Phosphorus and Suspended Sediment

Load Estimates for the Lower Boise River,

Idaho, 1994 – 2002

Scientific Investigations Report 2004–5235

Version 2.00, December 2005

Prepared in cooperation with the Idaho Department of Environmental Quality

U.S. Department of the Interior

U.S. Geological Survey
Phosphorus and Suspended Sediment 

Load Estimates for the Lower Boise River, 

Idaho, 1994 – 2002

By Mary M. Donato and Dorene E. MacCoy

Prepared in cooperation with the Idaho Department of Environmental Quality

Scientific Investigations Report 2004-5235

Version 2.00, December 2005 

U.S. Department of the Interior

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

Gale A. Norton, Secretary

U.S. Geological Survey

Charles G. Groat, Director

U.S. Geological Survey, Reston, Virginia: 2005

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

Box 25286, Denver Federal Center

Denver, CO 80225

For more information about the USGS and its products:

Telephone: 1-888-ASK-USGS

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

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

endorsement by the U.S. Government.

Although this report is in the public domain, permission must be secured from the individual copyright owners to 

reproduce any copyrighted materials contained within this report.

Suggested citation:

Donato, M.M., and MacCoy, D.E., 2005, Phosphorus and suspended sediment load estimates for the lower Boise 

River, Idaho, 1994–2002 (version 2.00): U.S. Geological Survey Scientific Investigations Report 2004-5235, 30 p.
iii

Contents

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

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

Method for Estimating Phosphorus and Suspended Sediment Loads.............................. 4

Input Data ................................................................................................ 4

Calibration Files .................................................................................... 4

Discharge Data .............................................................................. 5

Water-Quality Data ........................................................................... 5

Estimation Files .................................................................................... 5

Other Input Data .................................................................................... 7

LOADEST Modeling Results.............................................................................. 8

Regression Evaluation ........................................................................... 8

Estimated Loads .................................................................................... 13

Total Phosphorus .............................................................................. 14

Dissolved Orthophosphorus.................................................................. 16

Suspended Sediment ........................................................................ 19

Monthly Average Increases and Decreases of Constituent Loads Within Reaches ............ 20

Diversion to Glenwood ........................................................................... 20

Glenwood to Middleton ........................................................................... 20

Middleton to Parma................................................................................. 20

Temporal Trends .......................................................................................... 22

Comparison with Mass Balance Spreadsheet Load Estimates .................................... 23

Annual Flow-Weighted Concentrations ............................................................... 24

Implications for Total Maximum Daily Load Implementation ....................................... 25

Total Phosphorus.................................................................................... 25

Suspended Sediment .............................................................................. 25

Limitations and Advantages of the LOADEST Model ................................................ 26

Summary ................................................................................................... 28

References Cited.......................................................................................... 29
iv

Figures

Figure 1. Map showing location of the lower Boise River Basin and load 

estimation sampling sites, Idaho ......................................................... 3

Figure 2. Graphs showing relation between constituent concentration and 

discharge at four sites on the lower Boise River, Idaho, January 1994 

through September 2002 .................................................................. 6

Figure 3. Graph showing monthly average discharge at four sites on the lower 

Boise River, Idaho, January 1994 through September 2002 ........................... 8

Figure 4. Graphs showing relation between estimated and measured loads of 

total phosphorus at four sites on the lower Boise River, Idaho, 

January 1994 through September 2002 ................................................... 10

Figure 5. Graphs showing relation between estimated and measured loads of 

dissolved orthophosphorus at four sites on the lower Boise River, Idaho, 

January 1994 through September 2002 ................................................... 11

Figure 6. Graphs showing relation between estimated and measured loads of 

suspended sediment at four sites on the lower Boise River, Idaho, 

January 1994 through September 2002 ................................................... 12

Figure 7. Graph showing estimated daily average loads and measured instantaneous 

loads of total phosphorus in the lower Boise River at Parma, Idaho, 

January 1994 through September 2002 ................................................... 13

Figure 8. Graph showing estimated monthly average loads of total phosphorus at 

four sites on the lower Boise River, Idaho, January 1994 through 

September 2002.............................................................................. 15

Figure 9. Graph showing estimated average loads of total phosphorus, by month, at 

four sites on the lower Boise River, Idaho, January 1994 through 

September 2002.............................................................................. 15

Figure 10. Graphs showing estimated seasonal loads of total phosphorus, dissolved 

orthophosphorus, and suspended sediment at four sites on the lower 

Boise River, Idaho, January 1994 through September 2002 ........................... 16

Figure 11. Graphs showing estimated monthly average ratio of dissolved 

orthophosphorus to total phosphorus and monthly average discharge at 

four sites on the lower Boise River, Idaho, January 1994 through 

September 2002.............................................................................. 17

Figure 12. Graph showing estimated seasonal ratio of dissolved orthophosphorus to 

total phosphorus at three sites on the lower Boise River, Idaho, 

January 1994 through September 2002 ................................................... 18

Figure 13. Graph showing estimated average ratio of dissolved orthophosphorus to 

total phosphorus, by month, at four sites on the lower Boise River, Idaho, 

January 1994 through September 2002 ................................................... 18

Figure 14. Graph showing estimated monthly average suspended sediment load at 

four sites on the lower Boise River, Idaho, January 1994 through 

September 2002.............................................................................. 19

Figure 15. Graphs showing estimated monthly average increases and decreases of 

total phosphorus, dissolved orthophosphorus, and suspended sediment in 

three reaches of the lower Boise River, Idaho, January 1994 through 

September 2002.............................................................................. 21
Figure 16. Graph showing monthly average loads of total phosphorus at Parma, Idaho, 

estimated using LOADEST model and using mass balance spreadsheet 

method, April 15, 1996, through October 15, 1996 ....................................... 23

Figure 17. Graphs showing average annual flow-weighted concentrations and 

estimated loads of total phosphorus at three sites on the lower Boise River, 

Idaho, January 1994 through September 2002 .......................................... 25

Figure 18. Graph showing daily average discharge and measured and estimated 

daily average loads of total phosphorus at Parma, Idaho, July 1, 1996, through 

December 31, 1997........................................................................... 27

Tables

Table 1. Number of days per year and percentage of year for which daily loads were 

estimated at four sites on the lower Boise River, Idaho, January 19, 1994, 

through September 30, 2002 ............................................................... 7

Table 2. Regression coefficients and coefficients of determination (R2) for load 

models used to estimate total phosphorus, dissolved orthophosphorus, and 

suspended sediment at four sites on the lower Boise River, Idaho, 

January 14, 1994, through September 30, 2002 .......................................... 9

Table 3. Annual loads of total phosphorus, dissolved orthophosphorus, and 

suspended sediment at four sites on the lower Boise River, Idaho, 

January 1994 through September 2002 ................................................... 14

Table 4. Annual trends and significance level of trends in total phosphorus, dissolved 

orthophosphorus, and suspended sediment loads at four sites on the lower 

Boise River, Idaho, January 1994 through September 2002 ........................... 22

Table 5. Comparison between daily average loads of total phosphorus estimated by 

mass balance spreadsheet method and daily average loads and monthly 

average 95-percent confidence intervals estimated by LOADEST at three 

sites on the lower Boise River, Idaho, April 15 1996 through October 15, 1996 ...... 23

Table 6. Annual flow-weighted concentrations of total phosphorus, dissolved 

orthophosphorus, and suspended sediment at four sites on the lower 

Boise River, Idaho, January 1994 through September 2002 ........................... 24

Table 7. Estimated average annual loads, monthly average upper and lower 

95-percent confidence intervals, total flow volumes, and annual 

flow-weighted concentrations of total phosphorus at Parma, Idaho, 

May through September, 1994 through 2002 ............................................. 26

Table 8. Estimated average daily loads, monthly average upper and lower 

95-percent confidence intervals, total flow volumes, and annual 

flow-weighted concentrations of suspended sediment at Parma, Idaho, 

February 15 through June 14, 1994 through 2002 ....................................... 26

Figures—Continued
vi

Multiply By To obtain

mile (mi) 1.609 kilometer (km)

cubic foot (ft3) 0.02832 cubic meter (m3) 

cubic foot per second (ft3/s) 0.02832 cubic meter per second (m3/s)

pound, avoirdupois (lb) 0.4536 kilogram (kg) 

pound, avoirdupois, per day (lb/d) 0.4536 kilogram per day (kg/d)

ton per day (ton/d) 0.9072 metric ton per day

ton, short (2,000 lb) 0.9072 megagram (Mg) 

Concentrations of chemical constituents in water are given in milligrams per liter (mg/L).

Conversion Factors
Abstract 

The U.S. Geological Survey used LOADEST, newly 

developed load estimation software, to develop regression 

equations and estimate loads of total phosphorus (TP), 

dissolved orthophosphorus (OP), and suspended sediment (SS) 

from January 1994 through September 2002 at four sites on 

the lower Boise River: Boise River below Diversion Dam near 

Boise, Boise River at Glenwood Bridge at Boise, Boise River 

near Middleton, and Boise River near Parma. The objective 

was to help the Idaho Department of Environmental Quality 

develop and implement total maximum daily loads (TMDLs) 

by providing spatial and temporal resolution for phosphorus 

and sediment loads and enabling load estimates made by mass 

balance calculations to be refined and validated. 

Regression models for TP and OP generally were well 

fit on the basis of regression coefficients of determination 

(R2), but results varied in quality from site to site. The TP 

and OP results for Glenwood probably were affected by the 

upstream wastewater-treatment plant outlet, which provides a 

variable phosphorus input that is unrelated to river discharge. 

Regression models for SS generally were statistically well fit. 

Regression models for Middleton for all constituents, although 

statistically acceptable, were of limited usefulness because 

sparse and intermittent discharge data at that site caused many 

gaps in the resulting estimates.

Although the models successfully simulated measured 

loads under predominant flow conditions, errors in TP and 

SS estimates at Middleton and in TP estimates at Parma were 

larger during high- and low-flow conditions. This shortcoming 

might be improved if additional concentration data for a wider 

range of flow conditions were available for calibrating the 

model. 

The average estimated daily TP load ranged from less 

than 250 pounds per day (lb/d) at Diversion to nearly 2,200 

lb/d at Parma. Estimated TP loads at all four sites displayed 

cyclical variations coinciding with seasonal fluctuations in 

discharge. Estimated annual loads of TP ranged from less 

than 8 tons at Diversion to 570 tons at Parma. Annual loads of 

dissolved OP peaked in 1997 at all sites and were consistently 

higher at Parma than at the other sites.

