Reservoir-System Model for the 

Willamette River Basin, 

Oregon 

By James 0. Shearman 

RIVER-QUALITY ASSESSMENT OF THE WILLAMETTE RIVER BASIN, OREGON 

GEOLOGICAL SURVEY CIRCULAR 715-H 

1976 
United States Department of the Interior 

THOMAS S. KLEPPE, Secretary 

Geological Survey 

V. E. McKelvey, Director 

Library of Congress Cataloging in Publication Data 

Shearman, James 0. 

Reservoir-system model for the Willamette River Basin, Oregon. (River-quality assessment of 

the Willamette River Basin, Oregon) 

(U.S. Geological Survey Circular 715-H) 

Bibliography: p. 22. 

1. Willamette River watershed. 2. Stream measurements-Mathematical models. 3. Stream 

measurements-Data processing. 4. Reservoirs-Mathematical models. 5. Reservoirs-Data 

processing. I. Title. II. Series. III. Series: United States Geological Survey Circular 

715-H. 

QE75.CS no. 715-H[GB1225.07] 557.3'08s [627'.86'097953] 76-608239 

Free on application to Branch of Distribution, U.S. Geological Survey, 12 0 0 South Eads Street, Arlington, VA 2 2 2 o 2 
FOREWORD 

The American public has identified the enhancement and protection of river 

quality as an important national goal, and recent laws have given this commitment considerable force. As a consequence, a considerable investment has been 

made in the past few years to improve the quality of the Nation's rivers. Further 

improvements will require substantial expenditures and the consumption of 

large amounts of energy. For these reasons, it is important that alternative 

plans for river-quality management be scientifically assessed in terms of their 

relative ability to produce environmental benefits. To aid this endeavor, this 

circular series presents a case history of an intensive river-quality assessment 

in the Willamette River basin, Oregon. 

The series examines approaches to and results of critical aspects of riverquality assessment. The first several circulars describe approaches for providing 

technically sound, timely information for river-basin planning and management. Specific topics include practical approaches to mathematical modeling, 

analysis of river hydrology, analysis of earth resources-river quality relations, 

and development of data-collection programs for assessing specific problems. 

The later circulars describe the application of approaches to existing or potential 

river-quality problems in the Willamette River basin. Specific topics include 

maintenance of high-level dissolved oxygen in the river, effects of reservoir 

release patterns on downstream river quality, algal growth potential, distribution of toxic metals, and the significance of erosion potential to proposed future 

land and water uses. 

Each circular is the product of a study devoted to developing resource information for general use. The circulars are written to be informative and useful to 

informed laymen, resource planners, and resource scientists. This design stems 

from the recognition that the ultimate success of river-quality assessment 

depends on the clarity and utility of approaches and results as well as their basic 

scientific validity. 

Individual circulars will be published in an alphabetical sequence in the 

Geological Survey Circular 715 series entitled ~(River-Quality Assessment of 

the Willamette River Basin, Oregon." 

J. S. Cragwall, Jr. 

Chief Hydrologist 

III 
Cover: Willamette River as it winds through Portland, Oregon. Photograph taken by 

Hugh Ackroyd. 
CONTENTS 

Page 

1\bstract ______________________________________________________________________________ Ill 

Introduction-------------------------------------------------------------------------- 1 

1\cknovvledgment -------------------------------------------------------------------- 2 

Description of basin __ __ _ ___ _ _ __ __ __ _ _ _ ___ _ _ _ _ ____ _ _ _ _ __ __ __ _ _ __ __ _ _ __ ____ _ _ ____ __ _ _ _ _ 2 

~odeling objectives ------------------------------------------------------------------ 2 

~odel selection ---------------------------------------------------------------------- 5 

Input data preparation---------------------------------------------------------------- 7 

Strea~ftovv data------------------------------------------------------------------ 8 

Reservoir characteristics ---------------------------------------------------------- 8 

Diversion data____________________________________________________________________ 11 

Climatological data _ _ _ _ __ __ _ _ __ _ _ _ _ _ ___ __ _ _ __ _ ___ _ ___ __ _ _ _ _ _ _ __ _ _ ____ __ _ _ _ _ _ _ _ _ __ 11 

Other data ---------------------------------------------------------------------- 11 ~odel calibration _ _ ___ ___ _ _ ______ _ _ _ _ ___ _ _____ ___ _ _ ____ _ _ _ _ _ _ __ _ _ _ _ ___ _ _ _ _ _ __ _ _ _ _ __ __ 11 

~odel verification __ _ _ _ _ __ _ _ _ ___ __ __ _ _ ____ _ _ _____ _ _ _ ______ __ _ _ __ __ _ _ _ __ _ __ ____ _ _ _ _ __ __ 11 

1\pplications of the model______________________________________________________________ 18 

Summary and discussion -------------------------------------------------------------- 19 

References cited ---------------------------------------------~------------------------ 22 

ILLUSTRATIONS 

Page 

FIGURE 1. ~ap shovving study area______________________________________________________________________________ Ill 

2-13. Graphs shovving: 

2. Frequency-discharge relations at Salem, 1926-73 _________ ______________ ______________ _________ 5 

3. Frequency-discharge relations at Salem computed from short sequences of observed steamflovv ____ 6 

4. Relation betvveen annual minimu~ 7-day and annual minimu~ 30-day discharges, 1926-71 ______ 6 

5. Relation betvveen annual minimum 7-day and annual minimu~ monthly discharges, 1926-71 ____ 7 

6. Typical rule curve of reservoir operation in the Willamette River basin-------------------------- 9 

7. Evaporation relations for the Willamette River basin ------------------------------------------ 13 

8. Distribution of model errors, January 1970 through September 1973 ---------------------------- 16 

9. Comparison of design, observed, and computed reservoir-system storage ------------------------ 17 

10. Frequency-discharge relations at Salem, 1926-73, computed from simulated streamftovv for existing 

(regulated) conditions and from estimated streamftovv for natural (unregulated! conditions ____ 18 

11. Frequency-discharge relations at Salem, 1926-73, co~puted from simulated streamftovvs reflecting 

the existing reservoir system subjected to different estimated ftovv diversions ---------------- 20 

12. Frequency-discharge relations at Salem, 1926-73, computed from discharges for calendar months__ 21 

13. Frequency-discharge relations at Salem, 1926-73, co~puted from minimum multimonthly discharge 21 

TABLES 

Page 

TABLE 1. Selected information for the 13 reservoirs in the Willamette River basin reservoir system ---------------- II4 

2. Classification of streamftovv at Salem relevant to stage of reservoir-system development ------------------ 5 

3. Relationships used to extend unregulated, monthly streamftovv data through 1973------------------------ 8 

4. Rule curve storage data used in the IIEC-3 ~odeL _____________________________________________________ 10 

5. Flovv diversion data used in the IIEC-3 model -------------------------------------------------------- 12 

6. Summary of model verification results __________________________________________________________ ------ 14 

7. Classification of model errors into absolute error limits ------------------------------------------------ 15 

v 
Page 

TABLE 8. Summary of modeling errors with absolute value exceeding 15 percent ---------------------------------- 15 

9. Summation of total end-of-month storage and monthly storage charges specified by the design rule ct-rves for 

11 reservoirs in the Willamette River basin reservoir system __________ ------------------------------ 17 

10. Determination of tQm and tQ 7 at Salem for the simulated streamflows reflecting the 1970 and 2020 flow 

diversions -------------------------------------------------------------------------------------- 20 

CONVERSION FACTORS 

Factors for converting English units to metric units are listed below. In the text, the metric 

equivalents are shown only to the number of significant figures consistent with the values for the 

English units. 

Multiply English umt 

feet (ftl 

cubic feet per second lft3/sl 

miles (mil 

square miles (mi::lJ 

acre-feet ( acre-ft l 

acre-feet (acre-ftl 

thousands of acre-feet (acre-ftx 103) 

By 

Q.3048 

0.02832 

1.609 

2.59 

1.234x 10-6 

1.234x10-3 

1.234 

VI 

To obtam metric wut 

metres (ml 

cubic metres per second (m3/s) 

kilometres ( km) 

square kilometres (km2J 

cubic kilometres (km3J 

cubic hectometres (hm3J 

cubic hectometres <hm3) 
Reservoir-System Model For The Willamette River Basin, 

Oregon 

By James 0. Shearman 

ABSTRACT 

Evaluation of basin-development alternatives in terms of 

potential impacts on river quality for the Willamette River 

basin, Oregon, requires determining the low-flow characteristics of streamflow resulting from ( 1) unregulated !natural) 

conditions and !2) regulated (by existing or alternative reservoir system) conditions. Observed streamflow data that are 

presently ( 1973) available are insufficient for reliably estimating these required low-flow characteristics. This report describes ! 1 l a method for estimating monthly streamflow for 

natural conditions and !2) the use of a reservoir-system model 

to simulate monthly streamflow for regulated conditions. 

