data set, the behemoth that

consumed artists to Ph.Ds, interns to accountants; Audrey Roberts, Patrick Smith, Rene Davis,

Clint Morgan, Dawn Huguley, Barb Ralston, Terry Arundel, Val Saylor and Rich Lechleitner. For

assistance in field collections, thanks go to Robert Radtke (Upper Colorado Region, USBR), Kevin

Berghoff (Glen Canyon National Recreation Area), Jeanne Kern, Mike Horn (Technical Service

Center, USBR), and many others going back to 1964. Great thanks go to Margaret Matter,

generously lent to us from WAPA; also Peggy Benenati, Joe Shannon, Mike Yeatts, Glenn

Bennett, LeAnn Skrzynski, Nina Swidler, and Mike Yard for technical assistance, moral support,

brain storming, computer cajoling and coffee supplied to Ms. Hueftle.

Great appreciation to Dave Wegner for his historic and foresighted role in guiding Glen Canyon

Environmental Studies and the Lake Powell program into a comprehensive role in understanding

06/02/98 DRAFT Page ii of 62
upstream/downstream ecosystem interactions in the Grand Canyon and Lake Powell.

This document is dedicated to the memory of Dr. Charles L. Drake, who was one of the leading

scientists in early Lake Powell research. He was a man of great enthusiasm and vision and, with his

cohorts, began a legacy of holistic science. Dr. Drake died in July 1997.

06/02/98 DRAFT Page iii of 62
Abstract

The effects of the construction and operation of Glen Canyon Dam have been studied,

documented, and manipulated in the downstream environment in Grand Canyon. Glen Canyon

Environmental Studies led 13 years of study that focused on addressing and mitigating the adverse

impacts of the Dam on that ecosystem. While data has been collected from Lake Powell throughout

its filling history, there has been no concerted effort to evaluate the impacts of dam operations on

the physical, biological, and chemical processes of the reservoir and downstream releases. Using

data from the 33-year history of water quality monitoring on Lake Powell, primarily from Bureau

of Reclamation efforts, we will begin to demonstrate the effect of dam operations and other factors

on the water quality and hydrodynamics of Lake Powell. Of special importance are the historical

record reflecting the flood years of 1980 to 1986, modified operations of Glen Canyon Dam which

began in 1991, the results from the Experimental Beach/Habitat Building Flood of the spring of

1996, and most recently, the high sustained releases starting in February 1997. No special data

collection was designed to answer the question, but rather, the stock of existing data was analyzed

to provide the answers and to fuel the questions that formed the assessment. The results show that,

combined with other influences, dam operations have an undeniable effect upon the stratification

and mixing of the reservoir, and those effects are consequently passed downstream through the

dam. Not all aspects of dam operations could be answered or analyzed under the scope of the nine

months allowed for the assessment. The experimental flood demonstrated the effects of using

alternate structures for the release of water, in this case, the hollow jet tubes that are positioned 100

feet below the penstock withdrawal ports. The historic record of the 1980’s indicated that the

combination of high and repeated spring floods and high and sustained discharge from penstocks as

well as spillways and hollow jet tubes caused substantial mixing of the reservoir. The recent

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I

spring’s high-sustained releases that were not initially accompanied by high inflow demonstrate the

isolated effects of above average powerplant withdrawal.

06102198 DRAFT Page v of 62
I

Table of Contents

ACKNOWLEDGMENTS ..................................................... . II

ABSTRACT ................................................................ . IV

LIST OF FIGURES ........................................................ . VIII

I. INTRODUCTION .......................................................... . 1

A.SCOPEANDOBJECTIVESOFASSESSMENT ...................................... 1

B.PURPOSEOFASSESSMENT .................................................. 2

C.AFFECTEDRESOURCES .................................................... 4

II. BACKGROUND INFORMATION .......................................... . 5

A.BRIEFHISTORYOFLAKEPOWELLANDGLENCANYONDAMOPERATIONS ............. 5

B.RECLAMATIONMONITORINGPROGRAM ....................................... 7

C.DETAILSOFCURRENTMONITORINGPROGRAM .................................. 9

I. Quarterly Lake Wide Sampling. ............................................ . 9

2. Monthly Forebay Sampling. .............................................. . 10

3. Tailwater Monitoring ................................................... . IO

III. ASSESSMENT PROCESS AND DATA ANALYSIS ........................... . 11

A.DATASOURCES ......................................................... 11

l.LakePowell ........................................................... . 11

2. Glen Canyon Dam Releases .............................................. . 13

B.DATAMANAGEMENT ..................................................... 15

C.DATAANALYSIS ........................................................ 15

IV. RESULTS .............................................................. . 16

A.LAKEPOWELL .......................................................... 16

06102198 DRAFT Pageviof62
1. Introduction to Limnology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16

2. Seasonal Patterns seen at Lake Powell . . . . , . . . . . . . . . . . . . . . . 19

3. Historic Patterns seen at Lake Powell . . . . . . . . . . . . . . . . . . . . . . . . . . 23

B. GLEN CANYON DAM RELEASES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27

1. Historicaltrends . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27

2.SeasonalPattern.s . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29

V. DISCUSSION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30

A. VALUE OF EXISTING AND PAST MONITORING PROGRAMS . . . . . . . . . . . . . . . . . . . . . . . . . 30

B. ASSESSMENT OF FACTORS AFFECTING WATER QUALITY CONDITIONS . . . . . . . . . . . . . . . 3 1

1. Meteorological Conditions . . . . . . . . . . . . . . . . . . . . . . . . . 32

2.InflowHydrology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32

3. Glen Canyon Dam Operations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . , . 34

VI. CONCLUSIONS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 45

A. INFLUENCE OF DAM OPERATIONS ON LAKE WATER QUALITY . . . . . . . . . . . . . . . . . . . . . . . 45

1. Release volumes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . , , . . . . . . . . . . . . 46

2. Timing of Reservoir Releases . . . . . . . . . . . . . . . . . . . . . . . . . . 46

3. Operation of non-power release structures . . . . . . . . . . . , . . . . . . . . . . . . 46

4. Changes in Variability of Downstream Releases . . . . . . . . . . . . . . . . . . . . . . . . . . . . 47

B. INTEGRATION WITH DOWNSTREAM SYSTEMS . . . . . . . . . . . . . . . . . . . . . . . . . : . . . . . . . . 48

1. Grand Canyon Aquatic Ecosystem . . . . . . . . . . . . . . . . . . . . . . . . 48

2.LakeMead............................................................. 4 9

LITERATURE CITED . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 49

APPENDICES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 54

ASSESSMENTPROSPECTUS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 54

06102198 DRAFT Page vii of 62
I

List of Figures

Fig. 1: [Top] Thermal stratification and longitudinal zones of Lake Powell with structural

attributes of Glen Canyon Dam.

[Bottom] Conductivity plot with Lake Powell's circulation cells present for past

and present seasons.

Fig. 2a: Monthly longitudinal plots of one typical year's progression from stratification

of summer through seasonal changes back to summer. Some irregularities may

exist in the data, particularly in the dissolved oxygen. Penstocks and river outlet

levels indicated.

Fig. 2b: [Top] Sequence of 6 conductivity longitudinal isopleths that demonstrate the

most common annual scenario of winter bottom flows.

[Bottom] Example of 3 possible winter underflow currents: Underflow, overflow

and equi-potential flow-through for conductivity, dissolved oxygen, and

temperature.

Fig. 3a: Temperature, conductivity, and dissolved oxygen at the forebay of Lake Powell

from December 1964 to July 1997. Spillways, penstocks, and jet tubes indicated.

Sample sites annotated with dots. Qualitative hydrograph for outflow in blue,

inflow in red, spillways and jet tubes at their elevation.

Fig. 3b: Temperature, conductivity, dissolved oxygen at Wahweap (forebay) station in

Lake Powell from February 1992 to August 13, 1997, including results from

experimental flood (red line). Penstocks indicated at 1057.7 m &jet tubes at

1028.4 m elevation. Hydrograph of inflow is indicated by red line and outflow in

blue.

Fig. 4: Conductivity, temperature, and dissolved oxygen for 1965 and 1966. Before

during and after Lake Mead. Depth of penstocks (1057.7 m) and jet tubes

(1028.4 m) indicated by black lines.

Fig. 5: Longitudinal profiles of Lake Powell for February, March, April and May, 1973.

Drawdown of Lake Powell in anticipation of large spring flood and concerns of

inundation of Rainbow Bridge National Monument. Penstock discharges were

elevated from March 22, 1973 to May 1, 1973. Discharges during elevated

releases averaged 27 K cfs, accounting for 2.29 MAF

06/02/98 DRAFT Page viii of 62
Fig. 6:

Fig. 7:

Fig. 8:

Fig. 9:

The 198O's floods. 22 longitudinal profiles of conductivity during and after the

198O's floods. April 1983 to December 1990. Sample trip resolution decreases

through time.

The 1980's floods. 22 longitudinal profiles of temperature during and after the

198O's floods. April 1983 to December 1990. Sample trip resolution decreases

through time. Penstocks, jet tubes, outflow, inflow, spillway releases indicated

with sample date and station name.

23 longitudinal profiles of conductivity for the 199O's: September 1991 to June

1997, during Spike Flood and spring 1997 high sustained discharge. . Penstocks

& jet tubes indicated.

23 longitudinal profiles of temperature for the 199O's: September 1991 to June

1997, during Spike Flood and spring 1997 high sustained discharge. . Penstocks

&jet tubes indicated.

Fig. 10: 23 longitudinal profiles of dissolved oxygen for the 199O's: September 1991 to

June 1997, during Spike Flood and spring 1997 high sustained discharge. .

Penstocks & jet tubes indicated.

Fig. 11: Historical daily discharges at Lees Ferry.

Fig. 12: Historical daily temperature values at Lees Ferry,

Fig. 13: Historical daily salinity values at Lees Ferry.

Fig. 14: Daily salinity values at Lees Ferry since the closure of Glen Canyon Dam.

Fig. 15: Instantaneous specific conductance, temperature, and discharge below Glen

Canyon Dam.

Fig. 16: Instantaneous specific conductance, temperature below Glen Canyon Dam

showing effects of internal oscillations in Lake Powell during winter months.

Fig. 17: Glen Canyon Dam cross section and location of release structures.

Fig. 18: Research flow schedule during GCES Phase II studies.

Fig. 19: Effects of fluctuating releases on discharge water quality below Glen Canyon

Dam during Research Flow “E”.

M/02/98 DRAFT Page ix of 62
List of Tables

Table 1:

Table 2:

Table 3:

Major Features of Monitoring Program Phases

Lake Powell inflow, outflow, spillway and jet tube operations, and lake

elevations for selected years.

Spike discharges in acre-feet and percentage of total spike discharge

compared to the volume of the lake below those levels.

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I. Introduction

The Grand Canyon Protection Act of 1992 requires the Secretary of the Interior to evaluate the

impacts of Glen Canyon Dam operations on all affected resources. Although the primary

evaluation of these impacts are on resources downstream of the dam, concern has existed that

certain aspects of dam operations have the potential to affect various resource attributes upstream

of Glen Canyon Dam.

In January 1997, the Grand Canyon Monitoring and Research Center (GCMRC) presented to its

Planning and Transition Work Groups, a prospectus for assessing the effects of Glen Canyon

Dam? s operation on water quality resources in Lake Powell and Glen Canyon Dam releases

(Appendix A). This document will serve to define the scope and objectives of this study, and will

form the basis for review of the assessment process and results.

Three experienced reservoir limnologists from outside the federal government not currently

connected with GCMRC have provided critical review. Three federal limnologists provided

additional review from the Bureau of Reclamation and the U.S. Geological Survey. Final results of

the assessment will be reviewed by a Science Advisory Group and presented to the Adaptive

Management and Technical Work Groups.

A. Scope and Objectives of Assessment

This assessment is an effort to integrate existing data from current and past monitoring programs

on Lake Powell in order to evaluate the effects of various aspects of Glen Canyon Dam operations,

from 1965 to 1997, on reservoir and release water quality. Primary consideration will be made to

peaking power generation, operation of non-power release structures, and potential selective

withdrawal. An attempt will be made to identify other factors affecting Lake Powell such as the

existence and structure of Glen Canyon Dam, climatological factors, and internal hydrodynamic

06102198 DRAFT Page 1 of 62
processes, so that these impacts are not inappropriately associated with dam operations. The

assessment will rely mainly on data from the Bureau of Reclamation’s long-term limnological

monitoring program on Lake Powell, the current monitoring program, implemented in 1990 and

currently maintained by GCMRC, and information from other agencies and institutions.