The ratio of OP to TP varied considerably throughout 

the year at all sites. Peaks in the OP:TP ratio occurred 

primarily when flows were at their lowest annual stages; 

estimated seasonal OP:TP ratios were highest in autumn at all 

sites. Conversely, when flows were high, the ratio was low, 

reflecting increased TP associated with particulate matter 

during high flows. Parma exhibited the highest OP:TP ratio 

during all seasons, at least 0.60 in spring and nearly 0.90 in 

autumn. Similar OP:TP ratios were estimated at Glenwood. 

Whereas the OP:TP ratio for Parma and Glenwood peaked 

in November or December, decreased from January through 

May, and increased again after June, estimates for Diversion 

showed nearly the opposite pattern — ratios were highest 

in July and lowest in January and February. This difference 

might reflect complex biological and geochemical processes 

involving nutrient cycling in Lucky Peak Lake, but further data 

are needed to substantiate this hypothesis. 

Estimated monthly average SS loads were highest at 

Parma, about 250 tons per day (ton/d). Average annual loads 

from 1994 through 2002 were 13,000 tons at Diversion, 

33,000 tons at Glenwood, and 88,000 tons at Parma. Estimated 

SS loads peaked in the spring at all sites, coinciding with high 

flows. 

Increases in TP in the reach from Diversion to Glenwood 

ranged from 200 to 350 lb/d. Decreases in TP were small in 

this reach only during high flows in January and February 

1997. Decreases in SS, were large during high-flow conditions 

indicating sediment deposition in the reach. Intermittent data 

at Middleton indicated that increases and decreases in TP in 

the reach from Glenwood to Middleton were during low- and 

high-flow conditions, respectively. All constituents increased 

in the reach from Middleton to Parma, particularly in May 

1995. Periods of large increases in SS were interpreted as 

times of active erosion of sediment in the channel and (or) 

addition of sediment to the river from tributaries. 

Statistically significant downward temporal trends in load 

were determined for SS at Glenwood, for OP at Middleton, 

and for TP, OP, and SS at Parma. A significant upward trend in 

TP was determined at Diversion. 

Phosphorus and Suspended Sediment Load Estimates

for the Lower Boise River, Idaho, 1994 – 2002

By Mary M. Donato and Dorene E. MacCoy
Calculated annual flow-weighted concentrations 

highlighted the strong interaction between flow and particleassociated constituents such as TP and SS and enabled 

concentrations to be assessed despite the large range in flows. 

At Parma and Glenwood, where the OP:TP ratio was high, 

the flow-weighted concentration of TP was inversely related 

to load because of dilution and the strong effect of flow on 

the load calculation. At Diversion, where the OP:TP ratio 

was low, load and concentration were directly related because 

particulate matter was increased, not diluted, during high 

flows. Relations were similar for SS. 

LOADEST average daily TP load estimates indicated 

that reductions in load of 24 to 75 percent would have been 

necessary to meet the proposed goal of 565 lb/d set forth in the 

Snake River-Hells Canyon TMDL. Estimated average daily 

loads of SS at Parma from 1994 through 2002 exceeded the 

current lower Boise River TMDL load allocation of 101 ton/d 

at Parma except in 2001. 

The LOADEST model provided spatial and temporal 

resolution to help refine mass-balance spreadsheet calculations 

used for TP TMDL allocations between Diversion and 

Parma. LOADEST results compared favorably with previous 

mass-balance spreadsheet load estimates for TP and have an 

advantage over spreadsheet estimates in that they allow more 

detailed examination of loads at different timescales, whereas 

spreadsheet estimates are static.

Introduction

Water-quality and biological conditions of the lower 

Boise River have been affected by human activities such 

as agricultural land and water use, urbanization, and river 

channel alteration, as described by MacCoy (2004). The 

Lower Boise River Water Quality Plan, Inc., established in 

1992 and composed of public and private agencies, groups, 

and individuals, identified a need to assess water-quality 

conditions of the lower Boise River. To address this need 

and to provide a baseline from which to assess future trends, 

the U.S. Geological Survey (USGS), in cooperation with the 

Idaho Department of Environmental Quality (IDEQ), began 

an 8-year comprehensive assessment of the river in 1994. 

Data collection was completed in 2002. Water-quality and 

biological data from 1994 through 1997 were summarized in 

interim reports by Mullins (1998 and 1999). Water-quality and 

biological conditions for the entire study period, through 2002, 

were summarized by MacCoy (2004). 

In October 1994, the lower Boise River was listed as 

water-quality limited in accordance with Section 303(d) of 

the Clean Water Act (U.S. Environmental Protection Agency, 

1994, p. 6). This listing required the State of Idaho to develop 

total maximum daily loads (TMDLs) for the lower Boise 

River. Constituents of concern that must be addressed in the 

TMDL are nutrients, suspended sediment, bacteria, elevated 

water temperature, low dissolved-oxygen concentrations, oil 

and grease, and flow modification. 

The TMDL for suspended sediment in the lower 

Boise River was completed in 1999 (Idaho Department of 

Environmental Quality, 1999). The TMDL for phosphorus is 

currently undergoing final review and revision. In support of 

these efforts, the USGS used the LOADEST software (Runkel 

and others, 2004) to estimate and interpret total phosphorus, 

orthophosphorus, and suspended sediment loads at four sites 

on the lower Boise River: Boise River below Diversion Dam 

near Boise (USGS gaging station 13203510; hereafter called 

Diversion), Boise River at Glenwood Bridge at Boise (USGS 

gaging station 13206000; hereafter called Glenwood), Boise 

River near Middleton (USGS gaging station 13210050; 

hereafter called Middleton), and Boise River near Parma 

(USGS gaging station 13213000; hereafter called Parma) 

(fig. 1) from 1994 through 2002. Instantaneous loads for 

various constituents calculated from synoptic sampling data 

collected during that period were presented in earlier reports 

(Mullins, 1998; MacCoy, 2004). LOADEST augments 

this information by providing continuous time-series load 

estimates, thus offering better temporal and spatial resolution 

for constituent inputs. 

Modeling results can be used to refine mass-balance 

calculations that form the basis for allocating loads in the 

reaches between Diversion and Glenwood, Glenwood and 

Middleton, and Middleton and Parma. The results also will 

be useful for planning and implementing additional synoptic 

sampling to further improve the mass-balance calculations. 

This report briefly describes the LOADEST software and 

summarizes the results of the modeling work on the lower 

Boise River. 

Greg Clark and Paul Woods of the U.S. Geological 

Survey provided valuable technical advice about using the 

LOADEST model and contributed to the interpretation of the 

results.

   Phosphorus and Suspended Sediment Load Estimates for the Lower Boise River, Idaho, 1994 – 2002
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Figure 1. Location of the lower Boise River Basin and load estimation sampling sites, Idaho.

Introduction 
Method for Estimating Phosphorus and 

Suspended Sediment Loads

Total phosphorus (TP), dissolved orthophosphorus (OP), 

and suspended sediment (SS) loads in the lower Boise River 

were estimated with the USGS software called LOADEST, 

which uses time-series streamflow data and constituent 

concentrations to calibrate a regression model that describes 

constituent loads in terms of various functions of streamflow 

and time (Runkel and others, 2004). The software then uses 

the regression model to estimate loads over a user-specified 

interval. The model output includes statistical data to enable 

the user to evaluate the quality of the model. Model output 

also includes upper and lower 95-percent confidence interval 

(CI) values with the monthly average load estimates to provide 

an understanding of the precision of the monthly estimates. In 

this study, separate regression models were calibrated for TP, 

OP, and SS for each of the four sampling sites. 

The software performs calibration procedures and makes 

load estimates using four statistical estimation methods: 

Adjusted Maximum Likelihood Estimation (AMLE), 

Maximum Likelihood Estimation (MLE), Linear Attribution 

Method (LAM), and Least Absolute Deviation (LAD). The 

user chooses the most appropriate method for the data being 

analyzed. AMLE and MLE are suitable when the model 

calibration errors (residuals) are normally distributed; AMLE 

is the more appropriate method of the two when the calibration 

data set contains censored data (data that are reported as less 

than or greater than some threshold). LAM and LAD are 

useful when the residuals are not normally distributed. 

The software output includes the probability plot 

correlation coefficient (Vogel, 1986), Turnbull-Weiss 

likelihood ratio (Turnbull and Weiss, 1978), data for 

constructing a normal-probability plot, and standardized 

residuals. Because the input data in this study included 

censored data, and because the model calibration residuals 

were normally distributed within acceptable limits, the AMLE 

estimation method was selected in all cases. 

The software allows the user to choose between selecting 

the general form of the regression from among several 

predefined models and letting the software automatically 

choose the best model, on the basis of the Akaike Information 

Criterion (Akaike, 1981). The selection criterion is designed 

to achieve a good balance between using as many predictor 

variables as possible to explain the variance in load while 

minimizing the standard error of the resulting estimates. For 

this study, the software was allowed to choose the best model. 

The output regression equations take the following 

general form:

ln( ) (ln ) )ln )

[sin( )] [cos( )]

L a b Q c Q

d e

f g

= + +

+ +

+ +

2

2

2πT 2πT

T T

(1)

where

L is the constituent load, in pounds per day;

Q is stream discharge, in cubic feet per second;

T is time, in decimal years from the beginning of 

the calibration period; and 

a,b,c,d,e, 

f, and g

are regression coefficients.

Some of the regression equations in this study did not 

include all of the above terms, depending on the particular 

model chosen by the software. A complete discussion of the 

theory and principles behind the calibration and estimation 

methods used by the LOADEST software is given by Runkel 

and others (2004).

Input Data

Calibration Files 

The model calibration procedure performed by 

LOADEST uses instantaneous discharge data and concurrent 

instantaneous concentration data, provided by the user in a 

calibration file for each site. Data used in the calibration files 

for this study were collected at specific intervals between 1994 

and 2003 as part of the ongoing assessment of water-quality 

and biological conditions of the Boise River. Surface-water 

samples were collected monthly or bimonthly during the 

nonirrigation season (October through March) and as often 

as twice a month during the irrigation season (April through 

September). Further details of the water-quality analyses are 

given in a report by MacCoy (2004). 

The number and frequency of available calibration data 

varied from site to site. The total number of concentration 

measurements suitable for use in the calibration files also 

varied, depending on the constituent, but ranged from 55 at 

   Phosphorus and Suspended Sediment Load Estimates for the Lower Boise River, Idaho, 1994 – 2002
Diversion (SS) to 79 at Glenwood (TP and OP). Overall, more 

measurements were collected at Glenwood and Parma than at 

Diversion and Middleton. In addition, measurements were not 

evenly distributed in time throughout the estimation period. 