A 4-year period !1970-73) is used to verify the applicability 

of the U.S. Army Corps ofEngineers' HEC-3 reservoir-system 

model for the Willamette River basin. Modeling errors are 

computed as the differences between model results and observed streamflow at Salem, Oreg. These errors, although possibly subject to further reduction, are considered to be within 

reasonable limits. 

Simulations of monthly streamflows for a 48-year period 

( 1926-73) are made for four different levels of estimated flow 

diversions. Each of the simulations uses identical input data 

for definition of ( 1) 1926-73 hydrologic data and ( 2) operation 

and configuration of the existing reservoir system. Frequency-discharge relations computed from ! 1) simulated 

streamflows at Salem reflecting present regulated conditions 

and ( 2) estimated streamflows at Salem reflecting natural 

conditions are used to demonstrate potential applications of 

the reservoir-system model. Detailed examples illustrate application of these relations for (1) assessing the total impact 

that the existing reservoir system has had on dissolvedoxygen concentrations at Salem and (2) assessing the impact 

that the higher flow diversions projected for the future will 

have on dissolved-oxygen concentrations at Salem. Other potential applications of the reservoir-system model are briefly 

discussed. 

INTRODUCTION 

One objective of the intensive river-quality assessment study of the Willamette River basin, 

Oregon, was ''to develop and document methods 

for evaluating basin-development alternatives in 

terms of potential impacts on river quality* * *" 

(Rickert and Hines, 1975). The existing reservoir 

system in the Willamette River basin represents 

a very significant component of the overall basin 

development. Observed streamflow data that are 

available are not adequate to assess the impact of 

existing reservoir-system conditions on river 

quality (see subsequent section, "Modeling Objectives"). Furthermore, any modification of the 

existing reservoir system would likely have some 

impact on river quality. Especially significant 

impact could result from modification of (1) system operation, (2) flow diversions, (3) system configuration, and ( 4) various combinations of the 

preceding. River-quality assessment in the Willamette River basin, for either the existing or a 

modified reservoir system, thus requires a tool for 

simulating adequate streamflow data. This circular describes a reservoir-system model trat can be 

used to simulate the required data. 

Verification of the model is based on 4 years 

(1970-73) of observed streamflow at Salem, Oreg. 

A detailed discussion of the source and magnitude 

of the n1odeling errors for the verification period 

is presented. 

Monthly strean1flows in the Willame+.te River 

basin are simulated for four different levels of 

flow diversions. All of the simulations are based 

on input data which define (1) 48 years (1926-73) 

of hydrologic data and (2) operation ard configuration of the existing reservoir systen1. 

Frequency-discharge relations based on ( 1) estimated natural flows and (2) simulated flows are 

utilized to demonstrate potential model applications. All of the examples are based on Willamette River streamflow at Salem, Oreg. However, similar data are available for other points in 

the basin. Also, simple input data revisions make 

it possible to simulate streamflows for modified 

reservoir systems. 

The primary purpose of this circular is to demH1 
onstrate the applicability of reservoir-system 

modeling in river-quality assessment. In view of 

the fact that time and resources available for the 

study were limited, simplifying assumptions were 

made where practical to minimize input data 

preparation. The resultant limitations of the 

model used should be examined, and additional 

refinement of these data considered, before actually using the model for basin planning. 

ACKNOWLEDGMENT 

The advice and assistance of the staff of the 

U.S. Army Corps of Engineers, Portland District 

Office, the staff of the Texas Water Development 

Board, and Stuart McKenzie and Dannie Collins, 

colleagues in the U.S. Geological Survey, are 

gratefully acknowledged. 

DESCRIPTION OF BASIN 

The Willamette River basin, a watershed of 

nearly 11,500 mi2 (29,700 km2) (fig. 1l, is located 

in northwestern Oregon between the Cascade and 

Coast Ranges. Within the basin are the state's 

three largest cities, Portland, Salem, and Eugene, 

and approximately 1.4 million people, representing 70 percent of the state's population. The Willamette River basin supports an important 

timber, agricultural, industrial, and recreational 

economy and also extensive fish and wildlife 

habitats. 

The basin is roughly rectangular, with a 

north-south dimension of about 150 mi (240 kn1l 

and an east-west width of 75 mi (120 km). Elevations above sea level range from less than 10 ft 

<3 ml near the mouth of the Willamette River, to 

450 ft (137 m) on the valley floor near Eugene, 

and to about 10,000 ft (3,050 m) in the Cascades. 

The Coast Range generally varies in elevation 

from 1,000 to 2,000 ft (305 to 610 ml but includes 

peaks of greater than 4,000 ft ( 1,220 m). 

The main stem of the Willamette River forms 

at the confluence of its Coast and Middle forks 

south of Eugene and flows northward for 187 mi 

(302 km) through the 3,500 mi2 (9,060 km2) Willamette Valley floor. The first 135 mi (217 km) of 

the river is characterized by a meandering channel. Through most of the remaining 52 mi 

(84 km), the Willamette flows within well-defined 

banks, unhindered by falls or rapids except for 

the basaltic intrusion at Oregon City which 

creates the Willamette Falls. The 26-mi (42-kml 

reach below the falls is subject to nonsaline tidal 

effects, transmitted from the Pacific via the Columbia River. 

Until recently, the Willamette River has experienced acute water-quality problems related 

primarily to annually recurring low flow in summer and heavy organic-waste loading by pulp and 

paper industries. Low-flow augmentation by new 

reservoirs and new secondary waste-treatment 

plants has considerably improved the river quality in recent years. However, in light of expected 

urban and industrial growth, decisionmakers in 

the basin are now involved with evaluating alternatives for keeping future river-quality conditions at the present high levels. 

MODELING OBJECTIVE~ 

The primary objectives of this reservoir-system 

study for the Willamette River basin are to (1) 

estimate the magnitude and frequency of low 

flows at Salem, Oreg., for natural conditionsthat is, the low flows that would have occurred 

with no significant reservoir develorment in the 

basin; (2) estimate the magnitude ar<i frequency 

of low flows at Salem, Oreg., for the existing reservoir system with reservoir operations and flow 

diversions that reflect current conditions; and (3) 

develop practical, quantitative methods for estimating low flows at various control points (fig. 

1) in the Willamette River basin for alternative 

reservoir systems. 

The first two objectives are of spe,'?.ific interest 

because they are directly related to the testing 

and application of a DO (dissolved oxygen) planning model for the main stem of thE Willamette 

River at Salem, Oreg. This report discusses the 

application of a reservoir-system model to satisfy 

the second objective. The first objective is accomplished by using streamflow d::ta that are 

used as input to the model. The third objective is 

essential to satisfy specific needs of those agencies 

or individuals that are responsible for basin 

planning decisions. Not all of thes~ needs are 

presently defined. However, application of the 

reservoir-system model for estimating reduced 

low flows at Salem due to increased flow diversions in the basin is demonstrated, and other potential applications are briefly discussed. 

Critical DO conditions in the Willamette River 

basin have occurred only during low-flow periods 

and only downstream from Salem, Oreg. 

H2 
w 

(!J 

z 

<( 

a: 

~ <( 

0 

u 

II 

FIGURE 1.---Study area. 

H3 

EXPLANATION 

10 . 6 Control pomt at a reservoir 

• 40 Control point at a 

diversion 

1'0<43 FI b.. . '0' ow com 1nmg pomt 

CONTROL POINT LOCATIONf 

MAY NOT 8E EXACT 

I 

10 

10 

I 

I 

?0 !,Ill [ ~ 

I 

'0 KILOI\II£ T R£ S 

w 

Cl 

<( 

~ <( 

u 
(Gleeson, 1972). Therefore, simulation of the 

low-flow regime at Salem provides the necessary 

basis for DO modeling. DO modeling usually requires an estimate of the frequency of recurrence 

of a minimum flow rate for some duration of time. 

Minimum 10-·, 25-, or 50-year flow rates for consecutive 7-, 14-, or 30-day periods are most commonly used. Such low-flow estimates require a 

long sequence of homogeneous streamflow data. 

(Homogeneous, as used herein, implies data resulting from a system that is not radically altered 

over a period of time.) 

The Willamette River basin reservoir system, 

as defined for this study, is made up of 13 reservoirs. Selected information for the 13 reservoirs is 

summarized in table 1. Development of the system began in 1941 with the completion of Fern 

Ridge reservoir and ended in 1968 with the completion of Blue River reservoir. Total useable 

storage capacity is about 2 million acre-ft 

(2.47 km3) (Willamette Basin Task Force, 1969), 

about 12 percent of the approximately 17 million 

acre-ft (21.0 km3) (U.S. Geological Survey, 1973) 

average annual flow volume of the Willamette 

River at Salem. 