Several factors combine to limit the scope of this assessment. A relatively short time frame was

specified for completion of this study. Budgetary constraints limit the amount of financial and

human resources dedicated to this process. The quality and completeness of data from past

monitoring efforts may be insufficient for certain evaluations and may not have focused on

important affected resources. Some information has not yet been organized into a form that

facilitates analysis and some samples await analysis. Therefore, the scope of this study will be

limited to the analysis of those data which have been 1) consistently collected over a long period of

time; 2) are readily available for computer analysis; and 3) will most likely show the effects of dam

operations on the chemical and physical limnology of the reservoir.

In this context, the term water quality is used to include the various physical, chemical, and

biological attributes that characterize a body of water in terms of its hydrodynamic properties,

chemical composition, and the organisms that live in it. Its use is not intended to connote any value

judgments based on suitability for a given use.

B. Purpose of Assessment

The closure of Glen Canyon Dam in 1963 caused major changes to the physical, chemical and

biological characteristics of the Colorado River in Grand Canyon. These changes are well

documented and include the removal of sediment and the moderation of temperature, salinity and

other chemical extremes. Operation of the dam for peaking power generation resulted in the

removal of seasonal discharge variability and its replacement with daily discharge fluctuations

06102198 DRAFT Page 2 of 62
(Gloss et al. 1981, Ward and Stanford 1983, Stanford and Ward 1986, 1991, Angradi et al. 1992,

Stevens et al. 1997).

Concurrently, changes were also made to the Colorado River as it began to form Lake Powell

upstream of Glen Canyon Dam. The river was slowed and began depositing sediment in the

reservoir basin. Vertical temperature and chemical gradients appeared in the reservoir body due to

seasonal density variations of the inflows, climatic factors, and the mid-depth location of the

power-plant penstocks. Certain aspects of dam operations over the past 33 years are hypothesized to

have impacts to many of the reservoir's resources, especially water quality. Comprehensive

scientific assessments have not been conducted to determine the extent of these impacts.

Current and past monitoring programs on Lake Powell have been designed with fairly broad

perspectives in order to understand more about processes that affect various resource areas of

concern. Understanding and monitoring salinity trends and patterns in the Colorado River and its

reservoirs has long been important to maintaining adequate quality for downstream uses under

future water development conditions and has been a primary focus of Reclamation's long-term

monitoring program. Chemical changes occurring in a filling reservoir have implications to

maintaining fisheries and supplying nutrients to downstream environments. More recently it has

become important to understand the processes in Lake Powell that determine the physical,

chemical, and biological characteristics of downstream releases to the Grand Canyon ecosystem.

This was a primary objective for the revision of the Reclamation ‘s Lake Powell monitoring by the

Glen Canyon Environmental Studies office in 1990. Evaluation of a potential selective withdrawal

structure on Glen Canyon Dam will require an understanding of hydrodynamics, warming

processes, thermal budgets, and biological effects upstream and downstream of Glen Canyon Dam.

The effects of planned or unforeseen operational changes at Glen Canyon Dam may have far

reaching effects on varied physical, biological, and cultural resources in the Grand Canyon and on

06/02/98 DRAFT Page 3 of 62
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Lake Powell.

The challenge of any long-term monitoring program is to collect data focused on addressing

issues of current concern while being broad based and of sufficient quality to answer questions

which may arise in the future while maintaining a reasonable expenditure of resources. It is our

belief that data from past and existing monitoring programs on Lake Powell can be evaluated to

identify effects of various aspects of operation of Glen Canyon Dam and, at the same time, provide

valuable information to scientists and managers from a broad variety of resource areas. By

balancing information needs, resource expenditures, and monitoring program objectives, a valuable

evaluation tool for future adaptive management may be maintained.

C. Affected Resources

Many processes in Lake Powell are influenced by factors not directly related to dam operations

such as inflow hydrodynamics, climatological conditions, and the existence and structure of Glen

Canyon Dam. However, some water quality attributes of Lake Powell and downstream releases

may be affected by certain aspects of the operation of Glen Canyon Dam. These effects can be

evaluated from data developed in existing monitoring programs.

Three main interlinked resource categories may be affected by the operation of Glen Canyon

Dam and other factors. Physical and chemical conditions in Lake Powell address evaporative water

loss, temperature regime and heat budget, salinity levels, hydrodynamics and mixing patterns,

nutrient and trace element concentrations, and sediment deposition. These characteristics, in turn,

influence the biological resources of Lake Powell and the Colorado River below the dam. Affected

biological components may include primary productivity, algal and zooplankton abundance and

composition, and the dynamics of fish populations, waterfowl, and higher species. The third

affected resource category involves social and economic components such as power production,

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water delivery, cultural and historic resources, recreation and public health.

While direct linkages exist among these resource categories, identification and evaluation of

effects to all these resource categories is impossible within the current time constraints, budget

setting, and data limitations. The scope of this assessment will therefore mainly focus on the

various physical and chemical water quality attributes associated with Glen Canyon Dam

operations for which information has been or is currently being gathered.

II. Background Information

Concurrent with historical changes in dam operations and reservoir conditions, Reclamation has

maintained a water quality monitoring program on Lake Powell since 1965. By associating the

monitoring effort with historical dam operations, increased understanding can be gained of effects

of dam operations on reservoir resources.

A. Brief History of Lake Powell and G/en Canyon Dam Operations

Lake Powell has had a relatively short existence as an operating reservoir. Its history can be

described in terms of three major periods in Glen Canyon Dam operations.

1963-1980. The seventeen-year period from 1963 to 1980 resulted in the eventual filling of Lake

Powell to its normal pool elevation of 3700 ft. With minor exceptions, this period was

characterized by constantly increasing reservoir elevations, increasing depth of the penstock

withdrawal zone, and continual inundation of new areas of the reservoir basin. Stable stratification

patterns in temperature and salinity developed from the constant withdrawal at the penstock level.

1980-1990. Relatively full reservoir levels characterize the period from 1980 to 1990. A

succession of high runoff years in the early and mid 1980’s brought the reservoir 8 feet above its

normal pool level in July of 1983. Because of the need for increased releases from Glen Canyon

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Dam, the spillway structures and hollow jet bypass tubes were operated on several occasions. This

allowed significant amounts of water to be released from levels above and below the penstock

zone. These factors combined to cause nearly complete mixing of the reservoir in 1985, due to the

high volume of reservoir throughput and the operation of the alternative release structures. In the

late 1980’s, drought conditions returned to the upper basin and resulted in decreasing reservoir

levels and the return of strong chemical stratification below the penstock level.

1990-1997 The period from 1990 to 1997 was marked by a series of manipulations to the

operation of Glen Canyon Dam for scientific and environmental purposes. Before this time, the

dam was operated primarily for peaking power generation and water delivery to the Lower Basin

States. In 1990, Phase II of the Glen Canyon Environmental Studies and the development of the

Glen Canyon Dam EIS began. As part of the GCES Phase II Integrated Research Plan (USBR

1990), a series of research flows was initiated from June 1990 to August 1991. These flows ranged

widely in daily fluctuations and ramping rates, interspersed with periods of steady flow. In

November 1991, following the research flow period, the Secretary of the Interior implemented the

Interim Operation Criteria, which set limits on minimum and maximum discharge, daily range of

discharge, and hourly ramping rates. These criteria remained in place until October 1996, when the

Secretary signed the Record of Decision for the preferred alternative of the Glen Canyon DAM EIS

(U.S. Bureau of Reclamation 1995 and 1996).

Of significance during this latter period was the experimental beach/habitat building flow in

March and April 1996. This 7-day discharge of 45,000 cfs included a release of 15,000 cfs from the

river outlet works of Glen Canyon Dam. The operation of this structure released water from 100

feet below the penstock withdrawal zone and weakened the strong chemical stratification that had

previously built up below that level.

In February 1997, increases in Upper Colorado River Basin runoff forecasts prompted an

06/02/98 DRAFT Page 6 of 62
increase in Glen Canyon Dam releases that were sustained at levels above 20,000 cfs for the

remainder of the summer months. This represented a different operational pattern to handle the

forecasted runoff than in 1983, which experienced similar hydrologic conditions.

B. Reclamation Monitoring Program

Table 1. Major Features of Monitoring Program Phases

Frequency:

number of

stations

Parameters

chemistry

sampling

biological

sampling

inflow

monitoring

tailwater

Phase 1 1 Phase 2 1 Phase 3 Phase 4

8 8 8-10

Temp.

DO (Winkler)

Major Ions

50 f-t

Temp.

DO (Meter)

Major Ions

50 ft

Multiprobe

profiling (T, SC,

Major Ions

(Shipboard

50 ft

none none

none none

qualitative

olankton

selected sites

chlorophyll

phytoplankton

selected sites

none none below dam below dam

15-20

Multiprobe profiling

with datalogger

Major Ions

Nutrients

Variable

The Bureau of Reclamation initiated a water quality monitoring program on Lake Powell in 1964

to gather information on initial water quality conditions and to observe changes as the reservoir

filled and matured. This program has continued to the present. Based on sampling frequency,

spatial resolution of measurements, and changes in instrumentation, four distinct phases of

monitoring activity can be identified (Table II-l).

From 1965 to 1971, monthly sampling of the Glen Canyon Dam forebay and quarterly surveys of

the entire reservoir for temperature and salinity characterized monitoring activity. Measurements

and samples were collected at 50-foot depth intervals at seven locations on the reservoir.

06102198 DRAFT Page 7 of 62
From 1972 to 1981, the frequency of lake-wide surveys was increased to a monthly basis.

From 1982 to 1990, sampling activity steadily declined to single lake-wide surveys in 1988 and

1989. Despite the decline in sampling frequency, advances in instrumentation allowed the

collection of higher quality data at finer depth resolution. Continuous monitoring of temperature

and salinity of the tailwater was initiated during this period.

In 1990, concurrent with the implementation of GCES Phase II studies, Reclamation's Lake

Powell monitoring program was restructured. Monitoring frequency was returned to a level of

monthly forebay surveys and quarterly lake-wide surveys. During the Phase II Research Flow

period from 1990 to 1991, the monthly forebay surveys were conducted by the US Geological

Survey (Hart and Sherman, 1995). This restructuring resulted in a redistribution of resources to

allow the collection of data at a finer spatial resolution while reducing the number of samples

collected for chemical analysis. Sampling for nutrient chemistry and biological conditions was also

initiated. The objective of this phase of monitoring activity was to establish a program of basic data

collection that would balance cost with the ability to track changes in reservoir and release water

quality and evaluate the effects of Glen Canyon Dam operation on these resources

Various agencies and institutions during Lake Powell's history have conducted other work.

Studies have been conducted by educational consortiums and federal and state agencies on subjects

that include sedimentation, circulation patterns, trace element chemistry, remote sensing, and

public health issues (Potter and Drake, 1989).

C. Details of Current Monitoring Program

The current monitoring program was initiated in 1990 in response to the need to understand how

physical and chemical processes in Lake Powell and the operation of Glen Canyon Dam influence

the quality of water released to Colorado River in Grand Canyon. Based on limited personnel and

06102198 DRAFT Page 8 of 62
financial resources, efforts were made to incorporate existing technology to improve the overall

quality and resolution of measurements taken, eliminate unnecessary activities, and automate

routine data collection tasks. This program balances a broad based, high quality, data collection

program with limited resources.

1. Quarterly Lake Wide Sampling

Based on characteristic seasonal patterns and conditions, lake-wide sampling is conducted on

Lake Powell on a quarterly basis. Efforts are made to describe the physical and chemical conditions

of the major strata of the reservoir in the main Colorado River channel and the major tributary arms

of the San Juan and Escalante Rivers.

Sampling takes place over a week-long period and consists of measurements and chemical

samples collected at 20-25 established stations along the main channel and major tributary arms.

After initial surface observations are made, a profile of the physical parameters of temperature,

specific conductance, dissolved oxygen, pH, redox potential, and turbidity throughout the water

column is collected. This provides details of the density stratification patterns separating the

significant layers of the reservoir, location of inflow currents, dissolved oxygen patterns and

overall chemical conditions in the reservoir. Results are recorded on portable data logging

equipment for immediate viewing in the field and automated transfer to data management systems.