For example, in 1997 only two TP and OP concentration 

measurements were available for Diversion (both collected 

during high-flow periods), but nine measurements were 

available for 2002 and covered a wide range of flow 

conditions. 

After this report was completed, the SS data in the USGS 

database were updated to include new analyses of samples, 

which had been collected between 1994 and 2003, but which 

were not entered in the database until mid-2005. Although 

the new data were not used in our load estimates, new 

LOADEST SS load estimates based on the complete data were 

not significantly different from our results (Greg Clark, U.S. 

Geological Survey, oral commun., 2005).

Concentrations of TP and OP generally were highest in 

winter, when river flows are low. Conversely, concentrations 

were lowest in spring and early summer, when flows are 

highest. Suspended sediment showed the opposite pattern and 

was highest during high flows. The relations between SS, TP, 

and OP concentrations and discharge for the four sites are 

shown in figure 2.

The Glenwood site is about 1 mi downstream from 

an outlet of a wastewater-treatment plant that contributed 

an average of 330 lb/d of TP to the Boise River during the 

estimation period (City of Boise, written commun., 2002) 

Although discharge and concentration data for the effluent 

exist and attempts were made to adjust the Glenwood data 

to compensate for this known input, meaningful results were 

not obtained. Therefore, phosphorus concentrations for the 

Glenwood site include contributions from this source. 

Discharge Data

Instantaneous discharge was measured using standard 

USGS methods (Rantz and others, 1982) at the same time 

water-quality samples were collected. Instantaneous discharge 

measurements used in the calibration files usually were 

within a few percent of the daily average discharge recorded 

at the gage for that day, but a few measurements differed by 

more than 10 percent. One possible explanation for these 

discrepancies is a sudden change in discharge during a single 

day as a result of irrigation water-management practices 

(turning on or off a diversion), causing the daily average flow 

to be much different than the instantaneous measurement. 

Instantaneous loads were calculated using the onsite discharge 

measurements; the software used daily average discharge data 

to estimate daily, monthly, and seasonal loads (see section, 

“Estimation Files”).

Water-Quality Data

Depth- and width-integrated water samples for 

constituent concentration analysis were collected, processed, 

and preserved according to the methods described by Wilde 

and others (1999). Phosphorus and dissolved orthophosphorus 

were analyzed by the USGS National Water-Quality 

Laboratory according to the methods described by Fishman 

(1993); quality-assurance/quality-control protocols described 

by Pritt and Raese (1995) were followed. Suspended sediment 

was analyzed by the USGS Cascades Volcano Observatory 

Sediment Laboratory using methods described by Guy (1969). 

Concentrations of OP, TP, and SS were reported in milligrams 

per liter. Load estimates produced by LOADEST were in 

pounds per day for OP and TP and in tons per day for SS. 

LOADEST is capable of handling censored data with the 

AMLE method. Censored data were a substantial part of the 

input data only at Diversion, where nearly one-half the data for 

TP and OP were reported to be less than the detection limits. 

Minimum detection limits for TP and OP varied throughout 

the data-collection period because analytical methods changed 

during that time. Detection limits for TP ranged from 0.01 to 

0.06 mg/L; detection limits for OP ranged from 0.01 to 0.02 

mg/L.

In 9 of 88 sampling events at Glenwood, OP 

concentrations slightly exceeded the concurrent TP 

concentrations. This suggested a problem with these analyses, 

because OP should always be less than or equal to TP. 

However, it was not certain whether TP or OP was in error, so 

both OP and TP measurements for those dates were omitted 

from the calibration files. The remaining 79 data points were 

sufficient to perform the regressions. 

Estimation Files

Estimation files containing daily average discharge 

data, in cubic feet per second, were used by the software to 

estimate daily, monthly, and seasonal loads from January 19, 

1994, through September 30, 2002. The software estimates 

loads only for days for which discharge data are provided by 

the user. The number of days for which discharge data were 

available for each site are shown in table 1. The maximum 

possible number of days in the estimation period was 3,177.

Input Data 
1,000

100

10

1

0.1

0.01

600

60

6

0.6

0.06

300

30

3

0.3

0.03

100 300 600 1,000 3,000 6,000 10,000

1,000

100

10

1

0.1

0.01

600

60

6

0.6

0.06

300

30

3

0.3

0.03

100 300 600 1,000 3,000 6,000 10,000

Total phosphorus

Dissolved orthophosphorus

Suspended sediment

CONCENTRATION, IN MILLIGRAMS PER LITER

DISCHARGE, IN CUBIC FEET PER SECOND

DIVERSION GLENWOOD

MIDDLETON PARMA

Figure 2. Relation between constituent concentration and discharge at four sites on the lower Boise River, Idaho, January 1994 

through September 2002.

 Phosphorus and Suspended Sediment Load Estimates for the Lower Boise River, Idaho, 1994 – 2002
Daily average discharge data for the estimation input files 

were obtained from various sources. Complete daily average 

discharge data for the estimation period were available from 

USGS gaging station records for Glenwood; Parma lacked 

only 5 days. Discharge data for Middleton were available 

from USGS and Idaho Power Company data archives, but data 

were lacking for nine time periods totaling 1,025 of the 3,177 

days in the estimation period (including the entire year 1998). 

Complete data were available only for 2001; as a result, there 

are many omissions and discontinuities in the load estimates 

for Middleton. 

The USGS gaging station at Diversion was discontinued 

on September 30, 1994. Provisional discharge data for 

Diversion from October 1, 1994, through September 30, 

2001, were obtained from the Bureau of Reclamation online 

data server, Hydromet (http://www.usbr.gov/pn/hydromet/). 

These values are not direct measurements and were obtained 

by subtracting the measured discharge in the New York Canal 

from the discharge at the USGS gaging station 13202000 

(Boise River near Boise). Discharge data from October 1, 

2001, through September 30, 2002, are not available from 

Hydromet but were calculated by subtraction in the same 

manner, using discharge data from the New York Canal 

provided by Idaho Power Company and published data from 

USGS gaging station 13202000. 

Discharge from 1995 through 1999 was relatively high 

and peaked in February 1997 at approximately 8,000 ft3/s at 

Parma. In 1994 and from 2000 through 2002, discharge, even 

during spring runoff, was relatively low. A graph of monthly 

average discharge data for the four sites is given in figure 3. 

Other Input Data

Other routine input data necessary for LOADEST to 

perform the regression and load estimations, such as number 

of constituents and the reporting units of constituents, were 

provided in input files. In addition, four seasons, based on the 

following dates, were defined to allow the software to perform 

optional seasonal estimations: spring, March 1 through May 

31; summer, June 1 through August 31; autumn, September 

1 through November 30, and winter, December 1 through 

February 29. 

Year

Diversion Glenwood Middleton Parma

Number 

of days

Percentage 

of year

Number 

of days

Percentage 

of year

Number 

of days

Percentage 

of year

Number 

of days

Percentage 

of year

1994 347 95 347 95 347 95 347 95

1995 361 99 365 100 352 96 365 100

1996 366 100 366 100 205 56 366 100

1997 364 100 365 100 110 30 365 100

1998 365 100 365 100 0 0 365 100

1999 360 99 365 100 202 55 365 100

2000 346 95 366 100 310 85 366 100

2001 365 100 365 100 365 100 365 100

2002 266 73 273 75 261 72 268 73

Entire estimation 

period

3,140 99 3,177 100 2,152 68 3,172 100

Table 1. Number of days per year and percentage of year for which daily loads were estimated at four sites on the lower Boise River, Idaho, 

January 19, 1994, through September 30, 2002.

[Site locations are shown in figure 1]

Input Data 
LOADEST Modeling Results

Regression Evaluation 

Coefficients of determination (R2) for the best-fit 

regression models for loads of TP, OP, and SS for the four 

studied sites (table 2) tended to be somewhat inflated because 

of the form of the regression equation. Because load is a 

function of flow (see equation 1), a strong relation (high 

R2 ) is expected, unless there are very large variations in 

concentration. 

The relatively high R2 values indicate that, with few 

exceptions, the models for all three constituents successfully 

simulated the variability in constituent loads at most sites. 

An exception was Glenwood, where R2 values for OP and TP 

were 26 and 58 percent, respectively. Overall, results for SS 

exhibited the highest R2 values.

Although the R2 values for most constituents at 

Middleton were not notably smaller than at the other sites, the 

resulting load estimates were of limited usefulness because 

they were intermittent and contained large gaps owing to 

missing discharge data. The load estimates for Middleton are 

discussed in this report, with the caveat that they are based on 

limited and intermittent data. 

Measured instantaneous TP loads for all sites were 

plotted against estimated loads for the same day to visually 

assess the fitness of the model (fig. 4). Points falling above 

the 1:1 line are cases in which the estimated value was smaller 

than the measured value; points below the line are cases in 

which the estimated value was larger than the measured value. 

If the model were perfect, all points would fall on the line. 

In most cases, the points are close to and evenly distributed 

across the 1:1 line, indicating that the model neither 

systematically overestimated nor underestimated loads. At 

Diversion, more points lie above the line, possibly suggesting 

a slight negative bias (that is, the model systematically 

underestimated TP loads at this site). At Middleton and 

at Parma, points are more scattered at very large loads, 

suggesting that the model was not as successful in estimating 

TP loads in this range of values as it was at smaller values. 

The large amount of scatter in the graph for Glenwood reflects 

the low R2 and the poor fit of the model for the TP regression 

at this site (table 2). 







        









$IVERSION

'LENWOOD

-IDDLETON

0ARMA

.ODATA

9%!2

$)3#(!2'%).#5")#&%%40%23%#/.$

Figure 3. Monthly average discharge at four sites on the lower Boise River, Idaho, January 1994 through September 2002.

 Phosphorus and Suspended Sediment Load Estimates for the Lower Boise River, Idaho, 1994 – 2002
Table 2. Regression coefficients and coefficients of determination (R2) for load models used to estimate total phosphorus, dissolved orthophosphorus, 

and suspended sediment at four sites on the lower Boise River, Idaho, January 14, 1994, through September 30, 2002.