The U.S. Geological Survey has collected continuous streamflow data on the Willamette River 

at Salem since January 1923. These observed 

streamflow data reflect the total impact of the 

reservoir system defined above. Thus, the observed streamflows reflect natural (unregulated) 

flow conditions until1941 when minor regulation 

began. Regulation effects progressively increased 

until 1968, becoming especially significant in the 

mid-1950's when about half of the present storage 

capacity had been developed. 

Observed streamflow data at Sale1n are not 

adequate for estimating the low-flow cl'aracteristics required to satisfy the second objActive. Although the data sequence is long enoug'l, the data 

are not homogeneous. A compariscn of the 

frequency-discharge relations shown in figure 2 

supports this conclusion. These relations are 

based on 192~ 73 data sequences of monthly 

streamflow at Salem. Classification of these data 

into four shorter data sequences representing increased stages of reservoir-system development is 

shown in table 2. Figure 2A, based on observed 

data, exhibits the expected trend of higher annual 

minimum-monthly discharges with increased 

low-flow augmentation from the reservoir system. 

There is some overlap of the four classes of data as 

should be expected since the reservoir system 

does not entirely eliminate variations in 

reservoir-system inflqw. Figure 2B is based on the 

natural (unregulated) monthly flows that were 

estimated for reservoir-system model input (see 

''Streamflow Data" subsection of subsf'(]Uent section "Input Data Preparation"). The more complete mixture of the four classes of data in the 

latter figure illustrates the nonhomogeneity introduced into the observed data by rrogressive 

reservoir-system development. N onhcmogeneity 

is also introduced by other types of basin development (for example, urbanization, changing 

agricultural practices, and other changing land 

uses). However, these additional contributing factors are not as easily illustrated and are not as 

significant as those attributable to reservoirsystem development. 

Prediction of the effects of the present Willamette River basin reservoir system could be atTABLE 1.-Selected information for the 13 reservoirs in the Willamette River basin reservoir system 

Control 

point 

number 1 

10 

11 

2-! 

12 

13 

14 

15 

16 

18 

23 

25 

21 

22 

1See figure 1. 

Reservoir name 

Hills Creek _____________ _ 

Lookout Point ___________ _ Dexter3 ________________ _ 

Fall Creek ______________ _ 

Cottage Grove ___________ _ Dorena _________________ _ 

Cougar _________________ _ 

Blue River _____________ _ 

Fern Ridge _____ _ Detroit __________ _ 

Big Cliff' Green Peter _____________ _ 

Foster __________________ _ 

Strear;:. name 

Useable 

capacity 

lacre-ftl 

Middle Fork Willamette River ___________ ____ 240,000 _____________________ do______________________ 349,000 ______________________ do_________________ 4,800 

Fall Creek ltnbutarv to 

Middle Fork Willamette River!. Coast Fork Willamette River ____________ _ 

Row River !tributary to Coast 

Fork Willamette River!. South Fork McKenzie River _________________ _ 

Blue River \tributary to McKenzie RIVer! ____ _ Long Tom River ___ · __________________________ _ 

North Santiam River __________________ _ --- --- ________________ do ___________ _ 

Middle Santiam River ______________ _ 

South Santiam River __________________ _ 

115,000 

30,100 

70,500 

165,000 

85,000 

110,000 

340,000 

2,400 

333,000 

33,200 

2FC. flood control; N. navigation; I, irrigation; P, power; RR. reregulation. 3Small reregulatmg reservoir. 

H4 

Drainage Date Authorized 

area regulation purpose 

1mi2 1 began or use 2 

389 Aug. 1961 FC,N,l,P 

911 Nov. 1953 FC,NJ.P 

911 Nov. 1953 RR,P 

184 Jan. 1966 FC,NJ 

104 Oct. 1942 FC,N,I 

265 Oct. 1949 FC,NJ 

208 Sep. 1963 FC,NJ,P 

88 Oct. 1968 FC,N,I 

252 Nov. 1941 FC,N.[ 

438 Jan. 1953 FC.N,l,P 

438 Jan. 1953 RR,P 

277 Oct. 1966 FC,N,l.P 

-!94 Dec. 1966 RR,FC,P 
Class from Table 3 250 

4---- 200 

~ 3 1 5 150 

100 

3 

(A) 

3L--J--------~----~----~-----------J------------~----~----~--~--~ 

10 -

-

~ 

"""' 5f1.01 

3 

0 

(B) 

1 •) 

0 0 

1. 05 1.11 

_Class from Table 3 

3 

h 1 ~ ). 1 ...L ~~ -' k 3 4 1 

- 250 

- 200 

- 150 

- 100 2 ~<>o~IOO oo <A __.r-..._ 2 3 -1 1 r ~ v <l .......__ 

10 25 50 100 

RECURRENCE INTERVAL. IN YEARS 

FIGURE 2.-Frequency-discharge relations at Salem. 1926-73. A, Computed from observed streamflow. B. Computed 

from estimated natural (unregulated) streamflow. 

TABLE 2.--Classification of streamflow at Salern relevant to 

stage of reservoir-system development 

Class 

.3 -------------- 4 

Penod of record 

1926--41 

1942-52 

1953-66 

1967-73 

Percentage of present total useable 

storage capacity developed 

0 

6--11 

29-76 

95--100 

tempted by using a portion of the observed 

streamflow data which could be assumed to be 

homogeneous. The earliest data that are probably 

applicable would be for 1967 when 95 percent of 

the reservoir system had been developed. However, uncertainty exists because it is difficult to 

determine if the completion of Green Peter and 

,Foster reservoirs (in late 1966) and Blue River 

reservoir (in late 1968) created any significant 

nonhomogeneity in the flow data. Figure 3 illustrates the variability of frequency-discharge relations based on four short data sequences that are 

potentially applicable. Also, Hardison (1969) has 

shown that increasing the amount of adequate 

data improves prediction reliability. A data sequence of annual events for 50 years yields predictions that are roughly twice as reliable as a 

similar data sequence for 10 years. 

To satisfy the second modeling objective, an 

adequate reservoir-system model could be used to 

simulate streamflow data sequences. The model 

would require input data as follows: (1) data describing the present configuration anc operation 

of the reservoir system; (2) a long s'?.quence of 

hydrologic data representing system inputs and 

losses; and (3) data representing the p:-:-esent flow 

diversions in the basin. Model output would then 

represent a long sequence of homogeneous 

streamflow data for existing reserYoir-system 

conditions. Assuming that ( 1) the mo':lel results 

are representative of the actual system, (2) the 

errors associated with the results are within acceptable tolerances, and (3) the limitations of the 

model are understood and within reason, then the 

required low-flow estimates can be b2sed on the 

simulated data sequence. Furthermore, the same 

hydrologic data combined with different 

reservoir-system and (or) flow-diversio:':l data provides the capability to simulate vario11S data sequences that will satisfy the third objective. 

MODEL SELECTION 

The major considerations governing model 

selection were (1) modeling objectives; (2) availability of adequate data; (3) desired accuracy; and 

(4) available time, manpower, computational 

H5 
u 10 

s ~50 p:::; 

w ;:::, ;...., 0.. ;..-,U :300 ...:100 ...:l!J:.. ..,., w ~0 - p:::; E-<E-< E-<ooo G ,----- 1GO 

Zoz 5;:a Ozo ~~ 

::S..:t:u 0 1970-1973 1967-1973 ...... ::nw 100 e~o ~000 0 1969-Hl73 ::::; ~ z 6. 1968-1973 ~>-'0 

~~~ -uu • 1967-1973 ~~w 

~~~ ;;s 00 

...:< ~ 

...:< ~ W ww --< w --<v~J:.. 50 ;:::,~ ;::,p:::; 

~~ ~~ --<u --<u ga ga 0 0 

1. 01 1. 05 1.11 1. :35 5 10 25 50 100 

RECURRENCE INTERVAL, IN YEARS 

FIGURE 3.-Frequency-discharge relations at Salem computed from short sequences of observed streamflow. 