Based on the stratification patterns seen from the physical profile, depths for discrete chemical

sampling are determined, with the objective of characterizing the major ion and nutrient content of

the significant layers of the reservoir. Sample processing is performed on shipboard for later

analysis at a remote laboratory. Alkalinity titrations of these samples are also performed on

shipboard.

Biological sampling consists of chlorophyll sampling of surface samples, collection of discrete

06102198 DRAFT Page 9 of 62
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samples for phytoplankton and vertical tows for zooplankton.

2. Monthly Forebay Sampling

Monitoring of the forebay of Lake Powell is performed monthly at the long-term Wahweap

station. This site has been sampled monitored throughout Lake Powell’s history and is located in

the main channel at the confluence with Wahweap Bay, 2.4 channel kilometers upstream of Glen

Canyon Dam. Chemical and biological sampling similar to that for major stations on quarterly

surveys is also performed.

3. Tailwater Monitoring

Continuous water quality data collection is maintained at three locations in the Glen Canyon

Dam tailwater. The first and most long-term running station is a perforated pipe below the river

outlet works (a.k.a., hollow jet valves), immediately downstream of the Glen Canyon Dam

powerplant on the left wall. This site has been in operation since August 1980 measuring

temperature and specific conductance at intervals of two hours or less. Dissolved oxygen

monitoring was initiated November 1990; pH measurements were started July 1995.

A second station is near the USGS Lees Ferry stream gage (09380000), in operation since

October 199 1, recording temperature, specific conductance, and dissolved oxygen. pH

measurements were added August 1996.

Ill. Assessment Process and Data Analysis

A. Data sources

The assessment process consisted of integrating water quality data from various monitoring

phases to describe historical and seasonal patterns and trends in the water quality of the mainchannel reservoir body and the Glen Canyon Dam forebay, identify seasonal and long term

06102198 DRAFT Page 10 of 62
variability, and describe unusual conditions associated with reservoir operations or other factors.

Water quality data is present in the form of surface observations from a site visit, profiles of

physical parameters through the water column, and the results of laboratory analyses of chemical

samples. Hydrologic data was acquired to build a database of historical inflows, reservoir contents

and surface elevation, Glen Canyon Dam power-plant releases, and non-power releases from

alternate outlets on Glen Canyon Dam to associate water quality changes with hydrologic or

operation patterns of the dam.

1. Lake Powell

a. Physical Profiles

A large part of the assessment analysis is based on the evaluation of the profiles of physical

parameters collected on lake-wide surveys at each established station and monthly at the Wahweap

forebay station. These profiles provide fine detail of changes through the water column. The

increase to the number of main channel stations and the monthly frequency of forebay sampling

provides adequate longitudinal and temporal resolution, respectively, for the purpose of describing

vertical density gradients, longitudinal reservoir gradients, and the seasonal and temporal variation

seen in these patterns.

Temperature patterns lend information to warming processes, thermal content of the reservoir,

and density characteristics. Consistent temperature has been regularly collected through the

reservoir since monitoring began in 1964

Specific conductance is an indicator of a solution's ability to conduct electricity, a function of

the amount of total dissolved substances (TDS) in solution. It is much more readily determined in

the field than a laboratory analysis for TDS and is therefore used as an indirect measure of salinity.

As such, specific conductance measurements give an indication of the chemical makeup of a parcel

06102198 DRAFT Page 11 of 62
I

of water and can be used to identify its origin and density characteristics. Field measurements of

specific conductance were not taken during most of the 1970' s. Where absent, they were replaced

with lab-measured values.

Dissolved oxygen measurements can give an indication of biological and physical processes at

work in the reservoir, the amount of organic material carried by a water parcel, and its degree of

atmospheric exposure. Oxygen is produced as a byproduct of photosynthetic activity and is

consumed by respiratory and decomposition processes. High oxygen concentrations are therefore

seen near the surface in the early summer when photosynthetic activities are high. Oxygen

concentrations may be depressed at density gradient boundaries or in the deeper portions of the

reservoir due to buildup, at these locations, of autochthonous or allochthonous organic material

subject to bacterial decomposition.

b. Chemical Samples

The collection and analysis of chemical samples is valuable for determining the chemical

composition of the major ionic constituents that comprise the dissolved substances in a body of

water. This information may be used to determine a fingerprint of the water to identify its source,

the degree of saturation of a particular mineral, or suitability for a given use, such as irrigation.

Chemical analysis is frequently the only way to determine the quantity of certain substances in

solution, such as nutrient compounds.

C. Biological Samples

Biological monitoring in Lake Powell was initiated in 1990 to attempt to establish a link between

chemical and physical characteristics and higher trophic levels. The analysis of these samples has

not been fully completed. Furthermore, most biological activity in Lake Powell that would be

identified by the analysis of zooplankton and phytoplankton samples takes place near the surface

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and is probably less effected by normal dam operations than other chemical and physical indicators.

Therefore, although these samples may prove valuable in identifying water quality effects and

trophic linkages, their analysis has not been incorporated into this assessment.

2. Glen Canyon Dam Releases

a. Hydrologic Data

Data to quantify the amount of water discharged from Glen Canyon Dam comes from two main

sources. The most immediate source is powerplant releases from Glen Canyon Dam. These data are

the result of power-plant hydropower generation, integrated on an hourly period, and converted to

discharge in cubic feet per second (cfs) depending on generator rating curves and power-plant head.

These data form the basis for meeting the operational constraints set by the Glen Canyon Dam EIS

and Record of Decision. Data exist from 1965 to the present and separate powerplant release from

those of alternate spillway and river outlet works structures (U. S. Bureau of Reclamation, 192l1997).

The other source of hydrologic data is the U.S. Geological Survey (USGS) stream gage, Colorado

River at Lees Ferry, AZ (09380000). Discharge data is obtained every half hour from satellite

telemetered stream gage measurements applied to a discharge rating curve. These unit value data

begin in January 1985 and continue through the present. Daily values exist for the Lees Ferry gage

from October 1921 and are used to represent pre-dam conditions in the Colorado River (U.S.

Geological Survey, 1985-1996)

b. Time Series of Physical Parameters

The varying quality of Glen Canyon Dam releases were studied using data from two sources. The

USGS has maintained water quality monitoring for salinity and specific conductance at the

Colorado River at Lees Ferry gage (09380000) since 1944. These data exist in USGS daily values

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I

I

tables. Prior to 1977 single daily measurements for specific conductance were made (daily values

statistic code 00011). Since that time, specific conductance has been measured at specific intervals

with automated mini-monitor instrumentation; mean daily values are calculated from these data

(daily values statistic code (00003).

Reclamation began its water quality monitoring of the Glen Canyon Dam tailwater as part of

Phase I of the Glen Canyon Environmental Studies program in August 1988. This program was

begun with measurements of temperature and specific conductance at bi-hourly intervals. As

instrumentation improved, measurement of dissolved oxygen was added in June 1991; pH

measurements began in July 1995. Monitoring at Lees Ferry was initiated in October 1991. This

monitoring has been conducted with various versions of the Hydrolab Corporation's Datasonde

and Recorder submersible multi-parameter data logging instrumentation (Hydrolab, 1994). These

monitors are serviced on a monthly basis. The Grand Canyon Monitoring and Research Center

(GCMRC) currently maintains this program.

C. Chemical Analysis

The Colorado River at Lees Ferry gage (09380000) gage is maintained as part of the USGS

National Stream-Quality Accounting Network. Analysis of a broad range of water quality

parameters and contaminants is conducted on samples collected on a bimonthly basis.

As part of the monthly servicing of the tailwater instrumentation maintained by GCMRC, samples

are collected and analyzed for major ionic constituents and nutrient concentration and analyzed

according to standardized procedures (APHA, 1992). Biological samples similar to those collected

at reservoir stations are also collected.

6. Data management

Prior to the mid-1980s all data was stored as hard copy records in binders. Over time, more data

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can be generated and stored electronically, but a large quantity of historical data has remained in

hard copy form. During the past seven years, efforts have been made to enter these data into

electronic formats to facilitate data management and analysis. Recently a large project involving

entry of the past 34 years of chemical analyses was completed.

A relational database was designed and implemented in the mid-1980s to store and manage data

from physical profiles and laboratory analyses. This database has under gone several changes and

refinements. Currently available data is now served from an Ingres database management system.

Data is transported from this system to SAS software (SAS Institute Inc., 1996) for statistical

analysis, summarization, and graphical representation. Work is currently in progress to move this

database to the Grand Canyon Monitoring and Research Center’s Information Technology program

under the Oracle database management system.

C. Data Analysis

Three-dimensional analysis for depicting longitudinal changes through the reservoir and temporal

changes in the forebay was done with Surfer and Grapher software (Golden Software, 1996 and

1994), which performs interpolation, gridding, and contouring of three dimension data. With this

software, images may be generated which depict changes in temperature, for example, on a twodimensional framework of depth through the water column and distance from the dam, giving, in

effect, a cross-sectional view of water quality conditions throughout the reservoir body. This is also

used to display changes in a given parameter with depth at the forebay station over a sequence of

time. These three-dimensional analyses form the primary basis for describing water quality patterns

in the reservoir.

IV. Results

A. Lake Powell

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I

1. Introduction to Limnology

Interpreting the trends and history of Lake Powell’s requires the grasp of some basic limnological

principles as well as some specifics peculiar to Lake Powell (Wetzel 1975, Cole 1994). There are a

few components that drive the seasonal and yearly patterns of the lake; resulting in fairly

predictable horizontal and longitudinal stratification (Figs. 1,2a, 2b, 3a and 3b; Merritt and Johnson

1979, Gloss et al. 1980, Edinger et al. 1984, Stanford and Ward 1986, Potter and Drake, 1989,

Thorton et al. 1990).

It is important to first become familiar with the most common and pertinent limnological

terminology, then to build an understanding of the basic processes that drives the conditions found

in the reservoir. Only then can valid conclusions be drawn that differentiate inflow processes from

discharge dynamics.

a. Stratification:

4 Vertical Stratification

Lakes exhibit vertical stratification based on density gradients. (Fig. 1)

Epilimnion: The surface layer of the lake characterized by the least dense water resulting from

warmer temperatures and lower conductance. It is the most biologically active portion of the lake

due to light availability and higher oxygen concentrations.

Metalimnion: A boundary or steep gradient between the epilimnion and the hypolimnion.

Hvpolimnion: The dense, cold, saline water at the bottom of the lake. It is fairly stable and

resistant to mixing. It is dark and has low oxygen levels; consequently bacteria tend to be the

dominant life form.

Thermocline: A thermal boundary or gradient between water masses of different temperature.

Chemocline: A chemical boundary or gradient between water masses of different salinity.

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Meromictic: A condition of persistent high salinity in the hypolimnion that resists mixing

because of a strong chemocline.

Hypoxia: Low levels of oxygen associated with bacteriological respiration and chemical

reduction.

Anoxia: The absence of measurable oxygen in a water body. 

2) Horizontal Stratification

Horizontal or longitudinal stratification gradients are generally a reflection of the distance from

the riverine inflow.

Riverine conditions dominant in the reach where the river is still flowing within a channel even

though it may be just below the lake level. This condition consistently exists within the Cataract

Canyon reach.

A Transition zone occurs as the river conditions merge with the typical deep lake conditions.

Lacustrine conditions are characterized by pronounced vertical stratification as described above.

h. Hydrodynamic processes

Hydrodynamic processes are those that drive the stratification and mixing of the lake and include:

Density: The property of denser masses to sink is key to lake dynamics. Temperature and

dissolved chemicals are the fundamental components that determine density differences. Warmer

water is less dense than cold water (down to 4EC), and pure or “fresh” water is less dense than

water with many dissolved chemicals (reflected by measurements of electrical conductivity or

salinity--terms used here interchangeably).

Advective Flow: A lateral or pushing current, typically horizontal, driven by the momentum of

an inflowing current.

Convective circulation: As stratified water cools in the fall, the surface becomes denser than the

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underlying layers, eventually displacing it as the warmer water rises. This vertical density driven

mixing is enhanced by wind action. The depth of convective mixing is dependent on the volume of

the spring flood, as well as on the coldness of the winter. Sufficiently deep or aggressive mixing

may penetrate metalimnion.

Density currents: A water mass that seeks its own level based on differences in density from

adjoining water masses. Its movement also involves an advective component from river inflow.

This includes the bottom hugging winter flows in Lake Powell.