[Site locations are shown in figure 1. The regression equation is ln(L) = a + b(lnQ) + c(lnQ2) + d [sin(2πT)] + e[cos(2πT)] + fT + gT2: where L is the constituent 

load, in pounds per day or tons per day; Q is stream discharge, in cubic feet per second; T is time in decimal years from the beginning of the calibration period; 

a,b,c,d,e,f, and g are regression coefficients; R2 represents the amount of variance explained by the model. –. coefficient for term not used in the model]

Site name Regression coefficient R2

a b (percent) c d e f g

Total phosphorus

Diversion 3.433 1.150 0.208 -0.491 0.306 0.108 – 75

Glenwood 5.458 .463 – – – – – 58

Middleton 5.993 .495 – -.463 .321 – – 68

Parma 7.111 .490 -.170 -.087 .203 -.017 – 86

Dissolved orthophosphorus

Diversion 3.337 1.123 0.022 -0.370 0.124 0.019 -0.022 89

Glenwood 5.056 .308 – – – – – 26

Middleton 5.704 .374 – -.581 .325 -.044 – 71

Parma 6.803 .439 -.151 -.229 .172 -.027 – 85

Suspended sediment

Diversion 8.793 1.198 0.161 -0.323 0.409 -0.0003 0.045 71

Glenwood 9.810 1.577 – .003 .344 -.094 – 86

Middleton 10.093 1.345 – .290 .350 – – 82

Parma 12.048 .941 -.246 .491 -.671 -.074 – 81

The relation between estimated and measured OP loads 

at Diversion (fig. 5) suggests that the model underestimated 

large loads at this site but performed reasonably well at 

moderate loads. At Glenwood, scatter is very large; the R2

for this regression is lower than for all others. Note that the 

model selected by the software has only two terms (table 2), 

indicating that the load at Glenwood is best described simply 

as a function of discharge. Including terms that described load 

as a function of time did not improve the fitness of the model. 

The points for OP at Middleton are moderately scattered; 

the sparseness of the data contributed to time gaps in the 

results. Points for Parma are tightly clustered near the line, 

reflecting the relatively high R2 (85 percent) and overall fitness 

of the model. 

Similar relations between estimated and measured SS 

loads at Parma, Glenwood, and Diversion (fig. 6) indicate 

reasonably tight distributions near the 1:1 line over a wide 

range of loads. At Middleton, however, large loads were not as 

well modeled. 

LOADEST provides information on the errors associated 

with the load estimates, including upper and lower limits of 

the 95-percent CI for each monthly estimate. Confidence 

intervals varied with flow conditions in many cases. For 

example, at Parma, the largest CIs for all constituents (as a 

percentage of the estimated load) were associated with flows 

lower than about 800 ft3/s and higher than about 6,000 ft3/s. 

This is because the model is best suited to the overall flow 

conditions and is less successful at predicting loads under 

extreme flow conditions. 

Monthly average load estimates for SS generally were 

the least precise; the average width of the 95-percent CI, as a 

percentage of the mean estimated load of SS, ranged from 88 

percent at Parma to 129 percent at Diversion. In contrast, TP 

and OP average CIs at Parma were about 24 and 23 percent, 

respectively, of the estimated monthly average loads. 

LOADEST Modeling Results   
































             











   











     

,.4/4!,0(/30(/253-%!352%$,/!$).0/5.$30%2$!9

,.4/4!,0(/30(/253%34)-!4%$,/!$).0/5.$30%2$!9

$)6%23)/. ',%.7//$

-)$$,%4/. 0!2-!



Figure 4. Relation between estimated and measured loads of total phosphorus at four sites on the lower Boise River, Idaho, January 

1994 through September 2002.

10 Phosphorus and Suspended Sediment Load Estimates for the Lower Boise River, Idaho, 1994 – 2002




























           















   















         

,.$)33/,6%$/24(/0(/30(/253-%!352%$,/!$).0/5.$30%2$!9

,.$)33/,6%$/24(/0(/30(/253%34)-!4%$,/!$).0/5.$30%2$!9

$)6%23)/. ',%.7//$

-)$$,%4/. 0!2-!



Figure 5. Relation between estimated and measured loads of dissolved orthophosphorus at four sites on the lower Boise River, 

Idaho, January 1994 through September 2002.

LOADEST Modeling Results 11
16

15

14

13

12

11

10

9

8

15

14

13

12

11

10

9

8

7

6

6 7 8 9 10 11 12 13 14 15 8

15

14

13

12

11

10

9

8

7

6

9 10 11 12 13 14 15

6 7 8 9 10 11 12 13 14 6 7 8 9 10 11 12 13 14 15

6

7

8

9

10

11

12

13

14

LN SUSPENDED SEDIMENT ESTIMATED LOAD, IN POUNDS PER DAY

LN SUSPENDED SEDIMENT MEASURED LOAD, POUNDS PER DAY

DIVERSION GLENWOOD

MIDDLETON PARMA

1:1

Figure 6. Relation between estimated and measured loads of suspended sediment at four sites on the lower Boise River, Idaho, 

January 1994 through September 2002.

12 Phosphorus and Suspended Sediment Load Estimates for the Lower Boise River, Idaho, 1994 – 2002
Estimated Loads 

Although the model provided daily load estimates, 

they are not discussed in detail in this report because of 

their limited usefulness (see the section “Limitations and 

Advantages of the LOADEST Model”). Monthly and seasonal 

(as defined in the section “Other Input Data”) average loads 

were a more suitable way to synthesize the data because 

day-to-day variation (which can be misleading and cause 

invalid interpretation) tended to be minimized. Therefore, 

most of the discussion in the following sections refers to 

monthly, seasonal, and annual load estimates. In the case of 

the Middleton site, monthly averages were calculated only 

for months with complete data. This eliminated 38 of the 

105 months; therefore, Middleton data might not be entirely 

representative of conditions during the estimation period.

Although the model generally closely estimated loads 

under the predominant flow conditions, estimated loads 

for extreme flow conditions had larger errors, as discussed 

previously in the section “Regression Evaluation.” At Parma, 

for example, considerable errors in estimated versus measured 

loads were observed at high and low flows (fig. 7). The 

relation for SS at Middleton was similar. 

Annual loads for all sites for all constituents from 

1994 through 2002 were calculated to give an overview of 

loads throughout the estimation period. Note that data for 

2002 represent only 9 months, not a complete year, and that 

Middleton data for several years were incomplete. Annual 

loads were calculated as the sum of the daily loads in each 

year (table 3). Estimated annual loads of TP were largest in 

1997, the year of highest discharge. 







        









%STIMATEDDAILYAVERAGELOAD

-EASUREDINSTANTANEOUSLOAD

9%!2

4/4!,0(/30(/253,/!$).0/5.$30%2$!9

Figure 7. Estimated daily average loads and measured instantaneous loads of total phosphorus in the lower Boise River at Parma, 

Idaho, January 1994 through September 2002.

LOADEST Modeling Results 13
Total Phosphorus

Time-series graphs comparing monthly average TP loads 

at all four sites displayed the cyclical nature of the variation in 

loads with seasonal fluctuations in discharge, even though the 

dates of peak discharge were not the same every year 

(fig. 8). Overall, TP estimates were comparable at Middleton 

and Glenwood; loads at these sites rarely exceeded 1,000 lb/d. 

Loads at Diversion were smallest of all, averaging less than 

250 lb/d for the entire estimation period. Estimated loads at 

Parma were notably larger than at the other sites, averaging 

nearly 2,200 lb/d and exceeding that value nearly 30 percent 

of the time. This result was expected, because both flows 

and constituent concentrations are higher at Parma. Peak TP 

loads for the estimation period occurred during January and 

February 1997 at all sites, coinciding with maximum flows. 

Estimated loads at Parma during that time approached 5,000 

lb/d, whereas estimated loads at Glenwood and Diversion 

exceeded 1,500 lb/d. Loads at all sites were smallest in the late 

summer months throughout the estimation period. 

Average TP loads, by month, were calculated by 

averaging the monthly averages for each month of the year 

(for example, the average of all January monthly averages, all 

February monthly averages, and so on). In terms of average 

loads, by month, estimated TP loads at Parma exceeded 1,800 

lb/d from November through June and peaked in March at 

about 3,000 lb/d (fig. 9). The total load at Parma during the 

main irrigation season, May through September, exceeded 140 

tons. 

Seasonal loads of TP generally peaked in spring, 

decreased during summer and autumn, and increased again 

in winter (fig. 10), reflecting fluctuations in discharge as a 

result of the combined effects of seasonal runoff patterns and 

irrigation water management, the exact timing of which vary 

from year to year. 

Annual loads of TP at Parma ranged from 222 tons in 

2001 to 570 tons in 1997. In contrast, estimated annual loads 

at Diversion ranged from less than 8 tons (1994) to 118 tons 

(1997). Estimated loads at Glenwood ranged from 68 tons 

(1994) to 157 tons (1997). Data for Middleton were complete 

only for 2001; the estimated total load for that year was 92 

tons (table 3). 

Table 3. Annual loads of total phosphorus, dissolved orthophosphorus, and suspended sediment at four sites on the lower Boise River, Idaho, 

January 1994 through September 2002.

[Site locations are shown in figure 1. Annual loads, in tons, were calculated as the sum of the daily loads for each day of the year. Number of days per year for 

each site is shown in table 1. Numbers were rounded to three significant figures. –, no data. ]

Site name 1994 1995 1996 1997 1998 1999 2000 2001 2002

Total phosphorus

Diversion 8 22 63 118 43 59 23 14 119

Glenwood 68 96 138 157 115 117 89 69 162

Middleton 92 130 66 29 – 65 92 92 165

Parma 255 419 533 570 455 456 364 222 1181

Dissolved orthophosphorus

Diversion 6 15 33 51 26 29 15 8 19

Glenwood 54 67 86 93 76 77 65 55 146

Middleton 100 121 164 124 – 155 77 76 149

Parma 212 325 395 416 334 331 271 174 1131

Suspended sediment

Diversion 7,400 13,000 25,300 34,100 10,600 12,500 5,200 3,400 15,400

Glenwood 4,930 23,100 75,400 109,000 30,300 38,200 7,800 2,230 12,960

Middleton 2,400 16,600 12,000 11,670 – 12,280 2,710 1,940 12,540

Parma 35,700 116,000 160,000 167,000 116,000 104,000 56,100 17,600 123,100

1Totals represent fewer than 300 days of data.

14 Phosphorus and Suspended Sediment Load Estimates for the Lower Boise River, Idaho, 1994 – 2002














        

$IVERSION

'LENWOOD

-IDDLETON

0ARMA

.ODATA

4/4!,0(/30(/253,/!$).0/5.$30%2$!9

9%!2

Figure 8. Estimated monthly average loads of total phosphorus at four sites on the lower Boise River, Idaho, January 1994 

through September 2002.

