ANNUAL MINIMUM 30-DAY DISCHARGE, 

IN CUBIC METRES PER SECOND 

150 200 250 

Q 8 

z 

0 .. u - 200 ~ 

~~ OQ 

Om o::z 0:::~ <o ~~ ::r::u 

u~ uP-t ram 00 ~ Qp::; -~ 6 Q ~ ~~ 

~ ~ ~Pot 

~ u Cloo 

I - t-o:; ·~ t- p:) 

~B §~ ;:J~ ~~ 

~0 -u 

4 z 

Zoo --

p:) -Q ~ ;:J ~z HU H< <00 

;:J;:J §~ z zO z z::r:: < <~ 

z 

- 2 8 

ANNUAL MINIMUM 30-DAY DISCHARGE, 

IN THOUSANDS OF CUBIC FEET PER SECOND 

FIGURE 4.-Relation between annual minimum 7-day and annual minimum 30-day discharges, 1926-71. 

facilities, and funds. Manpower and funding limi- possibilities of developing a new model. Theretations, in conjunction with a relatively short fore, two existing reservoir-system m0dels (Jenproject duration (2.5 years), severely limited the nings and others, 1976) were considered for poH6 
tential application to the Willamette River basin 

reservoir system. These models are the HEC-3 

model developed by the U.S. Army Corps of Engineers ( 1968) and the SIMYLD II model developed by the Texas Water Development Board 

( 1972). 

The HEC-3 model exhibited major advantages 

as follows: ( 1) 8 of the 13 reservoirs in the Willamette River basin reservoir system are used for 

power generation, and power-release modeling is 

possible with HEC-3 but is not possible with 

SIMYLD II; (2) most of the minimum-flow requirements in the Willamette River basin are 

variable by season which can be expressed using 

HEC-3, but these requirements must be expressed as a yearly average in SIMYLD II; and (3) 

the more flexible expression of operating rules 

permitted by HEC-3 made it possible to define 

better the actual operating rules of the Willamette River basin reservoir system. These limitations of SIMYLD II could have been overcome 

by additional programing had sufficient time 

been available. 

The Portland District of the Corps of Engineers 

had used the HEC-3 model for preliminary 

analyses in the Willamette River basin and had 

already assembled a considerable amount of directly applicable input data. This advantage, 

combined with the fact that no revisions of the 

HEC-3 program were required, resulted in the 

selection of HEC-3 as the most directly applicable 

reservoir-system model for this study. 

INPUT DATA PREPARATIOP 

The computation time interval selected for the 

modeling effort was 1 month. Generally, a 

monthly basis would be inadequate for estimating low flows of short duration. However, the Willamette River at Salem exhibits a very Etrong relationship between 7 -day and 30-day annual 

minimum low-flow rates. Figure 4 shows this relationship as computed from 1926--71 st~eamflow 

data. Figure 5 shows a similar relationship 

between 7 -day and monthly annual ninimum 

low-flow rates. The standard errors of these two 

relationships are about 3 percent and 18 percent, 

respectively. The higher error of the latter is due 

to the fact that the minimum 30-day and 

minimum monthly flow rates can be sigrificantly 

different when the 30-day low-flow per~od overlaps calendar-month boundaries. However, this 

problem is considerably outweighed by the adANNUAL MINIMUM MONTHLY DISCHARGE, 

IN CUBIC METRES PER SECOND 

8 100 150 200 250 

~~ 200 t3o ~ p:;U 00 -<~ A P::z ::r:;rn -<o UP:: Gu ra ~ A Y = 0. 87X 01=4 ra~ 

~E-1 6 AA om A ~p:; -<~ A A 

0~ A <t::~ 

.~ A 150 

o1=4 

t-u ,rn 

t-~ 

:::>1=0 ~- ~p:; 

~:::> :::>E-1 

_u A ~~ z~ A -~ ~0 4 A ~~ ...:loo. A AA A ~1=0 

-< 0 A ...:l:::> :::>z 100 ..:r:U z~ A :::>z Z:::> 1. A z-

..:r:g z 

-< 

E-1 

~ 2 

2 4 6 8 10 

ANNUAL MINIMUM MONTHLY DISCHARGE, 

IN THOUSANDS OF CUBIC FEET PER SECOND 

FIGURE 5.-Relation between annual minimum 7-day and annual minimum monthly discharges, 1926-71. 

H7 
vantages of using monthly data in the model (that 

is, availability of data and computation reduction). Also, since the total travel time for flows to 

traverse the length of the basin is only on the 

order of days, the monthly time frame eliminates 

the need of any streamflow routing. 

STREAMFLOW DATA 

The HEC-3 model requires definition of the 

total streamflow that would occur at each control 

point under natural conditions (that is, unaffected by regulation, flow diversions, and so 

forth). The U.S. Army Corps of Engineers (written commun., 1974> provided adequate input data 

for unregulated, monthly streamflow at all control points for the period 1926-69. They had developed these data using correlation and waterbudget computations. It was deen1ed imperative 

to extend these natural streamflow data through 

the severe drought year of 1973 (which was also 

the first year of intensive data collection for DO 

modeling). 

Therefore, at each control point, an attempt 

was made to develop a relationship for extending 

the Corps data by correlating the Corps data with 

observed streamflow at an index station. An 

index station, in this context, is defined as a 

streamflow gaging station at which streamflow is 

unaffected by regulation and (or) diversions. A 

1 

satisfactory relationship could not always be ob-

. tained using a single index station, ir which cases 

two index stations were used. Selection of the 

applicable index station(s) for each control point 

involved a trial-and-error process to simultaneously minimize the standard error and maximize 

the correlation coefficient of the relationship. At a 

few control points the relationships developed by 

using gaging station data were judged inadequate 

because the standard error was too high and (or) 

the correlation coefficient was too lovr. To develop 

acceptable relationships at these control points it 

was necessary to use one (or two) upstream control points as the index station( s ). Table 3 summarizes the final relationships used for extending 

the unregulated streamflow for each control 

point. 

RESERVOIR CHARACTERISTICS 

Physical properties of the reservoirs plus power 

generation data and operating rules were obtained from the Portland District of the U.S. 

Army Corps of Engineers (written commun., 

1974). Figure 6 illustrates a typical operating 

rule curve that is applicable to each reservoir. 

The storage data used to define the individual 

rule curves for the HEC-3 model are tabulated in 

table 4. 

TABLE 3.-Relationships used to extend unregulated, monthly streamflouJ through 1973 

Control Fn·st Second Period of 

pomt mdex mdex SE3 record 

number' statwn" statwn2 I percent I R" used 

10 -------------- 1448 141 0.561 113~ 141 10.6 0.990~ 1959--69 

11 H7.5 1~65 .5.190 0 630 0.360 10.0 .9919 1940--69 

2~ 15 1 

1:2 ------------------------------- 15~5 152.5 :2.365 .690 .:213 16 6 .9910 19~0--69 

~1 1475 141 7.506 .9~.5 1"1 10.6 .9916 1940--69 

1:3 1525 141 .980 1.058 1"1 12.6 .9953 19~0--69 

u 15~5 ,., 1 ')')'1 1.005 ,., 9-! .9978 19~0--69 

~2 1.525 15~5 2.665 .62~ .478 10.4 9973 19~0--69 

~3 1.5:25 1475 16.223 .3~9 .605 9.6 .99~7 19~0--69 

1.5 1592 1'1 3.58 1.200 14 1 13.0 .984.5 19.58-69 

16 1611 1615 1 797 .868 .146 1~.2 9937 1964--69 

~~ 9~3 141 17 773 .674 14 1 HA 9738 19~0--69 

~.5 "9~~ 141 203 1.266 14 1 12.2 9889 1940--69 

18 16615 1670 1.291 .929 .176 9.8 .9982 19~1-69 

~6 ----------------------------- 166.5 1670 1.011 1.264 -066 88 .9983 1941-69 

~8 ---------- -- ---------- "9~.5 "9~6 2.419 .838 134 13.8 .9877 19~1-69 

21 1850 1870 3.061 .7.5.5 .230 1~ 6 .9922 19~8-69 

32 18.50 1870 ~.17.'1 .820 17.5 86 .9973 19~8--69 

39 18.50 1870 .5.181 730 .27.5 85 9974 1948-69 

:2:3 -- ---- 1790 1'1 6 719 919 (41 76 .9906 1940--69 

25 15 1 ~0 ---------- 1825 1790 10 11.5 .210 .703 12.5 .9867 1940--69 

~9 18.50 14 1 :29 36S .840 ,., 1.5.4 .988~ 19~0--69 

.50 6 948 "949 l.178 .88.5 .160 12.7 .9906 1941-69 

.51 "950 ,., 707 1.0.58 1"1 1.5.0 .98.59 19~1-69 

Month!) flow at a control pomt can be computed hy X =a }_·bzc. where X= monthly flow at the control pomt; Y =monthly flow at the first index station; and Z =monthlv 

flow at the second mdex statwn 

1See figure 1 for locatiOns and table 1 for reserYmr names 

°Four-digtt statwn numbers are the yyyy porhon of the USGS gaging-statwn number 14--yyyyOO. 

"SE, the standard error of estimate, and R. the correlatwn coefficient, are measures of the goodness of fit of the computed data to the observed data I Draper and Smith, 19661. 