Withdrawal currents: The current patterns established in a lake due to the operations of

withdrawal ports in a dam. In general, increases in discharge result in a 3rd power increase in kinetic

energy available for mixing, as KE-%-Q3 (Thorton et al. 1990); this extends the vertical draw of the

outlets. Hence, the increase from a discharge of 5000 cfs to 30,000 cfs increases mixing and

destratification by 216-fold, while total discharge only increased by 6-fold.

Wind Mixing: The vertical mixing of surface layers of a lake due to wind shear. Its effect is

deepest when combined with convective mixing on a homogeneous mass than on a strongly

stratified water body.

Diffusion, Dispersion and Entrainment: Diffusion is the passive mixing of water across a

concentration gradient. Dispersion is the active mixing of water across a concentration gradient

involving advective movement. Entrainment also involves advective movement that incorporates

water from adjoining masses of water.

2. Seasonal Patterns seen at Lake Powell

All the above properties come to bear on the mixing and stratification of Lake Powell’s waters.

Traditional categories of epilimnion, metalimnion and hypolimnion tend to oversimplify the

stratification of Lake Powell, which can have up to 4 or 5 chemoclines and several thermoclines

06102198 DRAFT Page 18 of 62
separating various flows. Differences in stratification occur in a temporal dimension as well as

vertical and longitudinal. The following describes the dominant seasonal inflows and circulation

cells of Lake Powell (bottom panel, Fig 1). Timing of these events can typically vary a month or

more in either direction. Isopleths for temperature, conductivity and dissolved oxygen for 1975 and

1976 (Figs. 2a and 2b) show a sequence of generally typical snapshots of water quality, as do the

isopleths of the longitudinal profiles (Figs. 4- 10).

a. Surface Processes:

Spring Flood: This is the yearly dominating lake event, dictated by the magnitude and timing of

the spring headwater snow-melt coming down Cataract Canyon and the San Juan River and into

Lake Powell from May to July. These waters are typically fresh (lower conductance/ salinity) and

initiate the warming processes in the lake since the rivers are the first to warm. The spring freshet

may begin injecting at the surface, but the bulk of the flood drops 10 to 20 meters below the surface

of the lake as spring progresses and the lake surface warming exceeds river temperatures. Though

not as warm as the lake’s surface, the volume of relatively warm flood water will dictate the

thickness of the epilimnion.

The fresh waters of the typical spring flood reach the dam by July as the last of the snow-melt

enters the reservoir. A small flood moves toward the dam more slowly than a large flood and its

signature may not be seen at the dam before it is dissipated by convective mixing in the fall.

Although riverine waters are well oxygenated, the organic and chemical nutrients within the flood

waters place a high oxygen demand that results in a precipitous drop in oxygen levels beginning in

July and continuing through fall. The trace of this dissolved oxygen sag, superimposed and just

below the flood plume, denotes the settling of detritus that fuels the hypoxic (or low oxygen

concentration) cycle.

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Convectively Mixed Epilimnion: Autumn surface cooling and convective mixing begins lakewide in September and reaches it deepest extent around January. The previous spring’s flood is

mixed with past years’ floods and may entrain the top of the hypolimnion; the conductance of the

convectively mixed epilimnion is a reflection of these layers. Almost any size flood will

significantly freshen this layer, but larger floods produce greater freshening. It is likely each mixing

event is entraining floods or the convectively mixed epilimnion from two or more previous years.

Not only does a large flood result in a much fresher convection cell, but, again, it also produces a

thicker and warmer body of water. The cooler temperatures and the mixing process result in

progressively elevated dissolved oxygen levels in this layer as winter progresses. This results from

the higher oxygen-carrying capacity of cold water as well as algal productivity that thrives on the

mixed and composted flood waters. As winter progresses into spring, oxygen levels increase,

typically exceeding super-saturation from February to May.

b. Bottom Processes:

In the winter, cold temperatures combine with advective currents from the inflows to produce

bottom hugging density currents. In Lake Powell these cold bottom plumes repeatedly divide into

three distinct masses depending on the time and meteorological conditions under which they form.

These bottom plumes are critical to the long term chemical and physical conditions found in the

hypolimnion (Edinger et al. 1984, Merritt and Johnson 1978, Johnson and Merritt 1979, Gloss et al.

1980, Stanford and Ward 1991). One year's events can sometimes be tracked for years if mixing is

unable to penetrate the hypolimnion.

Late Summer/ Fall SWARM (Saline WARM water): In late summer /early fall the river inflow

has become very warm and saline due to the reduced hydrograph, irrigation return flows and greater

solubility of salts in the geologic stratum of the watershed. Initially this flow intrudes into the

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spring flood below the lake’s surface, but as fall progresses, this mass becomes colder and

eventually sinks. The late summer salinity plume is convectively mixed with the colder bottom

hugging flow; which we have dubbed the fall SWARM bottom flow because of it’s high Salinity

and relative WARMTH (compared to other bottom layers). Oxygen levels are typically low due to

high oxygen demand from the inflow. This cell of water is the greatest contributor of salinity to the

hypolimnion on an annual basis. As the SWARM plug flows downlake, it incorporates-- through

diffusion, dispersion and entrainment --the previous winter’s underflows that have remained

relatively stagnant since the spring flood first appeared. When this saline mass of water reaches the

dam (between February and March), it contributes to the state of meromixis if of sufficient salinity

to resist subsequent mixing.

Deep Winter FRESCO: During the deepest cold of winter, convective mixing is most extensive.

In the inflow area, the winter river inflow is at its coldest, most oxygen saturated, and saline. This

cold saline winter inflow is mixed (convective mixing and entrainment) with the last of the spring

flood near the inflow. A relatively fresh plug of the lake’s Coldest most oxygenated water results.

The FRESCO bottom flow follows the swarm plug down the bottom of the lake, typically set in

motion in January and February.

The FRESCO'S downlake momentum will eventually reach the SWARM which has made contact

with the dam. The momentum of the FRESCO rocks the hypolimnion like a lever, moving the

uplake chemocline downward as the chemocline at the dam moves upward. It is at this time that the

highest conductivity, lowest temperature and lowest dissolved oxygen levels are discharged from

the penstocks, as seen in Lees Ferry water quality (Figs. 12-15). At this point uplake, the fresh cold

bottom plug typically has three options. 1) Hvpolimnetic Overflow: If the FRESCO flow is not

sufficiently cold or the chemical gradient between it and the meromictic salty warm bottom plug is

too great, its momentum will carry it over the hypolimnion where it is entrained by the penstock

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withdrawal plume suction from dam releases pull it up toward the penstocks. The penstocks will

continue to draw on this layer (as well as others) until the following winter when the next SWARM

reaches the dam. This is the most common scenario, occurring 14 of the past 33 years. 2)

Hvpolimnetic Underflow: If it is sufficiently cold and dense, the FRESCO will continue downlake

on the bottom, displacing the hypolimnion upward toward the penstocks and perhaps not stopping

until it reaches the dam if of sufficient magnitude. An underflow that reaches the dam is the most

efficient process for removing meromixis and restoring oxygen to the hypolimnion without nonpowerplant releases. This is also the least common scenario, occurring only 6 times since Powell’s

filling (1973, 1983, 1984, etc.). 3) Hvpolimnetic interflow: Several factors can contribute to this

process. If the advective forces are removed from the bottom currents through diversion to surface

flows in the spring, the bottom currents stall and the plumes substantially slow or cease to move

toward the dam. The discharge from the dam may continue to pull the FRESCO through the

SWARM at a depth that corresponds to the relative density and momentum of the respective flows.

Furthermore, if the chemical or thermal gradients are insubstantial, the FRESCO and SWARM

plugs may mix through diffusion and dispersion. This interflow is more common-occurring

approximately 12 of the past 33 years. The amount of meromictic removal is a function of the depth

of the interflow.

Late Winter SCOOL: The final of the three bottom hugging winter plumes, the late winter

SCOOL plug has higher salinity concentrations than FRESCO but is not as cold. There are less of

the previous spring’s floodwaters left to convectively mix with the saline but warming riverine

water. This Saline and COOL flow forms around March and follows the FRESCO plume down the

lake’s bottom. Its progress is typically stalled by April or May when warming and the advective

flow forces are diverted to the spring flood at the surface. Organic and chemical oxygen demands,

low light availability, and prolonged stagnation keep oxygen levels low in this cell. It typically does

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not move downlake until the following winter when the SWARM’s bottom hugging flow will

entrain it as its flows to the dam. An animated sequence of the longitudinal salinity profiles for

Lake Powell since 1965 can be found at website www.usbr.gov/gces. This demonstrates the inflow

and outflow dynamics under various tilling, drought and flood cycles.

C. Side Channel influences

While the most of the side channels have not been examined throughout Lake Powell’s history,

the San Juan and Escalante Rivers have been received some attention, primarily with the instigation

of the GCES program. The Escalante River contributes only -5% of the total inflow to Lake

Powell, yet produces a regular and pronounced effect on the main channel salinity and dissolved

oxygen levels (Figs. 8-10). Stagnant conditions and very low oxygen levels frequently produce an

oxygen sag and a salinity peak in the main channel year round, frequently at penstock levels. The

San Juan Arm, on the other hand, has greater discharge rates and does not suffer from the anoxic

conditions found in the Escalante arm. Occasionally the San Juan channel produces a bottom

flowing FRESCO plume in the winter that reaches the main channel of the lake, creating a rise in

oxygen concentrations that does not correspond to cold temperatures arising from the main channel.

3. Historic Patterns seen at Lake Powell

In the following discussion, We found it useful to use a ranking system to compare lake attributes

and reactions against such outside factors as total inflow or outflow for the year, coldness of winter,

hotness of summer. A table of these rankings is found in the appendix.

a. Filling Period (1963-mid 1970s)

While Lake Powell did not reach full pool until 1980, by 1973 to 1975 it reached a level of

normal fluctuations. By the mid-l 970’s most of the indicator parameters (temperature, conductivity

and dissolved oxygen) had stabilized. This early period of filling was characterized by low lake

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volume and reduced releases, resulting in a higher exchange rate for the water within the lake:

between 124% to 380%. This indicates that the entire lake’s water quality was more rapidly dictated

by inflow water quality than in subsequent periods, and such was the case in the 1960’s.

Interpretation of this period is hampered by erratic data collection--there are holes in temperature

and dissolved oxygen data at critical junctures, as well as irregularities in the temporal sampling.

Some conclusions can still be drawn from this period.

One of the signatures of this filling cycle was an 8th ranked flood (the 8th highest annual inflow

in Lake Powell’s history) in 1965. This tremendous volume of water appeared at the dam with the

lowest conductance values recorded since the lake’s Filling. This plume of fresh water shows clearly

in Fig. 3a and the longitudinal plots of Fig. 4, but doesn’t have the diluting effect that one could

expect from the mass it represents, for it is flushed from the lake within 2 years. It is apparent is

some of it is mixed into the hypolimnion by early 1966, but by 1967, the fall SWARM plug has

substantially salinized the lake. In spite of the high volume turnover rates for the 1960’s, this

extremely salty hypolimnion (values up to 1300 FS/cm) developed with only one season’s inflow,

but required 5 years to dilute to previous levels. One reason is the nature of the SWARM flow to

quickly dominate the hypolimnion if it is of sufficient salinity, while the diluting processes of the

hypolimnetic overflow or underflow is less efficient. Winter inflow salinity values today range from

800 to 1200 FS/cm. This cycle of events is a product of the low level of the lake, and the placement

of the penstocks and river outlet works, which were both functioning in this period.

In addition, the left river diversion tunnel, at the bottom of the dam, had not yet been sealed, and

was used for discharges when the lake level had not yet reached the jet tubes or penstocks or when

higher discharge could not be achieved through the penstocks and jet tubes alone (U.S. Bureau of

Reclamation, 1970, Martin 1989). At these low lake levels, both upper release ports drew from the

epilimnion, stripping out the spring floodwaters before they were integrated into the lake’s depths,

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while the diversion tunnel drew off the deepest hypolimnetic waters. The result was rapid turn-over

of the lake's volume in 1965 with pronounced refreshment of the hypolimnion. The diversion

tunnel was permanently sealed in September 1965. The bi-level epilimnetic withdrawals that

followed in 1966 and strong SWARM currents that followed in the late 1960's resulted in an

exceptionally persistent and very saline hypolimnion-meromixis-- that dominated the lake through

1971.