*ANUARY &EBRUARY -ARCH !PRIL -AY *UNE *ULY !UGUST 3EPTEMBER /CTOBER .OVEMBER $ECEMBER

$IVERSION

'LENWOOD

-IDDLETON

0ARMA

4/4!,0(/30(/253,/!$

).0/5.$30%2$!9

-/.4(

Figure 9. Estimated average loads of total phosphorus, by month, at four sites on the lower Boise River, Idaho, 

January 1994 through September 2002.

LOADEST Modeling Results 15
Dissolved Orthophosphorus

Monthly loads of OP generally peaked in early months 

of the year (January through May) throughout the estimation 

period, coinciding with times of high discharge. The largest 

monthly load, about 3,800 lb/d, was estimated at Parma in 

January 1997. Monthly OP loads at Diversion (654 lb/d) and at 

Glenwood (739 lb/d) were largest in February 1997. 

Annual loads of OP peaked in 1997 at all sites (table 3). 

Loads were largest at Parma and ranged from 174 tons in 2001 

to more than 400 tons in 1997, the year of highest discharge. 

Annual loads of OP at Glenwood averaged about 70 tons 

during the estimation period. Annual loads at Diversion ranged 

between 6 and 51 tons. Annual OP loads at Parma fluctuated 

by the largest amount throughout the estimation period, both 

in terms of range in absolute load and relative percentage of 

load. Although the magnitude of the OP load at Diversion 

was usually only about 5 percent of the OP load at Parma, 

large relative annual fluctuations in OP also were estimated at 

Diversion. 

The magnitudes of OP loads mimicked those of TP loads 

because OP is a component of TP, but the fraction of OP (the 

ratio of OP to TP) varied considerably throughout the year. 

Therefore, it was useful to view OP estimates as a ratio of OP 

to TP. Clear cyclical variations in the ratio are visible in 

figure 11. Peaks in the OP:TP ratio (that is, times when the 

OP:TP ratio is high) occurred primarily when flows were 

at their lowest annual stages; conversely, when flows were 

high, the ratio was low. This probably reflects increased TP 

associated with particulate matter during high flows. Although 

this pattern generally is true at most sites, the remarkably 

consistent variation of the ratio at Parma from year to year 

despite differences in annual flow suggests that seasonal 

(perhaps biological) processes affect the OP:TP ratio. 

At Parma, OP:TP ratio minima mimicked those at 

Glenwood but did not exactly coincide with peak flows for 

1996, 1997, and 1999 (several of the highest flow years). 

Instead, these minima occurred on the descending limb of the 

hydrographs. At Diversion, flow and the OP:TP ratio were 

inversely related during high flow years but, in 1994 and from 

2000 through 2002 (all low-flow years), high flows coincided 

with high OP:TP ratios, not with low ones. At Middleton, the 

OP:TP ratio was higher than 1 during the low-flow periods 

from 1994 through 1996. Although the calibration data did not 

include any cases where the OP value exceeded the TP value, 

the regressions resulted in estimates in which OP was higher 

than TP at Glenwood. As previously discussed, this situation 

cannot actually exist and must be interpreted as error in the TP 

estimate, the OP estimate, or both estimates. 

3,000

2,500

2,000

1,500

1,000

500

0

Spring Summer Autumn Winter 

2,500

2,000

1,500

1,000

500

0

600

500

400

300

200

100

 0

Diversion 

Glenwood

Middleton

Parma 

CONSTITUENT LOAD, IN POUNDS PER DAY 

SEASON 

CONSTITUENT LOAD, IN TONS PER DAY 

TOTAL PHOSPHORUS 

DISSOLVED ORTHOPHOSPHORUS 

SUSPENDED SEDIMENT 

Figure 10. Estimated seasonal loads of total phosphorus, 

dissolved orthophosphorus, and suspended sediment at four 

sites on the lower Boise River, Idaho, January 1994 through 

September 2002.

16  Phosphorus and Suspended Sediment Load Estimates for the Lower Boise River, Idaho, 1994 – 2002




































































































                 

$ISCHARGE

2ATIOOFDISSOLVEDORTHO


PHOSPHORUSTOTOTAL

PHOSPHORUS

.ODATA

2!4)//&$)33/,6%$/24(/0(/30(/2534/4/4!,0(/30(/253

9%!2

$)6%23)/. ',%.7//$

-)$$,%4/. 0!2-!

$)3#(!2'%).#5")#&%%40%23%#/.$

Figure 11. Estimated monthly average ratio of dissolved orthophosphorus to total phosphorus and monthly average 

discharge at four sites on the lower Boise River, Idaho, January 1994 through September 2002.

LOADEST Modeling Results 17
Estimated seasonal values of the OP:TP ratio were 

highest in autumn at all sites (fig. 12; Middleton is excluded 

from this discussion because of insufficient data). The ratio 

was highest at Parma during all seasons. In autumn, nearly 90 

percent of the TP was in the OP:TP ratio, but even in spring, 

that portion was not less than 60 percent. Similar, but slightly 

lower, values were estimated for the OP:TP ratio at Glenwood. 

The estimated values for Diversion were notably lower during 

winter and spring, when less than one-half the TP was in the 

dissolved state. 

Viewing average OP:TP ratio by month (fig. 13) further 

highlights the pronounced difference in ratio between 

Diversion and the other sites. Whereas the OP:TP ratio for the 

other sites was highest in November or December, decreased 

from January through May and increased again after June, 

estimates for Diversion showed nearly the opposite pattern: 

the estimated ratio was highest in July and lowest in January 

and February (also see fig. 11). Although the reason for this 

difference presently is not well understood, it might be a result 

of complex biological and geochemical processes involving 

nutrient cycling in Lucky Peak Lake, about 2 mi upstream of 

the site at Diversion. Additional information about thermal, 

biological, and chemical conditions in the lake throughout the 

year might shed light on the mechanisms contributing to this 

phenomenon. 

On an annual basis, the OP:TP ratio remained fairly 

constant during the estimation period at most sites. The 

average annual ratio at Parma was particularly consistent from 

year to year: the ratio varied only about 10 percent between 

1994 and 2002 (0.73 to 0.83). At Diversion, the average 

annual ratio remained close to about 0.60 until it decreased to 

about 0.40 in 2001. 





















 3PRING 3UMMER !UTUMN 7INTER

$IVERSION

'LENWOOD

0ARMA

2!4)//&$)33/,6%$/24(/0(/30(/253

4/4/4!,0(/30(/253

3%!3/.

Figure 12. Estimated seasonal ratio of dissolved 

orthophosphorus to total phosphorus at three sites on 

the lower Boise River, Idaho, January 1994 through 

September 2002.

















*ANUARY &EBRUARY -ARCH !PRIL -AY *UNE *ULY !UGUST 3EPTEMBER /CTOBER .OVEMBER $ECEMBER

$IVERSION

'LENWOOD

-IDDLETON

0ARMA

2!4)//&$)33/,6%$/24(/0(/30(/253

4/4/4!,0(/30(/253

-/.4(

Figure 13. Estimated average ratio of dissolved orthophosphorus to total phosphorus, by month, at four sites on the lower 

Boise River, Idaho, January 1994 through September 2002.

18  Phosphorus and Suspended Sediment Load Estimates for the Lower Boise River, Idaho, 1994 – 2002
1994 1995 1996 1997 1998 1999 2000 2001 2002 

0

200

400

600

800

1000

1200

1400

Diversion

Parma

Middleton

Glenwood

SUSPENDED SEDIMENT LOAD, IN TONS PER DAY 

YEAR 

Figure 14. Estimated monthly average suspended sediment load at four sites on the lower Boise River, Idaho, January 1994 through 

September 2002.

Suspended Sediment

Overall, estimated SS loads were largest at Parma 

(fig. 14), where the average daily load for the estimation 

period was about 260 ton/d. The maximum monthly loads 

at Parma occurred in May 1995, 1996, and 1997, when peak 

monthly loads ranged from 1,120 to 1,220 ton/d. In contrast, 

the smallest average daily SS loads, about 37 ton/d, were 

estimated at Diversion. Maximum monthly loads at Diversion 

occurred in March 1996 (about 220 ton/d) and in February 

1997 (about 260 ton/d). 

Seasonal loads of SS were largest in spring and smallest 

in autumn at all sites (fig. 14), corresponding with variations 

in discharge. The relative range in SS loads was greatest at 

Glenwood, where maximum loads exceeded minimum loads 

by a factor of about 30. In contrast, maximum loads at Parma 

exceeded minimum loads by only a factor of 8.

Total loads of suspended sediment from 1994 through 

2002 at Parma ranged from 17,600 tons in 2001 to 167,000 

tons in 1997, whereas annual loads at Diversion ranged 

from 3,400 to 34,100 tons for the same years (table 3). At 

Glenwood, annual SS loads ranged from 2,230 to 109,000 

tons. Average annual loads for the estimation period were 

13,000 tons at Diversion, 33,000 tons at Glenwood, and 

88,000 tons at Parma. 

LOADEST Modeling Results 19
Monthly Average Increases and 

Decreases of Constituent Loads 

Within Reaches

Monthly average increases and decreases of the 

constituent loads in the three reaches between the four 

modeled sites were calculated by subtracting the monthly 

average load at the upstream site from the monthly average 

load at the downstream site. Positive differences indicated an 

increase; negative differences indicated a decrease. Because 

the data for Middleton were incomplete, increases and 

decreases for the Glenwood-to-Middleton and Middleton-toParma reaches were based on fewer months of data than were 

increases and decreases for the Diversion-to-Glenwood reach. 

Therefore, data for these reaches might not adequately reflect 

the actual monthly average increases and decreases for the 

entire estimation period. Estimated monthly average increases 

and decreases of TP, OP, and SS for the reaches Diversion to 

Glenwood, Glenwood to Middleton, and Middleton to Parma 

from January 1994 through September 2002 are shown in 

figure 15. 

Diversion to Glenwood

TP and OP were increasing in the reach from Diversion 

to Glenwood during most of the estimation period. In that 

reach, daily decreases in TP that occurred during high flows 

in January and February 1997 were not reflected in the 

monthly averages. Increases in TP were largest in March 1998, 

but large monthly increases also were typical in April and 

May. The cyclical pattern of fluctuating increases followed 

the seasonal discharge patterns— increases were largest in 

spring and decreased throughout the summer. Minima usually 

occurred in winter and early spring. Monthly average TP 

increases ranged from about 200 to about 350 lb/d for the 

estimation period; monthly average OP increases ranged from 

250 to 300 lb/d. Annual TP increases in this reach averaged 60 

tons per year for the estimation period. 