"Best correlation obtained by usmg only one index station Monthly flows at these control points are computed by X =a yb. 

'These control pomt5 are at small reregulating reservoirs JUSt downstream from large resen·oirs and reqmre no streamflow data since there iS no additwnal mflow between the two reservoirs. 

"Correlation with gagmg-statwn data considered unacceptable. Correlation was made usmg one lor two 1 upstream control point Is 1 as mdex statwnls 1. 9xx denotes flows 

computed at control pomt xx usmg the relatwnship between control point xx and its mdex statwnl s 1 

HS 
--- ~---

JAN FEB MAR APR MAY JUNE JULY AUG SEP OCT NOV IEC 

@ Major Flood Period--reservoir held to minimum elevation (storage) 

for flood control. 

@ 

® 

Conservation Storing Period--reservoir filled for conservation 

purposes. 

Conservation Release Period--stored water released as needed. 

Minimum Reservoir Elevation (Storage)--minimum "storage level" 

specified for HEC-3. 

Top of Power Storage--next to lowest "storage level'' specified for 

HEC-3. ® Bottom of flood control pool for "dry conditions" (whereas H represents "ideal", "normal", or "average" conditions.) 

@ Intermediate "storage levels" may be used to control the rate of 

depletion below "ideal" for various degrees of "belowaverage" conditions, and among reservoirs in the system. 

@ Bottom of Flood Control Pool--next to highest "storage level" 

specified for HEC-3. 

([) Maximum Reservoir Elevation (Storage)--maximum "storage level" 

specified for HEC-3. 

FIGURE 6.-Typical rule curve of reservoir operation in the Willarnette River basin. 

H9 
TABLE 4.-Rule curve storage data used in HEC-3 

Control Storage Reservmr storage 1 thousands of acre-feet 1 

pomt level number' number" Jan Feb. Mar. Apr. May .June .July Aug Sept. Oct. Nov. Dec. 

I .356 6 } 340 325 295 238 

} 156 .s 156 250 290 330 350.6} 156 10 4 350.6 325 290 260 230 3 

156 

107 

7 456 6 'l 430 410 345 } 5 11 4 443.1} J 118 8 265 340 420 443.1 410 360 310 245 118.8 118.8 

3 

2 118.8 1 106.6 

24 1-7 All months: 25.4 

7 125 6 } 117.5 110 90 6.5 30 

10 58 84 109 117.5 l1o 100 95 65 35 15 12 10 

} 50 45 22.5 17.5 13 5 J 10 

7 32.9 6 

~ 31.75 30 25 18 

l39 5 31.75 26 18 10 13 4 J 2.9 10.8 19 27 31 75 31.75 16 8.4 5.9 f . 2.9 2.9 

3 11.9 10 7.7 5.9 \. 

I 2.9 

7 77.5 6 -~ 71.9 70 55 40 5 65 .57 40 25 )~ 14 4 J 24 42 5 60.5 71.9 35 35 25 14 s 3 30 24 19 14 

7 219 .. 27 6 } 207.97 200 190 165 115 

l6404 .5 6404 6404 15 4 127 162 195 207.97 200 190 160 130 105 J . 3 

64.04 

54 15 

7 89.1 6 t 82.8 77.5 67.5 52 5 I 5.8 16 4 ' .37.5 .57 76.5 82.8 } 80 70 50 30 J 4 

3 

4 

7 116.9 6 

5 ~ 18 4 ' -!3 81 101 2 101.2 1012 97.5 92.5 85 32.5 

3 

.-, } 

7 430 6 't 410 400 370 338 265 

}160 

5 21 -! J 160 280 336 392 HO ) -!00 350 300 255 205 160 

3 

160 

97 

7 61 6 l 38 

} 31 5 31 } 36 22 4 ' 38 46 53 .. 5 56 56 56 56 56 31 

3 

31 

27 4 

7 455 6 } 434 -!13 386 350 262 

} 1.55 5 1.55 284 355 422 43-! 155 23 4 } 425 395 345 295 245 3 

155 

115 

HlO 
TABLE 4.-Rule curue storage data used in HEC--3-Continued 

Control 

pomt 

number' 

25 

Storage 

len• I number2 

1-7 

. Jan Feb . 

1See figure 1 for locations and table 1 for reservOir names. 

Mar Apr 

Resenmr storage 1thousands ol acre-teet> 

Ma\' .June ,July Aug. Sept Oct NO\ Dec 

All months· 4.53 

2See figure 6 for storage-level definitwns. Storage level numbers m this table coinnde with lettered items m figure 6 as follows: Levels 1 through 3 corrEspond to hnes 

D. E. and F: levels 4 and 5 are within region G: and levels 6 and 7 correspond to hnes H and I. 

DIVERSION DATA 

The Corps also provided the diversion data 

(written commun., 1974) which they had obtained 

from the Bureau of Reclamation. These data reflect net depletions attributable to irrigation and 

municipal and industrial water-supply demands. 

Table 5 summarizes the estimated net depletions 

(withdrawals from the system) or return flows 

(returns of excess withdrawals to the system) for 

the years 1970, 1980, 2000, and 2020 at each diversion point (fig. 1). 

CLIMATOLOGICAL DATA 

Evaporation data used in the HEC-3 model are 

computed from the relationships shown in figure 

7. These relationships were obtained from the 

U.S. Army Corps of Engineers (written commun., 

1974). These data, reflecting total evaporation, 

could be adjusted to reflect net evaporation by 

adding average basin precipitation to each value. 

However, this represented a significant data 

preparation effort and was ignored for the purposes of this report. 

OTHER DATA 

The Corps (written commun., 1974) also provided the following miscellaneous data: ( 1) 

minimum desired flows at all control points, (2) 

maximum desired flows at all control points, and 

( 3) all of the data pertaining to shortage declaration (permissible depletion of desired reservoir 

storage to provide desired flows) in the reservoir 

system. 

MODEL CALIBRATION 

Many types of models (such as DO models and 

streamflow-routing models) must be calibrated. 

As discussed by Hines, Rickert, McKenzie, and 

Bennett (1975), this procedure is required to adjust certain model parameters so that model results (for a particular set of observed input data) 

adequately represent the observed Hreal world" 

results. HEC-3 contains no model parameters of 

this nature, thereby eliminating the need for 

calibration. 

MODEL VERIFICATION 

An important aspect of any modeling effort is 

model verification. That is, model results must be 

compared to observed, "real world" data to judge 

whether or not the model is a valid predictive 

tool. 

The present reservoir system of 13 reservoirs in 

the Willamette River basin was not ccmpleted 

until 1968. To avoid any anomalies that may 

have existed in the operation of the system with 

the addition of the last few reservoirs, tr. e period 

of 1970 through 1973 was selected for the verification procedure. The 1973 reservoir-sys-l;em configuration and operating rules were defined in the 

model and 1970 (table 4) diversions were assumed 

representative of water demands for the verification period. Unregulated monthly streamflow and 

evaporation data for 1970-73 were used to define 

the system inputs and losses. 

Table 6 (lines 6 and 7) summarizes the streamflow computed by HEC-3 (Qcl and the observed 

streamflow (Q0 ) for the Willamette Piver at 

Salem for the period 1970-73. Both monthly and 

average annual data are shown. Also tabulated 

Oine 8) is the total error of each of the computed 

values (total Qc error in percent of Q0 ). Differences between model results and observed average annual streamflow at Salem are considered 

very acceptable. Comparisons of nonthly 

streamflow values range more widely, from poor 

to very good. 

These total Q c errors are due to differences between the model and the real world for ( 1) inflows 

to and losses from the system and (2) definition 

and operation of the reservoir system. A massvolume balance of an entire reservoir syst.em may 

be expressed as 

IL-Q=~, (1) 

where IL represents an algebraic summation of 

all inputs to the system and all losses from the 

H11 
TABLE 5.-Flow diversion data used in the HEC-3 model 

Control 

pomt 

number' 

Net monthly depletwn !return fl.ow1 2 in cub1c feet per second 

41 

42 

45 

46 

48 

39 

40 

49 

.50 

51 

Year 

1970 

1980 

2000 

2020 

1970 

1980 

2000 

2020 

1970 

1980 

2000 

2020 

Jan 

1 

1 

4 

5 

3 

2 

34 

.53 

8:3 

124 

1970 ------------------- -- 1161 1980 ------------------------ 1291 

2000 ----------------------- 1461 

2020 ----------------------- 1801 

1970 

1980 

2000 

2020 

1970 

1980 

2000 

2020 

1970 

1980 

2000 

2020 

1970 

1980 

2000 

2020 

1970 

1980 

2000 

2020 

1970 

1980 

2000 

2020 

1970 

1980 

2000 

2020 

9 

8 

14 

19 

17 

9 

191 

7 

17 

16 

1191 

181 

6.3 

87 

109 

140 

1 

4 

0 

0 

1171 

1341 

1.341 

1181 

24 

1111 

1181 

29 

'See figure 1 for locatwns. 2Values m parentheses md1cate return flows 

Feb. 