Oxygen levels for this period remained fairly high. Although the early dissolved oxygen data was

sometimes noisy, the pattern of winter oxygenation predictable under the circumstances. Although

quite saline, the winter bottom flows were cold and well oxygenated. Furthermore, these flows were

entering a smaller lake and the cold, oxygenated FRESCO plume had less distance to travel to the

dam than in later years when the lake was deeper and longer.

b. Drought Cycles

For the purposes of this assessment, basin-wide drought is measured by the total inflow to the

lake and not local weather patterns. At least 5 major drought cycles can be observed in the long

term forebay plots in Fig. 3a, denoted by a low inflow (as well as outflow) hydrograph, decreasing

or low lake levels, and generally increasing salinity levels. Some of the droughts may have

consisted of only one or two year, such as 1964,1972,1976 to 1977 and 1981, yet it can be seen

they had an immediate and persistent effect upon the hypolimnion. Other droughts continued for

years, such as the 3 sub-normal years from 1966 through 1968, and the most recent drought from

1988 to 1992. These longer droughts produced greater meromixis in the hypolimnion, as well as

substantially higher levels in the epilimnion.

As happened in 1966, the effects of a strongly saline SWARM bottom current, such as typically

follows a dry year, has an immediate and strong influence on hypolimnetic conductance.

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Decreasing lake volumes characteristic of drought cycles exacerbated the concentration of salts in

the lake. Dissolved oxygen levels, on the other hand, could be elevated during drought if lake levels

dropped sufficiently to allow the winter oxygen plume a shorter approach to the dam.

Rapid releases that are not accompanied by matching inflow volume will result in a drop in lake

elevation. At this time, water stored in the more eutrophic side bays enter the mainstem (Thorton et

al. 1990). This process will contribute to meromixis during drought periods.

While the data during the 8O's flood was compromised, the temporal resolution dwindled to 1 or

2 trips per year for 4 years from 1988 to 1991. Inflow volumes indicate these were drought years,

but little information can be garnered from this period except a 5 year trend. While trends

manifested in the forebay data could be compared to similar periods to suggest the processes behind

the long and short term trends, there remain questions for this period. The monthly lake-wide trips

from 1971 to 1982 proved invaluable in discerning the patterns to the processes. This period of

decreasing resolution identifies the value of the quarterly lake wide sampling for determining lakewide processes that are manifested in the forebay data.

C. Flooding Cycles

While the salinization of the lake from a drought cycle can be manifested abruptly within the

hypolimnion, the freshening qualities of a flood may take several years to mix throughout the lake

(Fig. 3a). This relates to the density and mixing properties of the SWARM flows versus the

FRESCO flows. One sinks easily to the hypolimnion and mixes with previous cells and evades the

penstocks, the other tends to slide over the first and is evacuated at the penstocks. When a very

fresh flood follows a drought, the steeper salinity gradient poses a greater challenge to the mixing

and sinking forces.

The flood years of the 198O's were a dramatic example of the wettest series of years since 1921.

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The vast volume of fresh, oxygenated water, combined with the operation of the spillways and river

outlet works brought the lake to as complete a mixing as it has approached in its history. It was with

one of the worst cases of bad timing that the Bureau of Reclamation decided to scale back sampling

of Lake Powell at this time. This was a great loss from the perspective of understanding

limnological processes.

B. G/en Canyon Dam Releases

Since the filling of Glen Canyon Dam, water quality conditions in the Glen Canyon Dam

tailwater have changed substantially. Before Glen Canyon Dam the river was characterized by wide

fluctuations in discharge, sediment load, temperature and salinity content. Due to the presence of

the dam and the location of the penstock structures for powerplant releases there has been a marked

reduction in the variance of all these parameters. Discharge is now a function of predetermined

annual release volumes, monthly scheduled releases, and daily fluctuations in response to power

demands and operational constraints (Fig 11). Temperatures have been greatly reduced and

fluctuations now range between 7 and 12 EC, reduced from a pre-dam range of 0 to nearly 30 EC

(Fig. 12). Fluctuations in salinity have been similarly reduced (Fig. 13).

1. Historical trends

a. Temperature

Fluctuation in temperature patterns became reduced to their current levels by about 1973, at which

point the reservoir had filled sufficiently to isolate the penstock from the seasonally warming

surface waters (Fig. 12). Since that time, annual mean temperatures have been fairly stable between

8 and 10 EC. Increases in instantaneous and mean temperatures were seen in 1978 when the

reservoir was drawn down due to drought conditions and in the mid 8O's and a result of the

operation of the spillway release structures. Since the implementation of interim flows at Glen

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Canyon Dam annual mean temperatures and variation appear to have been reduced because of

constrained release patterns.

b. Salinity

Salinity patterns in the Colorado River have been representative of hydrological and climatic

conditions throughout the Colorado River basin. Although much moderated from pre-dam patterns

some cyclic patterns appear to be present in the relatively short existence of Glen Canyon Dam.

Annual salinity variations, like temperature, were brought to stable levels in 1973 after the reservoir

had reached a surface elevation above 3600 ft. Some notable variations occur (Fig 14). After initial

filling of the reservoir to the penstock level in 1965, the river outlet works and bypass tunnels were

operated, releasing a large volume of water to keep the reservoir at the penstock level. This routed

the early summer runoff quickly through the reservoir and replaced the existing, more saline water

near the dam with much more dilute water. This is shown by a marked decrease in salinity levels of

the Glen Canyon Dam releases in 1965. Peaks in salinity occurred later in the late 1960~s and again

in 1980 following drought years with associated reservoir drawdown and corresponding higher

salinity levels in the reservoir. Salinity was markedly reduced with the high water years of the mid8O?s at which time dilute water replaced most of the reservoir’s contents. After the low water years

from 1989-l 993 the reservoir was again drawn down, resulting in a peak in the overall salinity

trend. Since that time salinity levels have gradually declined and reached a level of nearly 400 mg/L

TDS in early 1997, the lowest level since 1984. If above average runoff and reservoir releases

continue, this level should decline further.

2. Seasonal Patterns

Seasonal patterns in the Glen Canyon Dam releases are a function of meteorological conditions,

internal reservoir mixing processes, inflow hydrology and the operation of the dam.

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a. Temperature

An asymmetric annual temperature pattern appears in Glen Canyon Dam releases, with

temperatures gradually warming through the year from a low point of 7 to 8 EC in February or

March to a maximum point in December of 10 to 12 EC. This is followed by a sudden drop to its

minimum value with a few months (Fig 15). This pattern is most likely due to the gradual

penetration of surface warming through the summer. Winter convective mixing in the upper layers

of the reservoir then takes place, drawing relatively warmer surface water to levels at or near the

penstock. The magnitude of the maximum release temperature appears to be a function of the

thickness of the mixed upper layer. This is the result of the volume of low-density inflow from the

previous season's runoff, which arrives near the dam and defines initial conditions for the winter

convection process, and the severity of the winter cooling and its effect on the convective process.

Only in certain years do withdrawals seem to be come from the mixed isothermal epilimnion. The

sudden drop in temperature which follows is most likely due to the continuation of the convective

process, but is also strongly affected by the upwelling of cold saline water from the hypolimnion

caused by the dense winter underflow currents in the reservoir. The period of sudden drop also

corresponds with high powerplant releases in the month of January that may augment the amount of

hypolimnetic water discharged while this upwelling occurs.

b. Salinity

Specific conductance cycles follow an opposite pattern compared to temperature (Fig 15). As

early summer inflows have more influence on the downstream reservoir, water at the penstock level

becomes progressively more dilute. Minimum salinity levels are reached concurrently with

maximum temperature patterns; maximum salinity occurs with minimum temperatures.

06102198 DRAFT Page 29 of 62
V. Discussion

A. Value of Existing and Past Monitoring Programs

The primary focus of this assessment is to identify the effects of Glen Canyon Dam's operation

on the water quality of Lake Powell and the Colorado River below Glen Canyon Dam. This

assessment was based on data from several sources and includes inflow hydrology, Glen Canyon

Dam release hydrology, measurements of physical water quality parameters from within Lake

Powell and dam releases, and the results of chemical analyses of water samples collected from the

reservoir and its releases. These data were evaluated in the context of the history of Lake Powell

with its initial filling and fluctuating reservoir levels, changing climatic patterns, and changing

operations criteria. Attention was given to seasonal patterns, operation of non-power release

structures, and other significant events such as special releases.

Much more data exists than what was readily available for analysis with the short time frame of

this assessment. Some data, such as dissolved oxygen and pH do not exist with sufficient long-term

quality to evaluate over the complete history of Lake Powell, but are valuable for shorter-term

analysis. Other data have only recently been collected and do not lend themselves to historic

comparison. Several aspects of the assessment were frustrated by the lack of consistent data

collection at a regular time interval. Resultant data gaps during significant parts of Lake Powell's

history have subjected some conclusions to inference and speculation.

Several aspects of the historical data set proved very valuable to the assessment. Monthly lakewide reservoir surveys from 197 1 to 1982 gave valuable information about the fate of advective

density currents through the reservoir. Consistent tailwater monitoring provided information about

variation in Glen Canyon Dam releases between monthly measurements. The current monitoring

program in the reservoir is characterized by quarterly lake-wide surveys and monthly forebay

06102198 DRAFT Page 30 of 62
surveys and includes measurements with higher resolution through the water column and along the

length of the reservoir. These data have given much information to inflow hydrodynamics, the

effects of Glen Canyon Dam release patterns, and seasonal dissolved oxygen dynamics mediated by

biological and hydrological processes.

When past monitoring programs have focused on specific aspects of Lake Powell water quality,

the resultant database is less amenable to analysis of long-term trends or patterns. The challenge of

designing any long-term monitoring program is to consistently collect those data that will be

valuable to long-term and short-term analysis, at a level of detail sufficient to accurately identify

trends and patterns, while maintaining a reasonable expenditure of resources to allow for the

continuation of data collection. The direction of a program such as this must be flexible to

accommodate changes in political and financial climate and respond to new information needs as

more knowledge is gained. It is felt that the current monitoring program on Lake Powell provides

consistent data collection with a sufficiently broad-based scope while keeping expenditures at a

reasonable level. Within this context, the program can be modified to meet changing information

needs and objectives.

B. Assessment of Factors Affecting Water Quality Conditions

The assessment process showed that the water quality in Lake Powell is affected by a variety of

factors, none of which can be truly separated from the others except, perhaps, by artificial scenarios

presented to a numerical hydrodynamic model of Lake Powell. Model development for Lake

Powell is not yet at a point where this type of modeling is possible. This section will describe some

of the important factors affecting reservoir and release water quality, primarily focusing on the

operation of Glen Canyon Dam.

1. Meteorological Conditions

06/02/98 DRAFT Page 3 1 of 62
Weather patterns play a significant role in determining inflow density, reservoir warming and

stratification, convective mixing processes, and variation in release quality, both on a daily and

seasonal basis. Warming or cooling of reservoir inflows from localized or seasonal meteorological

conditions can affect the density of these currents, the depth at which these currents enter the

reservoir, and the amount of diffusive mixing that occurs between the inflow and the receiving

reservoir water. The surface temperature of the reservoir is affected by these conditions and results

in setting up stratification during spring warming or breaking down this stratification through

convective mixing with the onset of colder winter conditions. The amount of cloud cover over the

reservoir affects how much radiant energy the reservoir receives to raise its heat content or stimulate

photosynthetic processes. Wind patterns from storm events can impede the onset of stratification or

accelerate convective mixing. Of significance to downstream releases, wind events can cause

internal seiches, or oscillations of stratification boundaries in the reservoir that can cause temporary

fluctuations in temperature or salinity levels in the tailwater (Fig 16).

2. Inflow Hydrology

One of the most important factors driving short-term and long-term processes in the reservoir is

the inflow hydrology, characterized by the volume and quality of inflows to Lake Powell and their

seasonal variation. Approximately 57% of the inflow volume occurs from April to June due to

snow-melt runoff (Gloss et al. 1981). This runoff is characterized very low salinity snow-melt that

has warmed on its course through the canyon lands. This combination results in a large volume of

low-density water that overrides the surface of the reservoir and extends downstream, reaching the

vicinity of the dam later in the year. The volume and duration of the runoff appears to dictate the

downstream extent and the thickness of this layer of water.