Annual increases in SS from Diversion to Glenwood were 

predominant from 1994 to 2002. Increases ranged from 327 

tons in 1995 to 2,500 tons in 1997, the highest-flow year in the 

estimation interval. Small annual decreases ranging from 40 to 

83 tons were estimated for 1994, 2001, and 2002. 

Glenwood to Middleton

Incomplete estimates for the Middleton site hindered 

adequate interpretation of increases and decreases except for 

a few time intervals. Small to moderate monthly decreases in 

TP were estimated during May through August in the reach 

from Glenwood to Middleton; large increases, sometimes 

more than 600 lb/d, were estimated for December and January 

(fig. 15). Orthophosphorus followed the same cyclical 

pattern. Suspended sediment exhibited only small increases 

or decreases during these periods. Increases, representing 

periods of sediment erosion or addition of sediment to the 

system, occurred from September through April; decreases, 

representing deposition, occurred from May through August.

Middleton to Parma

Again, increases and decreases for the reach from 

Middleton to Parma were difficult to assess because of 

incomplete results for Middleton. This probably is the reason 

that a cyclical fluctuating pattern of relative increases was not 

as well defined as it was in the other reaches. Data indicated 

increases, but no decreases, during the entire estimation 

period. Large increases were estimated in TP, OP, and SS in 

May 1995. Incomplete data indicated large increases of SS 

in June 1996 and of TP in December 1996. Large increases 

occurred during high-flow conditions, indicating active 

erosion or influx of sediment from tributaries. 

20 Phosphorus and Suspended Sediment Load Estimates for the Lower Boise River, Idaho, 1994 – 2002
1994 1995 1996 1997 1998 1999 2000 2001 2002 

900

700

500

300

100

-100

-300

-500

-700

90

70

50

30

10

-10

-30

-50

-70

3,000

2,500

2,000

1,500

1,000

500

0

1,200

1,000

800

600

400

200

0

-300

-200

-100

0

100

200

300

400

500

600

700

-300

-200

-100

0

100

200

300

400

500

600

700

Total phosphorus

Orthophosphorus

Suspended sediment

No data

TOTAL PHOSPHORUS AND DISSOLVED ORTHOPHOSPHORUS GAIN OR LOSS, IN POUNDS PER DAY 

YEAR 

SUSPENDED SEDIMENT GAIN OR LOSS, IN TONS PER DAY 

Diversion Dam to Glenwood 

Glenwood to Middleton

Middleton to Parma

Figure 15. Estimated monthly average increases and decreases of total phosphorus, dissolved orthophosphorus, and suspended 

sediment in three reaches of the lower Boise River, Idaho, January 1994 through September 2002.

Monthly Average Increases and Decreases of Contituent Loads Within Reaches 21
Temporal Trends

If loads change consistently as a function of time 

throughout the estimation period, LOADEST would determine 

that a trend is present. A trend is indicated in the modeled 

equation by the presence of a term with time as the variable 

(term fT in equation 1). If the coefficient of this term (f

in table 2) is positive, an upward trend is indicated; if the 

coefficient is negative, a downward trend in indicated. The 

size of the coefficient suggests the relative strength of the 

trend. Statistically significant (95-percent confidence level) 

downward trends were determined in SS at Glenwood, in OP 

at Middleton, and in TP, OP, and SS at Parma (table 4). A 

significant upward trend in TP was determined at Diversion. 

Other regression equations suggested trends, but the trends 

were not statistically significant at the 95-percent confidence 

level. 

The percentage of change per year produced by the trend 

can be calculated with the following equation:

Percentage of change per year = x 100 ( ) e 1 f − (2)

where

f Is the coefficient of the time term.

Annual percentage of changes are shown in table 4.

Table 4. Annual trends and significance level of trends in total phosphorus, dissolved orthophosphorus, and suspended sediment loads at four sites on 

the lower Boise River, Idaho, January 1994 through September 2002.

[Site locations are shown in figure 1. Annual trends: +. upward trend; -, downward trend, --, no trend found; p-value indicates statistical significance of trends; 

p-values greater than 0.05 are not statistically significant at the 95-percent confidence level. n.s., not statistically signficant; <, less than]

Constituent load

Diversion Glenwood Middleton Parma

Annual trend 

(percent) p-value Annual trend (percent) p-value Annual trend (percent) p-value Annual trend (percent) p-value

Total phosphorus +11 0.002 -- -- -- -- -2 0.031

Dissolved

orthophosphorus

n.s. .387 -- -- -4 0.015 -3 <.001

Suspended sediment n.s. .995 -9 0.008 -- -- -7 .014

22 Phosphorus and Suspended Sediment Load Estimates for the Lower Boise River, Idaho, 1994 – 2002
Table 5. Comparison between daily average loads of total phosphorus 

estimated by mass balance spreadsheet method and daily average loads 

and monthly average 95-percent confidence intervals estimated by 

LOADEST at three sites on the lower Boise River, Idaho, April 15, 1996 

through October 15, 1996.

[Site locations are shown in figure 1. Estimated load values: Spreadsheet—

Values taken from preliminary spreadsheet provided by Idaho Department 

of Environmental Quality (written commun., 2004). LOADEST—Values 

represent upper and lower limits of 95-percent confidence interval for monthly 

load estimates, April through October 1996. CI, confidence interval]

Site name

Estimated load

(pounds per day)

Spreadsheet

LOADEST

Mean

Upper 

95-percent

CI

Lower 

95-percent 

CI

Diversion 303 288 493 172

Glenwood 665 779 960 630

Parma 2,560 2,431 2,837 2,266



















!PRIL -AY *UNE *ULY !UGUST 3EPTEMBER /CTOBER

,/!$%34MEANLOADESTIMATE

,/!$%34LOWER
PERCENTCONFIDENCELIMIT

,/!$%34UPPER
PERCENTCONFIDENCELIMIT

3PREADSHEETLOADESTIMATE

4/4!,0(/30(/253,/!$

).0/5.$30%2$!9

-/.4(

Figure 16. Monthly average loads of total phosphorus at Parma, Idaho, estimated using LOADEST model and using mass 

balance spreadsheet method, April 15, 1996, through October 15, 1996.

Comparison with Mass Balance 

Spreadsheet Load Estimates

A mass balance spreadsheet method for estimating loads 

currently is being developed by the IDEQ to track phosphorus 

loads in the lower Boise River. This method is an accounting 

system of inputs and outputs of TP from Lucky Peak Dam 

to Parma for a particular time interval. Loads are calculated 

as the product of flow and TP concentration. Cumulative TP 

loads at about 90 points (including the four mainstem sites at 

Diversion, Glenwood, Middleton, and Parma) along the lower 

Boise River from Lucky Peak Dam to Parma are calculated 

from measured and estimated data, including inflow and 

outflow from irrigation diversions and returns, ground water, 

and tributaries. Because one goal of the LOADEST modeling 

was to refine the spreadsheet method, TP load estimates from 

a preliminary spreadsheet from April 15 through October 

15, 1996, were compared with LOADEST estimates for the 

same period (table 5). For the sites at Diversion and Parma, 

the LOADEST estimates of daily average TP load were about 

5 percent lower than the spreadsheet estimates; LOADEST 

estimates for Glenwood were about 17 percent higher than the 

spreadsheet estimates. LOADEST estimates for Middleton 

were incomplete because of lack of discharge data for this 

time interval. 

Although the similarity of the results for the daily 

average loads is not surprising (both methods use the same 

concentration and flow data for these sites), the LOADEST 

results provide detail that the spreadsheet does not. For 

example, a graph comparing the spreadsheet estimates with the 

monthly TP load estimates and 95-percent CIs at Parma from 

April through October shows that the average load (2,560 lb/d 

by the spreadsheet method) probably was exceeded during 

approximately the first 2 months of the period but decreased to 

less than about 2,000 lb/d during the remaining months 

(fig. 16). Note that the LOADEST monthly estimates include 

the entire months of April and October. The ability to examine 

loads readily at various timescales, rather than as a single 

static average, is a key advantage of the LOADEST estimation 

method over the spreadsheet method.

Comparison with Mass Balance Spreadsheet Load Estimates 23
Annual Flow-Weighted Concentrations

Because flow is a strong determinant of load, and because 

flow varied greatly from year to year, annual flow-weighted 

concentrations (FWCs) were calculated to better assess 

changes in concentration over the estimation period. Annual 

FWCs of TP, OP, and SS were calculated for Diversion, 

Glenwood, and Parma by dividing the annual constituent load 

for each year (in pounds) by the total annual flow (in cubic 

feet) and applying appropriate conversion factors to yield 

concentration (in milligrams per liter). Annual FWCs of TP, 

OP, and SS from 1994 through 2002 are given in table 6.

At Parma and Glenwood, where the OP:TP ratio was high 

(average 0.70 to 0.75), annual FWCs of TP were inversely 

related to load; that is, FWCs were smallest during high-flow 

years, even though the estimated loads were large (fig. 17). 

This is because the OP is diluted (low concentrations) by 

high flows, but the estimated loads were large because of the 

dominance of flow. In contrast, at Diversion, where the OP:

TP ratio was low (average 0.56), load and concentration were 

directly related, because phosphorus associated with entrained 

particulate matter is increased during high flows, not diluted. 

Similar relations between annual FWC and load were apparent 

for SS. Thus, annual FWCs highlight the strong interaction 

between flow and particle-associated constituents such as TP 

and SS. 

Table 6. Annual flow-weighted concentrations of total phosphorus, dissolved orthophosphorus, and suspended sediment at four sites on the lower Boise 

River, Idaho, January 1994 through September 2002.

[Site locations are shown in figure 1. mg/L, milligram per liter; –, no data]

Site name 1994 1995 1996 1997 1998 1999 2000 2001 2002

Total phosphorus (mg/L)

Diversion 0.009 0.014 0.022 0.033 0.021 0.028 0.019 0.019 10.020

Glenwood .137 .080 .056 .048 .068 .064 .105 .148 1.115

Middleton .319 .142 1.258 1.143 – 1.238 .299 .383 1.254

Parma .412 .266 .185 .153 .214 .204 .307 .384 1.342

Dissolved orthophosphorus (mg/L)

Diversion 0.007 0.009 0.011 0.014 0.013 0.014 0.012 0.010 0.010

Glenwood .108 .056 .035 .028 .045 .042 .077 .119 .087

Middleton .346 .132 .251 .118 .200 .249 .316 .189

Parma .343 .206 .137 .112 .157 .148 .228 .301 0.248

Suspended sediment (mg/L)

Diversion 8.4 8.1 9.0 9.5 5.2 5.9 4.4 4.5 15.9

Glenwood 10 19 30 33 18 21 9.2 4.8 15.5

Middleton 8.4 18 17.8 18.3 – 18.4 8.8 8.1 110

Parma 58 74 55 45 55 47 47 30 143

1Average represents fewer than 300 days of data.