3 

4 

3 

11 

33 

50 

84 

124 

I 131 

1271 

1431 

1711 

11 

10 

17 

24 

24 

17 

9 

33 

19 

17 

1111 

2 

66 

92 

11.3 

142 

3 

6 

.3 

4 

1141 

1.3.3r 

1281 

191 

45 

21 

53 

113 

Mar. 

4 

6 

3 

3 

3 

10 

33 

51 

79 

117 

1141 

1261 

1401 

1671 

10 

9 

16 

22 

21 

16 

9 

31 

18 

17 

191 

•) 

62 

1)5 

104 

131 

I 151 

1291 

r251 

181 

41 

28 

4B 

110 

Apr. 

3 

4 

3 

10 

37 

.56 

86 

128 

1151 

1281 

1461 

1751 

11 

10 

17 

23 

22 

17 

9 

3.3 

19 

17 

1101 ') 

65 

90 

109 

138 

1161 

1311 

r271 

191 

44 

27 

48 

105 

system, Q is the system output, and llS is the net 

summation of the storage change in all reservoirs 

In the system. The IL term is not directly observed (nor directly computed by the model) but 

can be estimated from observed (or computed) 

values ofQ and llS. Therefore, rewriting equation 

1 and subscripting with o for terms related to observed data and c for terms related to computed 

values (model results) yields 

(2a) 

and 

May 

6 

8 

14 

18 

15 

24 

47 

56 

67 

107 

215 

305 

1111 

1161 

1111 

1 

33 

54 

120 

117 

57 

84 

125 

126 

41 

49 

231 

232 

157 

2.36 

.342 

405 

5 

14 

17 

.35 

39 

90 

150 

194 

.300 

46.3 

877 

864 

June 

19 

24 

38 

52 

44 

74 

148 

177 

160 

245 

551 

764 

151 

3 

52 

151 

91 

58 

365 

349 

146 

227 

400 

-!42 

10.5 

126 

754 

768 

.374 

548 

890 

1,044 

11 

.3.3 

51 

12.3 

161 

.338 

568 

722 

883 

1,370 

2,797 

2,892 

July 

20 

26 

40 

54 

47 

78 

158 

187 

197 

306 

657 

931 

1211 

1211 

18 

108 

98 

174 

393 

377 

148 

234 

38.3 

410 

112 

139 

811 

829 

422 

631 

994 

1.185 

9 

.31 

4.3 

120 

175 

384 

591 

756 

966 

1,538 

2,971 

2,968 

Aug 

16 

21 

34 

46 

37 

64 

130 

15.3 

161 

254 

529 

740 

1211 

1271 

121 

62 

80 

1:39 

302 

285 

99 

140 

224 

211 

83 

102 

551 

543 

320 

475 

758 

898 

:3 

16 

22 

68 

107 

230 

377 

469 

631 

964 

2,134 

1.955 

Sept. 

5 

6 

11 

17 

9 

15 

.34 

.38 

86 

1.38 

278 

393 

1221 

1371 

1451 

1581 

33 

50 

122 

112 

46 

43 

22 

16 

47 

53 

179 

190 

174 

250 

.389 

478 

0 

4 

1.31 

14 

22 

47 

84 

129 

239 

.304 

668 

610 

Oct. 

0 

I 1 I 

0 

2 

121 

It' I 

1151 

1141 

42 

6f 

104 

158 

1201 

1381 

1701 

11151 

11 

8 

7 

9 

16 

1151 

1701 

1931 

21 

17 

15.31 

1541 

60 

78 

105 

1.35 

0 

1.31 

1111 

I 191 

1301 

1671 

1851 

1951 

1411 

11601 

12391 

12951 

Nov. 

I 11 

141 

181 

1101 

38 

59 

94 

137 

1141 

1301 

1521 

1921 

13 

10 

14 

13 

27 

10 

1351 

1341 

24 

2.3 

1341 

1211 

66 

91 

110 

141 

•) 

.3 

141 

171 

121 I 

1511 

1721 

1651 

28 

1521 

1831 

1881 

Dec. 

1 

1 

4 

5 

3 

2 

1 

5 

35 

56 

87 

131 

1161 

1311 

1501 

1871 

9 

8 

14 

18 

16 

1 

151 

161 

14 

15 

1181 

1171 

63 

86 

111 

137 

0 

141 

1221 

1491 

1381 

1.351 

14 

1471 

1161 

10 

Table 6 (lines 1-3) summarizes the values of 

IL0 , ILc, and the ILc error. The latter term is computed as 

ILc error= xlOO. (3) 

Also tabulated are the ilS0 and llSc values (lines 4 

and 5). The small reregulating reservoirs (Big 

Cliff and Dexter) are not included in llS because 

( 1) observed data are not available and (2) there 

was a constant zero storage change for these reservoirs in the HEC-3 model. Over 90 percent of 

(2b) the ILc values, which are based on the model reH12 
5 

9 

1. 

8 

2. 

7 3. 

00. 

~ 

::t: 

u 

25 

25 

z 6 

0 

~ 

"< 

~ 

0 

0.. 

"< 

> 5 ~ 

z 

< 0.. 

~ 

...:I 

::t: 

E-t 4 

z 

0 

~ 

1'1:;:1 

C) 

< 

~ 

1'1:;:1 3 

> 

< 

0: 

1'1:;:1 

2 

40 

AVERAGE MONTHLY AIR TEMPERATURE AT EUGENE, 

IN DEGREES CELSIUS 

10 15 

Evaporation rate, Er, in 

inches per month per acre 

of surface area is 

Er = 0. 7 Ep· 

Total reservoir evaporation, 

E, in acre-feet is 

E = A(Er)/12 where A is 

reservoir surface area. 

Evaporation is considered 

negligible for the months 

November through March. 

45 50 55 60 65 

AVERAGE MONTHLY AIR TEMPERATURE AT EUGENE, 

IN DEGREES FAHRENHEIT 

FIGURE 7.-Evaporation relations for the Willamette River basin. 

H13 

20 

200 

00. 

~ t~ 

.,.... 

~ 

,.-.; 

~ H 

~ 

150 ~ 

z 

g 

E-~ 

~ 

~=~ 

0 

~" 

~ 

> 

~ 

z 

<r: 

~< 

~ 

100 s 

t~ 

z 

0 

~ 

~ 

0 

~ 

~: 

~'t:l 

> 

~ 

0: 

~t:l 

50 

70 
TABLE 6.-Summary of model verification results 

Average 

Year Lme Type of data .Jan Feb Mar. Apr. May June .July Aug. Sept. Oct. Nov. Dec. annual 

IL0 !cubic feet per second I _____ 91,190 39,560 25,830 18,740 21,900 8,251 3,719 2,505 3.744 6,524 29,460 43,010 24,520 

ILc lcuhic feet per second!_ 81,570 .38,210 27.860 21,540 24,440 8,676 3.592 2,124 3,601 7,1K6 31,260 311,440 24,010 

3 ILc error !percent of!L,)i __________ -10.6 -3.4 79 15.0 11.6 5.1 -.34 -15.2 -3.8 10.2 6.1 -10.6 -2.1 

4 .:lS" nmverted to monthly flow_ 18,770 -17,490 7,419 5,025 2,771 -564 -3,350 -.5,025 -5,295 -6,206 -1,208 -3,599 -594 

1970 5 .:lSc rate !cubic feet per second! 11,720 -1,355 G,.304 Hl72 ::l,245 -451 -3,412 -4,856 -3,356 -5,274 -8,882 0 -75 

6 Q0 !cubic feet per second! _______________ 72,420 .57,050 Hl,410 1.3,710 19,130 S,t\15 7,069 7,5.30 9,039 12,730 30,710 46,610 25,110 

7 Qc !cubic feet per second! 69,850 39,56(1 21,560 16,670 21,200 9,127 7,004 6,834 6,9.57 12,460 40,140 38,440 24,090 

8 Q rota!.----~--------------------- --3.5 -30 7 17.1 21.6 10.8 3.5 -.9 -9.2 -23.0 -2.1 30.7 -17.5 -4.1 

9 efror due tulLe t>rrur !percent ofQ0 1 _ -13 3 -2.4 11.1 20.5 13.3 4.8 -1.8 -.5.1 -1.6 5.2 5.8 -9.8 -2.0 