This provides the initial conditions for convective mixing later in the winter. Convective mixing

06102198 DRAFT Page 32 of 62
involves cooling the surface of the reservoir, which increases its density. This water sinks and

mixes with deeper water of equal density with the assistance of wind action. The more winter

cooling the deeper the level of mixing. If the previous spring's runoff does not have enough

volume to reach the downstream portion of the reservoir, convection is mainly a result of the

cooling process. However, if a heavy spring runoff results in a large volume of snow-melt water

near the reservoir, this thick layer is already relatively homogenous. Winter cooling can then mix a

large portion of the epilimnion to depths at or below the level of penstock withdrawal. This results

in direct withdrawal of epilimnetic water and has been seen during winter months following high

runoff from the previous spring. Examples of this occurred in early 1996 and 1997.

Reservoir inflows during the winter months are characterized by cold water of higher salinity

compared to those of other times of the year. These conditions result in inflows with the highest

densities of the year entering the reservoir. Therefore, this water plunges when it meets the reservoir

and flows along the bottom of the reservoir. Its ultimate fate depends on preexisting conditions in

the hypolimnion. If water in the deepest portion of the reservoir is of greater density than the winter

inflow, the inflow will override the hypolimnion and eventually be discharged through the penstock

outlet. This appears to happen after a saline hypolimnion has been established, usually following

years of low runoff. If, on the other hand, the inflowing water is of greater density than the

hypolimnion, it will displace the deeper body of water and route it through the release structures.

Regardless of whether the inflow density current overrides or displaces the hypolimnion, there is

a consistent annual pattern of apparent upwelling of the more saline hypolimnion each winter into

the penstock withdrawal zone. This usually occurs shortly after the first of the year and may be the

result of the momentum of the winter inflow density current temporarily displacing the horizontal

upper boundary of the hypolimnion. This causes, in effect, an internal oscillation or seiche of this

boundary. The effect downstream is that during this period of hypolimnetic upwelling, dam releases

06/02/98 DRAFT Page 33 of 62
suddenly become colder and more saline. This usually coincides with high winter releases from

Glen Canyon Dam.

3. Glen Canyon Dam Operations

a. Presence and Structure of Glen Canyon Dam

Glen Canyon Dam has three structures from which water can be released (Fig 17). The majority

of Glen Canyon Dam releases are through eight penstock intakes which route water to the

powerplant turbines with a combined capacity of 940 ems (33,200 cfs). The penstocks withdraw

water from an elevation of 1058 m (3470 I%) above mean sea level, or a depth of 70 m (230 ft) when

the reservoir is at full pool with a surface elevation of 1128m (3700 ft). When release requirements

dictate, an additional 425 m3/s (15,000 cfs) can be discharged through the river outlet works which

are situated at an elevation of 1028 m (3374 ft), 30 m below the penstock structures. The third

means of withdrawal is from the spillways that have a combined capacity of 5890 m’/s (208,000

cfs) and withdraw water from elevations above 1122 m (3680 ft). Releases from the river outlet

works and the spillways bypass the power-plant and can not be used for hydropower generation. In

1965, high releases were routed through two bypass tunnels used during the construction of the

dam. The upper portions of these tunnels were subsequently plugged; the lower portions became the

lower spillway tunnels.

The location of the penstock intakes primarily defines the quality of Glen Canyon Dam releases.

Variation in temperature, salinity, and other water quality characteristics are greatly reduced from

pre-dam conditions (Figs. 11 and 13). The penstock withdrawal zone is deep enough that it is

isolated from the wide seasonal fluctuations of the epilimnion, except at times of epilimnetic

withdrawal as described in the previous section. Suspended sediment has essentially been removed

due to settling in upstream portions of the reservoir. The structure of Glen Canyon Dam also creates

06102l98 DRAFT Page 34 of 62
6.12 maf (7.55 km3) of inactive and dead storage in Lake Powell below the penstock elevation,

unavailable for hydropower generation. Cold saline water of high density can build up in this area

and remain relatively isolated from other mixing processes in the reservoir. Organic material

entering the hypolimnion will accumulate and decompose, gradually lowering dissolved oxygen

levels. This water will remain relatively stagnant until it is displaced by inflows of higher density or

flushed by high reservoir throughput. The process of intrusion of saline water into the hypolimnion,

followed by decreasing oxygen levels until the hypolimnetic water is replaced appears during three

periods in Lake Powell? s history, from 1967 to 1973, from 1978 to 1982, and from 1991 to 1997.

b. Annual Release Volumes

Annual release volumes from Glen Canyon Dam are dictated by the Criteria for Coordinated

Long-Range Operation of Colorado River Reservoirs (PL 90-537) and the development of an

annual plan of operation based on streamflow histories, water supply probabilities, anticipated

depletions, and other legal and institutional requirements. Glen Canyon Dam is legally required to

release a minimum of 10.2 km3 (8.23 maf) of water per year for downstream compact and treaty

requirements. Since construction of the dam, this amount has been exceeded during five separate

periods. In 1965, releases were increased following 2 years of low releases during the initial filling

stages of Glen Canyon Dam. In the spring of 1973 a court order directed the drawdown of Lake

Powell to the 1094 m (3,590 ft) elevation to avoid impinging on Rainbow Bridge National

Monument. During this period there was maximal powerplant production at Glen Canyon Dam. For

a third time, after initial filling of Lake Powell was achieved in 1980, releases again exceeded 10.2

km3 (8.23 maf). For the period of 1983 to 1987 annual releases were well above this level

corresponding with wet conditions in the Upper Colorado River Basin. From 1995 to 1997, annual

releases again exceeded the minimum requirements. Each of these periods was followed by a

06/02/98 DRAFT Page 35 of 62
reduction in the salinity of the hypolimnion.

C. Monthly Release Volumes

Monthly release volumes are determined as part of the development of the Annual Operating Plan

and are based on anticipated power demands, forecasted inflows, and other factors such as storage

equalization between Lake Powell and Lake Mead. Typically, high releases occur in the month of

January and again in August, in response to increased power demands during these months. Of

significance is that high release volumes do not always concur with peaks in reservoir inflow. An

important effect of this timing is that high releases in January occur during the upwelling of saline

hypolimnetic water normally seen shortly after the first of the year. This upwelling and its

subsidence appear to be a function of the winter density currents impinging on this body of water.

The significance of the high January release volume is that more saline water is removed from the

reservoir during this upwelling than if releases were lower. Therefore, the timing of these high

releases facilitates the replacement of the hypolimnion near the dam.

d. Daily Release Volumes

Under existing operating criteria dictated by the recent Glen Canyon Dam EIS and Record of

Decision releases cannot exceed 25,000 cfs except under emergency conditions or when required

for flood control. Furthermore, during high release months, daily fluctuations cannot exceed 8000

cfs per day. Prior to June 1990, Glen Canyon Dam releases fluctuated widely, averaging between

13,000 and 16,000 cfs per day.

Water quality monitoring at an adequate time interval was not in place directly below the dam

until after August 1988. Therefore, evaluation of water quality changes due to daily fluctuations is

not possible for this period. However, the research flow period of June 1990 to August 1991

contained several flow scenarios representative of past operations in which wide fluctuations

0610219% DRAFT Page 36 of 62
occurred. During this time, measurements of temperature and specific conductance were being

taken at intervals of 2 hours or less. A common occurrence was a distinct fluctuation in daily

temperature and specific conductance as shown for Research Flow E during late September 1990

(Fig 19).

e. Use of alternate release structures

Besides the penstocks, Glen Canyon Dam is equipped to release up to 256,000 cfs. The 8

penstocks account for a capacity of 33,200 cfs, discharging at 1057.66 meters (3470 feet) amsl or

70.1 meters below full pool. The four hollow jet tubes or river outlet works are rated to release

15,000 cfs at a depth of 99.4 meters below full pool or 1028.4 meters amsl (3374 feet). The

spillways can each release 104,000 cfs, provided the lake can adequately submerge the outlet ports

at an elevation of 1111.9 meters (3648 feet) or 15.86 meters below full pool. With concerns of

possible spillway damage, safety and power revenue losses, the alternate release ports have been

used sparingly.

1) 1965-l 966 releases for filling Lake Mead.

The early filling stages of Lake Powell came at the expense of Lake Mead's pool level. When the

operation of Mead's penstocks was in jeopardy, Glen Canyon Dam was forced to substantially

increase downstream discharges starting in February of 1965. The river outlet works and diversion

tunnels were operated in conjunction with the penstocks to discharge up to 58,000 cfs; 40% of the

year’s discharge came from the river outlet works and left diversion tunnel between February and

August (Fig. 1l), and for 3 months the bypass releases exceeded the penstocks. The effects of this

discharge can be seen in the isopleths (Fig. 4). Much of the spring flood was withdrawn by the high

penstock and river outlet works releases drawing horizontally across the lake, while the diversion

tunnel directly released the most extreme hypolimnetic waters. By October of 1965, the spring

06/02/98 DRAFT Page 37 of 62
flood had been drawn downstream from the Bullfrog station, 170 kilometers above the dam. It was

unusual that such a large spring flood (gth highest of 33 inflows to Lake Powell) was no longer

present as far uplake as the inflow in fall. In this instance, the operation of these outlets emulated

the spillways by drawing from the fresh flood waters of the epilimnion, while the hypolimnion was

evacuated from the lake's bottom. Dissolved oxygen appears to be elevated as a result of the high

throughput and a hypolimnetic underflow.

2) 1980's flood years

The 1980s included one of the most distinct flooding periods in the southwest since detailed

records began in the early 1920's. From 1979 to 1988, the Colorado River basin received 6 of the

10 greatest flood events in Lake Powell' s history (Table 2). The flooding of the mid-eighties is

attributed to the weather phenomenon of El Nino, a pattern that is currently building in the Pacific

Ocean and which may compete with that of 1983. The results to Lake Powell were 5 years of back

to back above average flooding. Unfortunately this unusual period was accompanied by decreasing

sampling; from 5 lake-wide trips in 1983, to 4 in 1984, 3 in 1985 and 1986, 2 in 1987 and 1989, and

1 each in 1988, 1990 and 1991. With less than 4 trips per year, the inferences become uncertain.

The period preceding the flood years included a strong drought in 1977 that produced a strong

chemocline below the penstocks, followed by 8’h and 1 2th ranked high floods in 1979 and 1980 that

significantly refreshed the hypolimnion. In 1981 a fairly strong drought introduced another saline

SWARM cell to the hypolimnion that set the stage for the series of floods. Lake levels were already

high preceding the 1983 flood, about 5 meters short of full pool.

In 1983, the lake reached full pool by June 8th, but it was nearly a month before the inflows

peaked on July 1”‘. The spillways were opened on June 2”d to be followed by the river outlet works

on June 7th. Cavitation and erosion within the spillway tunnels lead to their shutdown on August lst,

06/02/98 DRAFT Page 38 of 62
and it was August 1 lth before the jet tubes were shut down. The reservoir appeared to be poised for

a deeply centered hypolimnetic overflow (or interflow) previous to the flood. Through the course of

the withdrawals from all 3 depths, a pattern of equi-potentialflow-through

overrides the interflow pattern. We use the term equi-potentialflow-through to describe the

conditions that accompany high discharges through multiple outlets. The resulting horizontal

stratification pattern is characterized by weak vertical thermal and chemical gradients that are

relatively uniform from the inflow to the dam. It is assumed velocities across the reservoir are

relatively homogeneous as they approach the dam. Hence, the SWARM cell of the hypolimnion is

almost entirely eliminated by the end of summer, leaving a small isolated cell of the cold FRESCO

cell in front of the dam.

Table 2: Lake Powell inflow, outflow, spillway and jet tube operations, and lake

elevations for selected years.

Dissolved oxygen values become very unstable at this time due to faulty and erratic instrument

readings. Some of the more reasonable values and patterns suggest a high degree of oxygenation of

the hypolimnion accompanied by modest metalimnetic dissolved oxygen sags following the spring

flood. Under more typical release patterns we would expect a major flood event that follows

drought conditions to introduce a large quantity of organic carbon creating that would create a high

06/02/98 DRAFT Page 39 of 62
biological oxygen demand (BOD). While there are repeated episodes of metalimnetic hypoxia

throughout the 198O's, they are not severe, nor do they translate to subsequent hypoxia in the

hypolimnion. Spillway withdrawals have intercepted part of the spring flood along with the high

organic concentrations it entails. This effect could be real or it could be an artifact of poor data

quality and low temporal resolution, but epilimnetic releases in the 1960's produced similar

patterns. Dissolved oxygen values present more questions than they answer, which may be

addressed if the current scenario of continued high anticipated flows is manifested.