24 Phosphorus and Suspended Sediment Load Estimates for the Lower Boise River, Idaho, 1994 – 2002
























































































        

!NNUALFLOW
WEIGHTEDTOTAL

PHOSPHORUSCONCENTRATION

%STIMATEDANNUALTOTAL

PHOSPHORUSLOAD

4/4!,0(/30(/253#/.#%.42!4)/.).-),,)'2!-30%2,)4%2

9%!2

4/4!,0(/30(/253,/!$).4/.30%29%!2

$)6%23)/.

',%.7//$

0!2-!

Figure 17. Average annual flow-weighted 

concentrations and estimated loads of total 

phosphorus at three sites on the lower Boise 

River, Idaho, January 1994 through September 

2002.

Implications for Total Maximum Daily 

Load Implementation

Total Phosphorus

The current Snake River-Hells Canyon TMDL (Idaho 

Department of Environmental Quality, 2004) suggested TP 

allocations for upstream tributaries to meet TP loads. On 

the basis of average flow and TP concentrations from May 

through September, the lower Boise River contributed the 

largest load (estimate of about 2,450 lb/d) of all the tributaries 

evaluated. To reach a target TP concentration of 0.07 mg/L in 

the Snake River, TP loads in the lower Boise River must be 

decreased to 532 lb/d. A TMDL for TP for the lower Boise 

River currently is being developed; an implementation plan 

subsequently will be developed. 

LOADEST average TP load estimates from May through 

September, 1994 through 2002, ranged from 740 lb/d in 2001 

to 2,550 lb/d in 1997; the target of 532 lb/d was exceeded 

every year. These estimates indicate that reductions in load of 

about 28 to 76 percent would have been necessary to meet the 

proposed goal during this period. 

 Because load and flow are strongly related, large 

estimated loads would be expected in high-flow years. 

However, annual FWCs of TP were inversely related to 

load estimates and flow measurements at Parma; that is, 

TP concentrations at Parma were lower in high-flow years 

than in low-flow years, as discussed in the section “Annual 

Flow-Weighted Concentrations.” Annual flow-weighted 

concentrations of TP for this time interval ranged from 0.41 

to 0.35 mg/L. LOADEST TP load estimates, 95-percent upper 

and lower CIs, and annual FWCs are given in table 7. 

Suspended Sediment

The lower Boise River TMDL for suspended sediment 

was completed and load allocations determined at Parma, 

Middleton, and several tributaries to the lower Boise River 

in 1999. Lower Boise River suspended sediment allocations 

were based on the 30-day low-flow period February 15 

through June 14, the period during which the most significant 

loads of sediment are generated. According to the lower 

Boise River TMDL, the SS load allocation is 4.4 ton/d at 

Middleton and about 101 ton/d at Parma, on the basis of an 

upper-limit concentration of 50 mg/L (Idaho Department 

of Environmental Quality, 1999). The allocation at Parma 

incorporates a 37-percent decrease from 1995 calculated loads. 

 LOADEST daily average SS loads at Parma from 

February 15 to June 14, 1994 through 2002, ranged from 86 

ton/d in 2001 to 968 ton/d in 1996. Estimated annual loads 

exceeded the allocation at Parma except in 2001. In all years 

except 1999 and 2001, low-flow years, the annual FWCs of SS 

exceeded 50 mg/L. Daily average SS loads, 95-percent upper 

and lower CIs, and FWCs at Parma, estimated by LOADEST, 

are given in table 8.

Implications for Total Maximum Daily Load Implementation 25
Table 7. Estimated average annual loads, monthly average upper and 

lower 95-percent confidence intervals, total flow volumes, and annual 

flow-weighted concentrations of total phosphorus at Parma, Idaho, May 

through September, 1994 through 2002.

[Site locations are shown in figure 1. 95-percent confidence intervals: 

Values represent upper and lower limits of 95-percent confidence interval 

for monthly load estimates for May through September, 1994 through 2002; 

Abbreviations: CI, confidence interval; lb/d, pound per day; m/ft3, millions of 

cubic feet; mg/L, milligram per liter]

Year

Estimated 

average

load

(lb /d))

Lower 

95-percent 

CI

Upper 

95-percent 

CI

Total flow 

volume 

(m/ft3)

Annual 

flowweighted 

concentration 

(mg/L)

1994 1,140 970 1,330 59,970 0.35

1995 2,450 2,190 2,740 230,200 .20

1996 2,330 2,090 2,600 247,800 .17

1997 2,550 2,280 2,840 268,000 .17

1998 2,450 2,180 2,760 316,800 .14

1999 2,130 1,920 2,360 166,700 .23

2000 1,520 1,370 1,680 96,570 .29

2001 740 624 874 42,180 .32

2002 1,240 1,110 1,380 76,800 .30

Table 8. Estimated average daily loads, monthly average upper and 

lower 95-percent confidence intervals, total flow volumes, and annual 

flow-weighted concentrations of suspended sediment at Parma, Idaho, 

February 15 through June 14, 1994 through 2002.

[Site locations are shown in figure 1. 95-percent confidence intervals: 

Values represent upper and lower limits of 95-percent confidence interval 

for monthly load estimates for May through September, 1994 through 2002. 

Abbreviations: CI, confidence interval; ton/d, tons per day; m/ft3, millions of 

cubic feet; mg/L, milligram per liter]

Year

Estimated 

average

load

(ton/d))

Lower 

95-percent 

CI

Upper 

95-percent 

CI

Total flow 

volume 

(m/ft3)

Annual 

flowweighted 

concentration 

(mg/L)

1994 161 245 79 52,900 87.7

1995 562 783 338 167,000 97.1

1996 968 1,340 543 466,000 60.3

1997 931 1,360 522 499,000 53.7

1998 624 918 358 300,000 59.8

1999 585 819 338 343,000 49.2

2000 313 392 181 137,000 66.3

2001 86 119 49 50,800 49.0

2002 114 157 70 59,300 55.1

Limitations and Advantages of the 

LOADEST Model

Modeling results for Diversion, Glenwood, and Parma 

demonstrate that LOADEST can be used to gain insight to the 

timing and distribution of constituent loads in the lower Boise 

River over a range of timescales. Although the model output 

includes estimated daily loads, daily values have limitations. 

The primary reason for this is that the software is unable to 

accommodate sudden changes in streamflow and constituent 

transport that occur within a day as a result of watermanagement practices, which results in large errors, primarily 

during extremely high flows. Thus, an estimate for a particular 

day could be unrepresentative of actual loads. However, 

because the model performs well for the prevalent conditions, 

the overall effect of these inaccuracies is diminished as longer 

time periods are examined. Therefore, monthly, seasonal, and 

annual timescales are useful intervals in which to examine 

load estimates. 

As with any model, the quality of the results (or lack 

thereof) is determined by the quality of the input data. 

LOADEST estimates loads only for days for which discharge 

data are available. Because there were many gaps in the 

Middleton discharge data, load estimates for long, continuous 

periods are absent. In addition, input data should represent 

samples collected over a wide range of flow conditions in 

order for the model to be calibrated over the complete range 

of flow conditions. In this study, the best available data were 

used but, in many cases, sampling events did not represent a 

sufficiently wide range of flow conditions. For example, data 

for only two samples were available for Diversion in 1997, 

both during high springtime flows. The lack of low-flow 

samples during 1997 could affect the reliability of the results 

for that year at Diversion and also could bias the overall 

model results. Ideally, deliberate and regular sampling before, 

during, and after high flows would have improved the model’s 

ability to estimate loads accurately over a wider range of flow 

conditions. 

The TP regression equation for Glenwood indicates 

that loads were related only to flow and that terms involving 

harmonic functions or time were not important (table 2). This 

site was the only case where the regression equations took 

this form. Also, this simple regression model, although it was 

statistically the best, was able to explain only 58 percent of 

the variation in the data. This probably is related to the known 

source of phosphorus at the outlet of the wastewater-treatment 

plant 1 mi upstream. This source of phosphorus is independent 

of river discharge and could have introduced a large amount of 

variability that the model was unable to account for. 

26  Phosphorus and Suspended Sediment Load Estimates for the Lower Boise River, Idaho, 1994 – 2002
Another problem, described by Clark (2003) and Woods 

(2001), involves errors resulting from hysteresis effects, that 

is, different measured constituent concentration values for the 

same discharge value. Hysteresis is an important factor for TP 

and SS. As Clark (2003) pointed out, certain samples from 

the Coeur d’Alene River Basin that were collected during the 

ascending limb and near the peak of the hydrograph typically 

contained higher concentrations of sediment than did samples 

collected during the descending limb at the same discharge. 

This is because material that had accumulated in the stream 

channel prior to spring runoff becomes mobile as stream 

velocities rise; later, concentrations measured at the same 

discharge were low because the channel had been flushed of 



















 *ULY !UG 3EPT /CT .OV $EC *AN &EB -AR



















 !PR -AY *UNE *ULY !UG 3EPT /CT .OV $EC

$ISCHARGE

-EASUREDLOAD

%STIMATEDLOAD

-/.4(

4/4!,0(/30(/253,/!$).0/5.$30%2$!9

$)3#(!2'%).#5")#&%%40%23%#/.$

Figure 18. Daily average discharge and measured and estimated daily average loads of total phosphorus at Parma, Idaho, 

July 1, 1996, through December 31, 1997.

accumulated sediment. However, the model does not account 

for hysteresis; as a result, changes in load resulting from rapid 

changes in streamflow may not be modeled accurately. An 

example of this phenomenon is shown for Parma in 

figure 18. During July 1996 through December 1997, 

measured and estimated TP loads during low or moderate 

flows were in reasonable agreement, but the load measured 

during high flow was underestimated by the model by about 

40 percent. On the descending limb of the hydrograph, 

measured loads were slightly overestimated by the model. This 

is one important reason that daily load estimates can be highly 

inaccurate; monthly, seasonal, and annual averages are more 

representative. 