10 due to reservou·-storage error __ 9.7 -21'1.3 6.1 1.1 -2.5 -1.3 9 -4 2 -21 5 -7.3 24.9 -7 7 -21 

IL0 I cubic feet per second! ______ 86,830 39,ROO -~1.8011 38,140 21'1,610 19,430 S,831 4,126 6,768 7,757 3.5,500 63,830 32,640 

ILc tcubic feet per second I_ 79,720 :36,770 51,8:30 40,010 28,8.50 19,170 8,170 3,909 6.70.5 8,580 39,900 .'i6,340 31,680 

3 ILc error I percent of IL0 1 _____________ -8 2 -7.6 .1 4.9 8 -13 -7.5 -5.2 -.9 10.6 12.4 -11.7 -2.9 

4 .:lS0 converted to monthly flow_ 9,735 263 6,145 4,822 4,545 -21 -612 -5,595 -5,922 -8,453 1,815 -8.296 -138 

1971 5 .:lSc rate !cubic feet per second I __ 9,043 2,622 6,428 6,289 1,912 43 -1,410 -3,.5.52 -3,267 -9,160 -8,882 0 0 

6 Q0 !cubic feet per second! 77,100 39,G40 4.5.61)0 33,320 24,070 19,450 9,443 9,540 12.690 16,210 33,690 72,130 32,770 

I Qc lcuhic teet per second! _ _ ______ 70,680 34,150 45,41111 33,720 26,940 19,130 9,580 7.461 9,97:l 17,740 48,780 56,340 31,6SO 

8 Qc { ~~: to JL~-~;,~;;(p~;~~,;t~f-Q~~-=== -8.::l -13 6 -.5 1.2 11.9 -l.fi 1.5 -21.8 -21.4 9.4 44.8 -::n.9 -3.3 

9 -9.2 -7.7 .1 5.6 1.0 -1.3 -7.0 -2.3 -.5 5.1 13.1 -10 4 -2 9 

::r:: 10 error due to reservmr-storage error ----- 9 -6.0 -.6 -4.4 10.9 -.3 8.4 -19.5 -20 9 4.4 31.1'1 -11.5 -.4 

1--' 

*"" 1 IL 0 !cubic feet per second I _______________ 78,040 56,320 79,060 37,100 29.160 15,760 6,469 4,0.52 4.374 4,864 10,070 38,420 30,240 2 ILc !cubic feet per second!_ 73,750 56,610 76,700 39,150 28,840 14.480 5,2t1.5 3.492 4,164 5,119 10,590 35,220 29,360 

3 ILc error I percent of IL0 1 -55 .5 -3 0 55 -1.1 -8.1 -18.3 -13.8 -4.8 5.2 5.2 -S.3 -2.9 

4 .:lS11 converted to monthly flow_ 8,911 5.348 5,393 4,107 3,944 5 -980 -4.527 -7,996 -6,086 -6,030 68.5 219 

1972 5 .:l8c rate !cubic feet per second! 405 12,180 6,42R 6,289 1,920 -18 -2,206 -3,796 -3,388 -7,951 -8,882 0 0 

6 

~~ ~~~:~ f::i ~=~ ~=~~~t =====--~=-===== 69,130 50,970 73,670 32.990 25,220 1.5,760 7,449 8.579 12,370 10,950 16,100 37,740 130,020 

7 73,340 44.430 70,270 ~2.860 26,920 14,500 7,491 7,28R 7,552 13,060 19,470 35,220 29,360 

8 Qc {~~:\o i£:"-~~;~;(p~~~~,;t-oYQ~,-~-=- 6.1 -12 8 -4.6 -.4 6.7 -8 0 6 -15 0 -38 9 19 3 20 9 -6 7 -2 2 

9 -6.2 .6 -3.2 6.2 -1.3 -8.1 -15.9 -6 .. 5 -1.7 2.3 3.2 -8 .. 5 -2.9 

10 error due to reservoir-storage error -- 12.3 -13 4 -1.4 -6.6 8.0 .1 16.5 -!-i.5 -37.3 17.0 17.7 18 .7 

IL0 1cub1c feet per second I __________ 37,950 16.420 22,870 20,190 11,500 6,875 3,522 2,489 4,966 (21 

ILc 1cub1c feet per second!_ 34,420 18,300 24,660 24.540 13,160 6,767 3,111 2.093 4,722 6,653 63,080 74,960 23,060 3 ILc error I percent of IL0 1 -9.3 11.-i 7.8 21.5 14.4 -1.6 -11.7 -15.9 -4.9 (21 

4 .:lS11 converted to monthly flow ___________ 296 3,572 6,65R 7,109 3,797 336 -2,536 -4,091 -3,081 (2) 

1973 5 .:lSc rate !cubic feet per second! 0 ::l,282 5,489 6,003 2.725 -188 -2,889 -3,907 -1,278 -1,571 -7,441 2,157 183 6 Q0 !cubic teet per second! ___________ 37,6.50 12,8.50 16,210 13,080 7,701 6,539 6,058 6,580 8,047 13,970 70,440 85,930 "23,600 7 Qc !cubic feet per second I 34,420 1.'5,020 19,170 18,540 10,440 6,955 6,000 6,000 6,000 8,224 70,520 72,800 -122,880 

R Q rota! ----------------------- -S.6 1li.9 18.3 41.7 35.6 6.4 1.0 -8.8 -2.5 4 -411 .1 -15.3 -3.1 9 ei~;·or due to ILc error I percent of Q0 1 -9 4 14 6 110 333 21 6 -1 7 -6.8 -6.0 -3.0 (2) 

10 tlue to reservOir-storage error _ .8 2.3 7 2 8.5 13.9 8.0 58 -2 8 -22.4 1"1 

'Adjusted to a 28-day February since HEG-3 1gnores leap year. 2Final reservOir-storage data unavailable at hme of analysis. 3 0ctober through December values are prelimmary and subject to revision. 

"October through December results are based on preliminary observed streamflows at all ga~mg statwns 
suits, are within ±15 percent of the lL0 values 

which are based on observed data (table 7 and 

fig. 8). This is especially gratifying considering 

the vast opportunity for discrepancies in the ( 1) 

correlations for natural inflows, (2) diversion 

data, and (3) evaporation data. The errors in the 

annual average flow for 1970-73 are all less than 

3 percent. 

Additional insight to the modeling error can be 

gained by subtracting equation 2a from 2b and 

rearranging to obtain 

Furthermore, multiplying through by 100/Q0 results in 

(

Q -Q ) CQO 

0 

X 100 

The three bracketed terms in equation 5, fron1left 

to right, represent (fig. 8) (1) total Qc error, (2) Qc 

error due to !Lc error, and (3) Qc error due to 

reservoir-storage error. Table 6 (lines 8-10) 

summarizes these errors (with some roundoff error). Generally, errors due to reservoir-storage 

error are more significant than those due to ILc 

error (tables 7 and 8 and fig. 8), especially when 

the total Q c error is large. 

Most of the error related to reservoir storage is 

due to differences in actual reservoir operation as 

compared to the reservoir operatior in the 

reservoir-system model. The summation of both 

the storage changes and the total storage for 11 

reservoirs in the Willamette River barin reservoir system (Big Cliff and Dexter are orritted) for 

the rule curves defined for the model are tabulated in table 9. Figure 9 illustrates the operation 

differences between the model and the actual reservoir system. 

Operational differences are a commor problem 

in reservoir-release modeling. Rule curves are 

formulated during the reservoir's design phase in 

such a way that benefits from the reservoir will 

be maximized over a long period. Es"entially, 

they reflect the operation of the reservoir for ((average'' hydrologic conditions. Departures of the 

hydrologic conditions from ('average'' frequently 

necessitate deviations from the design rule curve. 

A good example of such a case is the limited snowfall in the winter of 1972--73. Even by holding 

spring flows relatively low, it was only p'1ssible to 

fill the reservoir system to about 92 percent of 

desired capacity. Therefore, to meet derired flow 

objectives in July through September, system 

storage was depleted to less than 85 :r: 8rcent of 

desired capacity in August. The HEG-3 model (in 

its present form) was not capable of such anticipation and only filled the system to about 76 percent 

and depleted it to less than 65 percent of desired 

capacity. 

Design rule curves (or parts the reo[) may also 

become outdated if new objectives are considered. 