The reservoir conditions preceding the 1984 flood are significantly fresher than the previous year.

A very cold winter and low chemical gradient created a good opportunity for a thorough underflow

winter current. This process may have been initially augmented by high river outlet withdrawals

(running above recommended capacity for 2 months) and eventually discharges out-competed the

underflow currents, entraining much of the convectively mixed epilimnion. Although monitoring

resolution was inadequate to capture the exact process of flow, another equi-potentialflow-through

appears to have eliminated any significant trace of a hypolimnion. The spillways were not operated

until August to test repairs, again at an opportune time to draw off high BOD from the spring flood.

Temporal data resolution is problematic beyond 1985. There was no winter sampling and it can

only be inferred that a hypolimnetic underflow occurred. It is in early summer of 1985 that the Lake

Powell reached its most isotonic state, with conductivity varying only 76 FS/cm from top to bottom

at the dam (contrasting with 635 FS/cm in 1965 or 285 FS/cm at this writing). There were no

spillway releases in 1985, but the river outlets discharged at capacity (15,000 cfs) for over a month

in May and June. The reservoir experiences equi-potential flow-through once again.

A repeat of 1985 conditions was approximated in 1986. The hollow jet tubes were operated above

capacity for a month in May and June. A hypolimnetic underflow is indicated by temperature and

dissolved oxygen trends, with a less pronounced equi-potentialflow-through. Slightly high salinity

06lO2l98 DRAFT Page 40 of 62
values began setting up in the hypolimnion.

Throughout this period that utilized the alternate releases / non power structures, surface warming

did not appear to be significantly affected by spillway releases. This can be attributed to a number

of items. As mentioned earlier, epilimnetic thickness and heat content appears to be controlled by

volume of inflow more than any other factor. Furthermore, the intensity of warming at the surface

of the lake is more dependent on small scale warming trends. In the face of these first two factors,

spillway discharges during flooding events did not appear to have an appreciable effect on heat

content of the lake or maximal heating of the surface. Caution must be applied due to poor temporal

data resolution.

3) The Spike Flow, 1996

In spring of 1996, the first habitat/ beach building experimental flood was applied to demonstrate

the use of controlled dam releases to enhance habitat in the Grand Canyon. The experimental floods

were originally intended for use in low flow years so that subsequent high flows would not degrade

any benefits derived from the high flows, such as newly formed beaches and backwaters. The

conflict between pure scientific experimentation and concerns of water managers situated the first

experimental flood between several high inflow years. This has brought a different perspective to

the results of the spike flood as well as depriving lake researchers from a more ideal hydrodynamics

experiment.

Table 3. Spike discharges in acre-feet and percentage of total spike discharge compared

to the volume of the lake below those levels.

Dam Outlet Acre-Feet of Spike

Penstocks 506,072

Jet tubes 216,742

‘Total for Spike 722,814

% of Spike

4.41%

1.89%

6.30%

% of volume turnover

16.48%

102.24%

23.54%

The years preceding the spike flood included 2 high floods (1993 ranked 1 2th, 1995 ranked 6’h)

0610219% DRAFT Page 41 o f 62
that were beginning to break down a resistant chemocline that developed in the drought years of

1988 to 1992. The winter of 95-96 was fairly mild and the winter FRESCO plume of cold

fresher oxygenated water was well established in a hypolimnetic overflow pattern by early March.

The SWARM plume was relatively fresh due to a dilute fall inflow. It had hit the dam and was

being levered up by the force of the FRESCO cell when the hollow jet tubes were opened on March

26th. Seven continuous days with the jet tubes and penstocks at capacity (45,000 cfs) produced a

marked and immediate evacuation of the hypolimnion at those levels, as evidenced in Figs. 3b, 8-

10. The discharge from the jet tubes was equivalent to the volume in the lake below the jet tubes,

while the volume discharged from the penstocks was approximately 16.5% of the hypolimnion

below the depth of the penstocks (see table II). The spike flood enhanced and reinforced the flow

patterns already in action, seen in the cropping of the overflow in the longitudinal plots (Figs. 8-l 0)

and the accelerated conclusion of the hypolimnetic upwelling seen in the forebay plot in figures 3a

and 3b. Complete analysis of the spike flood results are reported in Hueftle et al. (1998, in review).

f. Unique use of the penstocks

1) 1973 Rainbow bridge drawdown

In March of 1973 a high snowpack presaged a substantial rise in lake elevation. Concerns over the

reservoir impinging on Rainbow Bridge National mounted. As a result, 41 days of steady penstock

withdrawals averaging 28,300 cfs (23% of the year's discharge) brought the lake level to 1094 m

(3590 feet). A stay on a court order allowed the reservoir to continue filling by May 1. Reservoir

conditions before the release included a very cold hypolimnetic underflow and a relatively high

winter inflow volume, resulting in high oxygen and low salinity winter underflows. This cold flow

wedged the salty SWARM cell to the level of the penstocks where it was released downstream

(figure 5). This episode demonstrates the effectiveness of hypolimnetic underflow as a refreshing

06102l98 DRAFT Page 42 of 62
agent when combined with well-timed dam discharges. The freshening effect of this combination

persisted in the hypolimnion for several years.

2) 1997 High Steady Flows

The winter of 1996- 1997 resulted in another year of above average snowpack on a watershed that

had received 4 years of heavy precipitation in the last 5 years. With lake levels still high from the

preceding year, dam releases were raised to an average of 27,000 cfs by February 1 sth with the

intention of avoiding power plant bypasses. For the next 5 months (and counting) releases were

maintained at levels between 21,000 cfs and 27,000 cfs, providing a strong, steady pull on the

reservoir at penstock levels.

A lake-wide sampling trip within a week of the elevated releases revealed a relatively cold

oxygenated FRESCO plume poised for a possible hypolimnetic underflow or a deep interflow.

Three months later the upper portion of the hypolimnion had been almost entirely stripped out. The

resulting hypolimnion had the lowest conductivity and the highest oxygen levels that the forebay

had experienced since the 1987. The June 1997 isopleths (figures 3b, S-10) demonstrate a deep

hypolimnetic flow that was drawn up vertically 60 meters toward the penstocks.

The effects of these strong sustained releases were enhanced by the conditions created by the

spike flood releases the previous spring. The chemocline had been weakened by the refreshment of

the meromixis that resulted from the jet tubes releases in 1996. In addition, the SWARM formed in

the fall of 1996 appears to be relatively low in salinity because of a low salinity inflow and the

dominance of a fresh convectively mixed epilimnion, further weakening the chemocline. Thus the

deep winter interflow and high sustained releases were optimally positioned to draw off the

remaining meromixis at greater depths. If projections of a strong El Nino weather pattern occur in

1998, it can be expected that even deeper mixing and oxygenation could occur next year, possibly

emulating conditions of the mid 1980's.

06/02/98 DRAFT Page 43 of 62
VI. Conclusions

I

A. influence of dam operations on lake water quality

As discussed in the preceding section, many factors are significant to determining the water

quality of Lake Powell and Glen Canyon Dam releases. Data for this evaluation is based on a 33-

year data set from a broadly focused long-term monitoring program conducted by the Bureau of

Reclamation, Glen Canyon Environmental Studies, and the Grand Canyon Monitoring and

Research Center. There exists a complicated linkage between climatic and meteorological factors,

Colorado River basin hydrology, physical hydrodynamic processes, the presence and structure of

Glen Canyon Dam, and the operation of the Dam itself.

It is impossible to discern the extent and degree that each of these factors has on determining

reservoir and release water quality because seldom, if ever, do these factors operate independently

of one another. Glen Canyon Dam has changed the overall quality of the Colorado River in Grand

Canyon, the dam operates to supply required amounts of water to the Lower Basin States and

generate hydropower with current constraints, and global-scale climatic patterns determine

decadal-scale patterns in water availability.

The focus of this assessment is to identify the effects of the operation of Glen Canyon Dam on

reservoir and release water quality. Within the scope of evaluating readily available data within a

short time frame, several factors that affect patterns and changes in the physical and chemical

characteristics of water in Lake Powell and Glen Canyon Dam releases were identified that can be

attributed to the operation of Glen Canyon Dam. These factors range from those determined mainly

by the Long-Range Operating Criteria to those affecting day-to-day operations under the current

constraints of the Glen Canyon Dam EIS and Record of Decision. Other effects may be ascertained

as other sources of data are developed, additional analyses are performed, and the ability to model

WO2198 DRAFT Page 44 of 62
operating scenarios and environmental factors is enhanced.

1. Release volumes

Glen Canyon Dam must operate in response to hydrologic conditions in the Colorado River

basin and downstream water delivery requirements. At the same, dam safety considerations must

be met, hydropower generation must be maintained, and recent operating criteria must be followed.

Reservoir releases must respond to these outside factors and will follow overall patterns in

Colorado River Basin hydrology. The existence of Lake Powell and the amount of water passing

through the system moderate the salinity levels in Lake Powell and water released from Glen

Canyon Dam. High sustained releases as seen in 1973 and 1997 act to increase mixing of the

reservoir beyond that caused by other processes.

The effects of reservoir drawdowns can be seen in the buildup of a saline body of water in the

deepest portions of the reservoir, which shows progressive loss of dissolved oxygen until its

eventual replacement during wet hydrologic cycles.

2. Timing of Reservoir Releases

Differential monthly volumes released from Glen Canyon Dam can have a significant effect on

salinity on dam releases and the amount of salinity stored in Lake Powell. While the dam is not

operated for this purpose, high releases in January draw larger volumes of saline water from the

reservoir during a period of hypolimnetic upwelling into the zone of penstock withdrawal.

3. Operation of non-power release structures

a. River Outlet Works

One of most striking effects of Glen Canyon Dam operations is the use of alternate release

structures to route releases exceeding power plant capacity for flood control, storage equalization,

surplus deliveries, or experimental ecosystem enhancements. River outlet works structures were

06/02/98 DRAFT Page 45 of 62
operated in 1965, during the mid- 1980's, and again in the spring of 1996, as part of the

Experimental Beach/Habitat Building Flow. The operation of these structures released water from

deeper levels in the reservoir, enhancing advective flow at these levels, and promoting the mixing

of the reservoir and reduction of the meromictic hypolimnion and hypoxia.

b. Spillways

The spillway structures were operated during 1983 and 1984 and showed enhanced routing of

warm, epilimnetic water from snow-melt runoff through the reservoir. They also result in the

release of warm dilute water downstream that may be of very different quality than that normally

discharged from the dam. By routing the dilute snow-melt downstream before subsequent mixing

to the lower depths of the lake, spillway releases could increase the reservoir salinity budget. In the

same manner, the dissolved oxygen sag that normally accompanies the riverine snow-melt could be

routed out of the reservoir.

Operation of the spillways can also be viewed as a simulation of operation of potential selective

withdrawal structures due to their location in the warm water near the surface of the reservoir.

4. Changes in Variability of Downstream Releases

Since the implementation of the Interim Operations Criteria in November 1990 and the

subsequent EIS Record of Decision, daily fluctuations and the rate of change of these fluctuations

have been significantly reduced. Compared to previous conditions, it is apparent that mean annual

temperature and the annual variability of temperature in Glen Canyon Dam releases have been

reduced.

Evidence of daily fluctuations in temperature and specific conductance of dam releases in

response to high fluctuating flows is less apparent under the current operating criteria that under

historical operation scenarios.

06/02/98 DRAFT Page 46 of 62
These changes have significance to the future potential of the study of seasonally adjusted steady

flows as stated in the US Fish and Wildlife Service's Reasonable and Prudent Alternative to the

Glen Canyon Dam EIS. These flows could be expected to exhibit less variation in release water

quality than currently exists.

B. Integration with Downstream Systerns

1. Grand Canyon Aquatic Ecosystem

Chemistry data for nutrient concentrations collected in Lake Powell and in downstream releases

have only been collected since 1992. In addition to the lack of historical data to make comparisons,

levels of phosphorous compounds are often below standard detection levels. Therefore, a complete

analysis of chemistry data from Lake Powell was beyond the scope of this assessment. However, it

has been noted that nutrient concentrations in the deeper portions of Lake Powell can be three to

four times higher than those in the epilimnion. This could have significance for the aquatic

ecosystem downstream of Glen Canyon Dam. Recent studies have noted a shift in the community

structure of primary productivity below Glen Canyon Dam which has been concurrent with a

gradual reduction in salinity levels as well as shifts in dam operations (Benenati, P.L et al. in

press). An unknown portion of the dissolved mineral load on Glen Canyon Dam releases is

comprised of nutrient compounds and the observed salinity reductions may indicate reduced

nutrient delivery to the aquatic ecosystem. Changes in dam release timing and magnitude impact

community structure and biomass of primary producers such as Cladophora glomerata. The

community has shifted to other algal forms since the high sustained releases of 1995, many of

which are a poorer substrate for the aquatic food base which supports the fishery.