Limitations and Advantages of the LOADEST Model 27
Summary

The U.S. Geological Survey software program 

LOADEST was used to develop regression equations 

and to estimate loads of total phosphorus (TP), dissolved 

orthophosphorus (OP), and suspended sediment (SS) at four 

sites on the lower Boise River from January 1994 through 

September 2002. The equations and loads were based on 

water-quality and discharge data from the following sites: 

Boise River below Diversion Dam near Boise, Boise River at 

Glenwood Bridge at Boise, Boise River near Middleton, and 

Boise River near Parma. 

The regression models generally were well fit on 

the basis of R2 values, but the fitness varied from site to 

site. Results for all constituents for the site at Middleton 

were of limited usefulness because discharge data for the 

estimation period were incomplete and intermittent; data were 

unavailable for about one-third of the estimation period. 

Comparisons between instantaneous loads used in the 

calibration procedure and estimated daily loads for the same 

days indicate that the models successfully simulated TP 

and SS loads under the predominant flow conditions but, 

in some cases, larger errors were associated with flows that 

were higher or lower than the predominant flow conditions 

for the estimation period. Load estimates probably would 

have been improved for all sites if constituent sampling had 

taken place over a more complete range of flow conditions, 

including rising limbs, peaks, and falling limbs of high-flow 

hydrographs. 

The model estimated daily, monthly, seasonal, and annual 

loads for all constituents. Although the daily load estimates 

provided some valuable information, monthly, seasonal, and 

annual load estimates were the most representative because the 

longer timescales tended to minimize day-to-day variability in 

daily estimates that otherwise might have been misleading.

Regression models for TP and OP for Glenwood were the 

least successful in explaining variation in the data, as shown 

by the low R2 values compared with those for other studied 

sites. This most likely is due to the proximity of the Glenwood 

site to a municipal wastewater treatment plant outlet, which 

provides a variable phosphorus input that is unrelated to 

discharge. Results for SS were statistically well fit at all sites, 

probably because of the strong correspondence between SS 

and discharge. 

The estimated average daily TP load at Diversion was 

less than 250 pounds per day (lb/d). Overall, TP estimates 

at Glenwood were similar to those at Middleton and rarely 

exceeded 1,000 lb/d. Estimated TP loads at Parma were 

notably larger than at the other sites, averaging nearly 

2,200 lb/d and exceeding that value nearly 30 percent of the 

time. TP loads for the estimation period were largest during 

January and February 1997, coinciding with maximum 

flows. TP loads at all four sites displayed cyclical variation 

coinciding with seasonal fluctuations in discharge, even 

though the dates of peak discharge were not the same every 

year. Estimated TP loads generally peaked in spring, decreased 

in summer and autumn, and increased again in winter at 

all sites, reflecting the combined effects of seasonal runoff 

patterns and irrigation water management. Estimated annual 

loads of TP for the estimation period ranged from less than 

8 to 118 tons at Diversion, from 68 to 157 tons at Glenwood, 

and from 222 to 570 tons at Parma.

Annual loads of OP peaked in 1997 at all sites. Dissolved 

orthophosphorus loads were largest at Parma. The fraction 

of OP (the ratio of OP to TP) varied considerably throughout 

the year at all sites and was a useful way to gain insight to 

phosphorus cycling and processes in the lower Boise River. 

Peaks in the OP:TP ratio occurred primarily when flows 

were at their lowest annual stages, typically in autumn at all 

sites. Conversely, when flows were high, the ratio was low, 

reflecting increased TP associated with particulate matter 

during high flows. The estimated OP:TP ratios for Diversion 

were notably lower during winter and spring, when less than 

one-half the TP was in the OP:TP ratio. Parma exhibited the 

highest OP:TP ratio during all seasons, at least 0.60 in spring 

and nearly 0.90 in autumn. Similar, but slightly lower, values 

were estimated for the OP:TP ratio at Glenwood. On an annual 

basis, the OP:TP ratio remained fairly constant during the 

estimation period at most sites. The average annual ratio at 

Parma was consistent from year to year and varied about 10 

percent between 1994 and 2002. 

Whereas the OP:TP ratios for Glenwood and Parma 

peaked in November or December, diminished from January 

through May, and increased again after June, estimates for 

Diversion showed nearly the opposite pattern—the ratio 

was highest in July and lowest in January and February. The 

difference might reflect complex biological and geochemical 

processes involving nutrient cycling in Lucky Peak Lake, but 

further data are needed to substantiate this hypothesis. 

Estimated SS loads peaked in the spring at all sites, 

coinciding with high flows. Estimated average monthly SS 

loads were largest at Parma, about 250 tons per day (ton/d). 

In contrast, the monthly average load at Diversion was about 

37 ton/d. At Parma, the peak monthly average loads occurred 

in May and June from 1995 through 1998, and averaged about 

970 to 1,200 ton/d. Average annual loads from 1994 through 

2002 were 88,400 tons at Parma, 33,000 tons at Glenwood, 

and 13,000 tons at Diversion. 

28  Phosphorus and Suspended Sediment Load Estimates for the Lower Boise River, Idaho, 1994 – 2002
Monthly average TP increases in the reach from 

Diversion to Glenwood ranged from about 200 lb/d to 

about 350 lb/d from 1994 through 2002; monthly average 

OP increases ranged from 250 to 300 lb/d. Total annual TP 

increases in this reach averaged 60 tons per year for the 

estimation period. Total phosphorus decreases occurred 

only during high flows in January and February 1997. 

Large SS decreases (360,000 tons in 1997) were estimated 

because abundant sediment stored in the channel upstream of 

Diversion is mobilized during high-flow periods but is mostly 

redeposited before reaching Glenwood. During moderateand high-flow conditions, smaller decreases, and even slight 

increases, were estimated. 

Incomplete data for the Middleton site hindered 

investigation of the increases and decreases in the reaches 

between Glenwood and Middleton and between Middleton and 

Parma. The intermittent available load estimates for the reach 

from Glenwood to Middleton suggested cyclical increases and 

decreases in monthly average TP, corresponding to changes in 

discharge. Increases were estimated in December and January; 

small to moderate decreases were estimated from May 

through August. The reach from Middleton to Parma exhibited 

increases but no decreases for all constituents. May 1995 

marked a time of particularly large increases in TP, OP, and SS 

in this reach. The results also indicated large increases of SS 

in June 1996 and of TP in December 1996. These periods of 

large increases suggested erosion of sediment in the channel 

and (or) addition of sediment to the river from tributaries. 

Statistically significant downward temporal trends in 

all constituent loads were determined at Parma. A significant 

upward trend in TP was determined at Diversion. Other 

regression equations suggested trends, but the trends were not 

statistically significant at the 95-percent confidence level.

Because flow strongly affects the calculation of load, 

annual flow-weighted concentrations (FWCs) were calculated 

to facilitate assessment of changes in concentration over the 

estimation period despite a large range in flows. At Parma and 

Glenwood, where the OP:TP ratio was high, annual FWCs of 

TP were inversely related to load. This is most likely because 

the OP was diluted by high flows, but calculated loads are 

large as a result of the dominance of flow. In contrast, at 

Diversion, where OP was a smaller fraction of TP, load and 

FWCs were directly related, because entrained particulate 

matter is increased during high flows, not diluted. Relations 

between FWCs and loads of SS were similar. Thus, FWCs 

bring to light the strong interaction between flow and particleassociated constituents such as TP and SS. 

The Snake River-Hells Canyon TMDL for TP calls for 

loads in the lower Boise River at Parma to be capped at 532 lb/

d from May through September. LOADEST TP load estimates 

from 1994 through 2002 ranged from 740 to 2,550 lb/d and 

indicated that decreases in load of 28 to 76 percent would have 

been necessary to meet the proposed goal during this period. 

The present SS load allocation to Parma for lower Boise River 

TMDL purposes is about 101 ton/d (Idaho Department of 

Environmental Quality, 1999). Estimated loads from 1994 

through 2002 ranged from 86 to 968 ton/d and exceeded the 

allocation at Parma except in 2001. 

The LOADEST model is a useful tool for providing 

spatial and temporal resolution to help refine mass balance 

spreadsheet calculations used to allocate loads of phosphorus 

and SS in the lower Boise River. The LOADEST daily average 

TP load estimate from April 15, 1996, through October 15, 

1996, compared favorably with the load estimate made by 

mass balance spreadsheet calculations for the same period, but 

monthly average load estimates showed that the loads changed 

considerably within that time interval. Therefore, a key 

advantage of the LOADEST method is the ability to examine 

loads at a variety of timescales, whereas spreadsheet estimates 

are static. 

References Cited

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criterion: Journal of Econometrics, v. 16, p. 3-14.

Clark, G.M., 2003, Occurrence and transport of cadmium, 

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daily loads: Boise, Idaho Department of Environmental 

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References Cited 29
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in the lower Boise River, Ada and Canyon Counties, Idaho, 

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U.S. Geological Survey Open-File Report 95-443, 35 p.

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U.S. Geological Survey Water-Supply Paper 2175, 313 p.

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statistic for testing goodness of fit with randomly censored 

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waters still requiring total maximum daily loads 

(TMDLs)—paragraph 303(d) list for the State of Idaho 

Decision Document, October 7, 1994: U.S. Environmental 

Protection Agency, 13 p. plus app.

Vogel, R.M., 1986, The probability plot correlation 

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distributional hypotheses: Water Resources Research, v. 

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p. 587-590.

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samples, in National field manual for the collection of 

water-quality data: U.S. Geological Survey Techniques of 

Water-Resources Investigations, book 9, chaps. A4 and A5, 

[variously paged].

Woods, P.F., 2001, Concentrations and loads of cadmium, 

zinc, and lead in the main stem Coeur d’Alene River, 

Idaho—March, June, September, and October 1999: U.S. 

Geological Survey Open-File Report 01-34, 33 p.

30 Phosphorus and Suspended Sediment Load Estimates for the Lower Boise River, Idaho, 1994 – 2002
Manuscript approved for publication, October 19, 2004

Prepared by the U.S. Geological Survey Publishing staff, 

Idaho Water Science Center, Boise, Idaho

Linda Channel

Richard Helton

Washington Water Science Center, Tacoma, Washington

Bobbie Jo Richey

Linda Rogers

For more information concerning the research in this report, contact the 

Director, Idaho Water Science Center

U.S. Geological Survey, 

230 Collins Road

Boise, Idaho 83702

http://id.water.usgs.gov
SIR 2004–5235, Version 2.00 Donato and MacCoy

Printed on recycled paper

Phosphorus and Suspended Sediment Load Estimates for the Lower Boise River, Idaho, 1994 – 2002