Other authors have reported alterations of downstream aquatic ecology in conjunction with dam

operations, including Ward and Stanford (1983), Angradi et al. (1992), Ayers and McKinney

06/02/98 DRAFT Page 47 of 62
(1996) Stevens et al. (1997).

2. Lake Mead

Releases from Glen Canyon Dam have relatively stable physical characteristics, within the

observed range of fluctuation, compared to pre-dam releases. As such, the water entering Lake

Mead has more uniform density characteristics. The patterns of seasonal overflow and underflow

seen in Lake Powell inflows are not present in Lake Mead, which has affected the behavior of this

large body of water since the presence of Glen Canyon Dam (Evans and Paulson 1983). It is

possible, under future hydrologic conditions, operational scenarios, or the use of a selective

withdrawal structure that this pattern could be modified to produce occasional periods of inflows

routing through the upper layers of Lake Mead, which could have an enhanced effect on the

biology of the reservoir.

06/02/98 DRAFT Page 48 of 62
I -

Literature Cited

Angradi, T.R., R. W. Clarkson, D. A. Kinsolving, D. M. Kubly, and S. A. Morgenson. 1992.

Glen Canyon Dam and the Colorado River; responses of the aquatic biota to dam operations.

Prepared for the Bureau of Reclamation, Upper Colorado Region, Glen Canyon Environmental

Studies, Flagstaff, AZ. Cooperative Agreement No. 9-FC-40-07940. Arizona Game and Fish

Department, Phoenix, AZ.

APHA, 1992. Standard methods for the examination of water and wastewater. 1 8’h edition.

APHA, Washington, D.C.

Ayers, A. D. and T. McKinney. 1996. Water chemistry and zooplankton in the Lake Powell

forebay, Glen Canyon Dam discharge and tailwater. Prepared for the Bureau of Reclamation, Upper

Colorado Region, Glen Canyon Environmental Studies, Flagstaff, AZ. Cooperative Agreement No.

9-FC-40-07940. Arizona Game and Fish Department, Phoenix, AZ.

Benenati, P.L, D. Blinn, J.P. Shannon. 1998, Response of primary productivity flow regimes and

water quality changes in the Colorado River, Lees Ferry, AZ. (In press) Cole, G. A. 1994.

Textbook of Limnology. Waveland Press. Prospect Heights, Illinois.

Edinger, J. E., E. M. Buchak, D. H. Merritt. 1984. Longitudinal-Vertical Hydrqdynamics and

transport with chemical equilibria for Lake Powell and Lake Mead. Pages 213-222, in R. H. French

(ed): Salinity in watercourses and reservoirs. Salt Lake City, Utah: Butterworth, Boston.

Evans, T.TD., and L.J. Paulson. 1983. The influence of Lake Powell on the suspended sedimentphosphorus dynamics of the Colorado River inflow to Lake Mead. In V. D Adams and V. A

Lamarra (eds): Aquatic Resources Management of the Colorado River.

Gloss, S. P. L. M. Mayer, and D. E. Kidd. 1980. Advective control of nutrient dynamics in the

epilimnion of a large reservoir. Limnology and Oceanography 25: 219-228.

Gloss, S. P., R. C. Reynolds Jr., L. M. Mayer, and D. E. Kidd. 198 1. Reservoir influences of

salinity and nutrient fluxes in the arid Colorado River Basin. Pages 16 18-l 629 in H. G. Stephan

(ed.), Proceedings of the Symposium of Surface Water Impoundments. American Society of Civil

Engineers, New York.

Golden Software, Inc. 1994. Grapher, version 1.24.

Golden Software, Inc. 1996. Surfer, version 6.04.

Hart, R.J., and K.M. Sherman, 1996. Physical and Chemical Characteristics of Lake Powell at the

06/02/98 DRAFT Page 49 of 62
I

Forebay and Outflows of Glen Canyon Dam, Northeastern Arizona, 1990-91. U.S. Geological

Survey Water-Resources Investigations Report 96-40 16.

Hydrolab Corportation. 1994. Hydrolab Surveyor 3 Multiparameter Water Quality Monitoring

Instruments, Operating Manual. Austin, TX, U.S.A.

Hueftle, S.J., and L. E. Stevens. 1998. Experimental flood effects on the limnology of Lake Powell

reservoir, southwestern USA. Ecological Applications. (In review).

Johnson, N. M. and D. H. Merritt, 1979. Convective and advective circulation of Lake Powell,

Utah-Arizona, during 1972- 1975. Water Resources Research. 15(4):873-884.

Martin, R. 1989. A story that stands like a dam, Glen Canyon and the struggle for the soul of the

west. Henry Holt and Company, New York, New York.

Merritt, D. H. and N. M. Johnson. 1978. Advective Circulation in Lake Powell, Utah-Arizona.

Lake Powell Research Project Bulletin # 6 1. Institute of Geophysics and Planetary Physics,

University of California, Los Angeles,CA.

Potter, L. D. and C. L. Drake, 1989. Lake Powell: Virgin Flow To Dynamo. University of New

Mexico Press. Albuquerque, New Mexico. 328 p.

SAS Institute Inc. 1996. Version 6.12. SAS Institute, Inc., Cary, NC, 275 13.

Stanford, J. A. and J. V. Ward. 1986. Reservoirs of the Colorado River system. Pages 375-383 in:

The Ecology of River Systems, ed. B. R. Davies and K. F. Walker.

Stanford, J. A. and J. V. Ward. 199 1. Limnology of Lake Powell and the Chemistry of the

Colorado River. Pages 75-101 in Colorado River Ecology and Dam Management.

Stanford, J. A., J. V. Ward, W. J. Liss, C. A. Frissell, R. N. Williams, J. A. Lichatowich and C.

C. Coutant. 1996. A General Protocol for restoration of regulated rivers. Regulated Rivers:

Research and Management, 12:39 l-41 3.

Stevens, L. E., J. P. Shannon and D.W. Blinn. 1997. Colorado River Benthic Ecology in Grand

Canyon, Arizona, USA: Dam, Tributary and Geomorphological Influences. Regulated Rivers:

Research and Management, 13: 129- 149.

Thornton, K.W., B. L. Kimmel, F. E. Payne, ed. 1990. Reservoir Limnology: Ecological

Perspectives. John Wiley and Sons, Inc. New York, NY.246 pp.

U. S. Bureau or Reclamation. 1921-1997. Hydromet database. Online data retrieval. Upper

Colorado Region, Salt Lake City, UT.

U. S. Bureau or Reclamation. 1970. Glen Canyon Dam and Power-plant, Technical Record of

06/02/98 DRAFT Page 50 of 62
Design and Construction. A Water Resources Technical Publication. United States Bureau of

Reclamation. Washington, D.C.

--. 1990. Glen Canyon Environmental Studies Phase II draft integrated research plan. Bureau of

Reclamation, Salt Lake City, UT.

--. 1995. Operation of Glen Canyon Dam. Final environmental impact statement. Bureau of

Reclamation, Salt Lake City, UT.

--. 1996. Record of Decision for Operation of Glen Canyon Dam Final Environmental Impact

Statement. Department of Interior. Department of Interior. Washington, D.C.

U.S. Geological Survey, 1985-96. Water Resource Data for Arizona, Water Year 1985-1996.

Water Data Report. AZ-85-1 to 96-l. U.S. Geological Survey, Department of Interior. Washington,

D.C.

Ward, J. V. and J. A. Stanford. 1983. The intermediate disturbance hypothesis: an explanation

for biotic diversity patterns in lotic ecosystems. Pages 347-356, in T. D. Fontaine & S. M. Bar-tell

(eds), Dynamics of Lotic Ecosystems. Ann Arbor Science, Ann Arbor.

Wetzel, R. G. 1975. Limnology. W. B. Saunders Co., Philadelphia, PA. 767 pp.

06/02/98 DRAFT Page 51 of 62
Appendices

Assessment Prospectus

A. Table A-l

Ranking of Inflow and outflow hydrographs, seasonal temperatures, and ratings of winter

underflow currents.

Year

1963

1964

1965

1966

1967

1968

1969

1970

1971

1972

1973

1974

1975

1976

1977

1978

1979

1980

1981

1982

1983

1984

1985

1986

1987

1988

1989

1990

1991

1992

1993

1994

1995

1996

1997

(to 8/20/97)

Total:

outflow Inflow

Rank Rank

35 33

34 30

9 9

32 22

33 26

19 20

14 15

15 14

16 16

11 23

10 5

24 19

12 13

17 24

25 34

18 21

21 8

8 11

22 31

20 17

3 2

1 1

2 4

4 3

5 10

26 25

31 32

27 35

28 29

30 28

29 12

23 27

13 6

7 18

6 7

35 35

. n

undert ,- lw (uccurred 3 times in 33 years, 9.1%)

Winter Summer Summer Winter

Coldness Hotness Hotness Underflo

14

24

18

5

1

11

12

6

10

7

17

13

20

19

30

4

26

31

21

23

28

17

24

30

10

22

1

14

18

13

20

2

15

3

8

6

16

19

26

29

16

15

3

27

2

9

8

22

25

28

(moderate

to cool)

32

9

23

29

12

4

11

27

21

25

5

7

15

17

9

4

21

2

12

7

8

13

6

5

19

11

14

18

20

16

25

24

23

22

10

1

3

U

P

P

P

P

0

0

0

u+

0

P

0

Pu

P

o+

0

0

P

PO

ew

eqp

eqp

u?

---pu?

---(o?)

---(o?)

---(o?)

Pu

Pu

o+

0

0

P

30 25 33

p= Hypolimnetic partial under- or overflow (interflow) (Occurred 12 times in 33 years, 36.4%)

o=Hypolimnetic overflow (Occurred 14 times in 33 years, 42.4%)

eqp = Equi-potential flow-through (Occurred 3 times in 33 years, 9.1%)

06/02/98 DRAFT Page 1 of 62
Appendices

Assessment Prospectus

A. Table A-l

Ranking of Inflow and outflow hydrographs, seasonal temperatures, and ratings of winter

underflow currents.

Year outflow Inflow

Rank Rank

35 33

34 30

9 9

32 22

33 26

19 20

14 15

15 14

16 16

11 23

10 5

24 19

12 13

17 24

25 34

18 21

21 8

8 11

22 31

20 17

3 2

1 1

2 4

4 3

5 10

26 25

31 32

27 35

28 29

30 28

29 12

23 27

13 6

7 18

6 7

Winter Summer Summer Winter

Coldness Hotness Hotness Underflo

1963

1964

1965

1966

1967

1968

1969

1970

1971

1972

1973

1974

1975

1976

1977

1978

1979

1980

1981

1982

1983

1984

1985

1986

1987

1988

1989

1990

1991

1992

1993

1994

1995

1996

1997

(to 8/20/97)

Total:

14

24

18

5

1

11

12

6

10

7

17

13

20

19

30

4

26

31

21

23

29

16

15

3

27

2

9

8

22

25

28

(moderate

to cool)

32

28

17

24

30

10

22

1

14

18

13

20

2

15

3

8

6

16

19

26

9

23

29

12

4

11

27

21

25

5

7

15

17

9

4

21

2

12

7

8

13

6

5

19

11

14

18

20

16

25

24

23

22

10

1

3

U

P

P

P

P

0

0

0

u+

0

P

0

Pu

P

o+

0

0

P

PO

eqP

eqp

eqp

u?

---pu?

---(o?)

---(o?)

---(o?)

Pu

Pu

o+

0

0

P

35 35 30 25 33

u=Hypolimnetic underf NW (Occurred 3 times 133 years, .l%)

p= Hypolimnetic partial under- or overflow (interflow) (Occurred 12 times in 33 years, 36.4%)

o=Hypolimnetic overflow (Occurred 14 times in 33 years, 42.4%)

eqp = Equi-potential flow-through (Occurred 3 times in 33 years, 9.1%)

06/02/98 DRAFT Page 1 of 62