TABLE 7.-Model errors classified by absolute error limits 

Percentage of months during the period January 1970 through September 

1973 for wh1ch model errors are within absolute error lim1ts of- Type of model error' 

5 

percent 

EJL _____________ 31.1 

EQT _____ 26.7 EQJL _____________ 37.8 

EQR _____ 35.6 

10 

percent 

6.±.4 

.±8 9 

75.6 

64.4 

15 20 percent percent 

91.1 97 8 60.0 71.1 91.1 933 75.6 84.4 

25 30 35 

percent percent percent 

100.0 

84A 86.7 911 

97.8 97.8 100.0 

93.3 9.56 97.8 

'These symbols are used lin thts table and in table 8 and figure 81 to mdtcate absolute magmtude of the model errors as follows: EJL -[Lc error 1lme 3, table 61 

EQr-total Qc error llme 8, table 61 

EQJL -Qc error due to ILc error 1lme 9, table 61 

EQR -Qc error due to reservOir-storage error I line 10. table 61 

TABLE B.-Summary of modeling errors U.'ith absolute value exceeding 15 percent 

Number 

of 

observatwns 

Average 

absolute 

error 

I percent I { 

EQT --- EQJL ---

EQR ---

Jan. 

.. 

0 

-----------

Feb 

.. 

Q 

23 . .8 

8 .. 5 

15.3 

Mar 

4 

2 

2 

17.7 

110 

6.7 

H15 

Apr 

4 

•) 

2 

31.7 

36.9 

.±.8 

May 

4 

1 

1 

35.6 

21.6 

13.9 

Month 

June July 

4 .. 0 u 

Aug. 

4 

1 

21.8 

2 .. :3 19 .. 5 

Sept. 

.. 

4 

27.3 

1.7 

2.5.5 

.±0 

percent 

95.6 

100.0 

Oct. 

19.3 

2.3 

17.0 

Nov 

32 .. 1 

7.4 

2HI 

45 percent 

100.0 

Dec. 

19.7 

10.1 

9.6 
100 

0 

w 0 

0 w w 0 w 80 w u w X u w X 

0::: w 

0 0::: 

0 0 

w 0 .....:1 GO w 60 ..:r: .....:1 

;:::; ..:r: 0 ;:::; w G' 

;a w 

...:l ~ 

w E-< 

G' w 40 w 40 

~ w E-< ~ ~ E-< 

0 ~ 

E-< 0 z :20 E-< w 20 z u w 0::: u w 0::: 0.. w 

0.. 

0 0 15 25 

ElL' IN PERCENT EQT' IN PER CENT 

100 

0 

w 0 

0 w w 0 

w w u 80 w 80 X u w :X: 

w 0::: 0::: 0 0 

0 

w 0 

.....:1 w ..:r: 60 .....:1 60 ;.:.., ..:r: 

G' ;:::; 

w G' 

w ;a ;a ~ 0::: G' G' w 40 w 40 w w g ~ 

E-< ~ ~ ~ 

0 0 

E-< E-< z 20 z 20 w w u u 0::: 0::: w w 0.. Pot 

0 

25 30 35 15 20 40 

EQIL' IN PERCENT EQR' IN PERCENT 

FIGURE B.-Distribution of model errors, January 1970 through September 1973. A, ILc error. B, Total Qc error. C, Qc error 

due to ILc error. D, Qc error due to reservoir-storage error. 

H16 
TABLE 9.-Summation of total end-of-month storage and monthly storage changes specified by the design rule curces for 11 

reservoirs in the Willamette Riuer basin reseruoir system 

Svstem January February March April May June st~rage' 

End-of-month total ___ ------------------ 715.74 1.417.30 1,812.50 2,186.70 2,306.82 2,306.82 

Change dunng month -- ------------------ 0 701.56 395.20 :374.20 120.12 0 

System July August September October November December storage 1 

End-of-month total 2.224.00 2.067 .. 00 1.809.00 1,24-!.20 715.74 715.74 

Change during month -82.82 -157.00 -258.00 -564 .. 80 -.528.46 0 

'Storage quantities, shown m thousands of acre-feet, represent summatwn of indi\·idual destgn rule curves for the enttre system, wtth the exceptton of Big Cliff and Dexter reregulatton reservoirs. 

--Design Rule Curves 

o Observed Data 

I!!. HEC-3 :.Iodel Results 

2000 

rn 

w 

c:: 

Ew 

~ 

~ u 

w 

~ (:::) 

~ 

~ 

w 

0 

<C 

~ Ern 

E= 

z 

0 

~ 

I 

i:;.. 

0 

I 

c 

z 

~ 

...:l 

<C 

E0 

EFIGURE 9.-Comparison of design, observed, and computed reservoir-system storage. 

This has occurred in the Willamette River basin 

where increased flows in late August and in September are now used to stimulate fall fish runs. 

Table 6 readily reveals this effect with Q 0 consistently higher than Qc in August and September. 

Revision of the rule curve definitions in the model 

would have eliminated this particular bias, but 

detailed information was not available to make 

the revision for this study. 

Other general possibilities for deviation from 

the design rule curve at individual reservoirs are 

(1) floods which require temporary use of the flood 

control pool, (2) special operational procedures to 

optimize short-term power production in the 

basin (especially if a portion of the total generation capacity is inoperable), and (3) terr.porary 

storage depletion and (or) discontinuation of releases for maintenance reasons. 

Another important factor to consider in the discussion of the HEC-3 model errors is the combination of temporary flood storage and the 

monthly time incren1ent. Several instances can 

be found where the end-of-month deviatior above 

the rule curve (fig. 9) is due to a flood occurring 

late in the month. An examination of ol:'served 

daily data reveals that the system storage was 

depleted to the design rule curve shortly into the 

next month. Evacuation of the flood wat£-v-s any 

H17 
sooner would have created unnecessary flooding 

below the reservoirs. 

In summary, the verification results were considered acceptable. The HEC-3 model duplicated 

the actual system fairly well when considering 

that ( 1) some of the errors are due to the monthly 

time increment <which tend to balance out over 

two or more months) and ( 2) the rule curve defined in the model could be altered to more closely 

reflect current operating procedures. The ILc 

error analysis provided confidence in the input 

data used to define the natural streamflows, the 

diversions, and the evaporation for the system as 

a whole. 

APPLICATIONS OF THE MODEL 

The frequency-discharge relations shown in 

figure 10 can be used to estimate the low-flow 

characteristics required to satisfy the first two 

modeling objectives. The lower relation is based 

on the unregulated, monthly streamflow at Salem 

for 1926-73. These streamflow data (developed to 

satisfy part of the model input requirements) are 

an estimate of the natural flows that would have 

occurred at Salem had the reservoir system never 

been developed. Simulated monthly streamflows 

at Salem were used to determine the upper relation. These simulated streamflow data are the 

output from the model applied as described in the 

verification procedure except that unregulated 

streamflow data and evaporation data for 1926-

73 were input to the model. Thus, these simulated 

flows represent the homogeneous, 48-year record 

of monthly streamflow that would have occurred 

1. 01 1. 05 1.11 1. ~5 

at Salem had the existing (1973) reservoir system, subjected to present (1970) flow diversions, 

been in operation since 1926. Estimates of annual 

minimum low-flow rates based on these relations 

should be significantly more reliable than those 

based on available data <Hardison, 1969), assuming that model results are of acceptable accuracy. 

A comparison of these relations provides an 

impressive picture of the increase in the annual 

monthly minimum low flows made possible by the 

reservoir system. These relations can also provide 

the critical low-flow data required for input to the 

DO model. Assume, for example, that n1inimum 

DO concentrations for the Willarnette River at 

Salem occur during the annual minimum 7-day 

low-flow period. The relation shmvn in figure 5 

can be used (as illustrated in a suh-;<equent example) to estimate annual minimum 7 -day low flows 

from annual minimum monthly low flows. Such 

estimates of 7 -day low-flow rates, for both regulated and unregulated conditions, can be input to 

the DO model to compute the anrual minimum 

DO concentration for the entire range of low-flow 

recurrence probabilities. The resultant 

frequency-DO concentration relations provide a 

means of assessing the total impact of the existing reservoir system on minimum DO concentrations at Salem relative to the minimum DO concentrations that would have occurred at Salem 

under natural conditions for the same 48-year 

period. 

The third modeling objective wc-s to develop a 

model which could be used to estimate 

streamflows for various planning and manage-

/ 

Regulated 

5 10 30 100 

HECURRENCE INTERVAL, IN YEARS 

FIGURE 10.-Frequency-discharge relations at Salem, 1926--73, computed from a simulated streamflow for existing !regulated) conditions and from estimated streamflow for naturallunregulated) conditions. 

H18 
ment alternatives. Such alternatives or conditions might include ( 1) different demands on the 

system (such as 1980, 2000, or 2020 flow diversions), (2) different system-operation schemes 

(such as deemphasis and (or) placing more emphasis on one or more of the various authorized 

uses), (3l addition of reservoirs to the system, (4) 

the occurrence of more severe hydrologic conditions (such as more extren1e droughts than have 

been experienced), and (5) combinations of the 

above, ad infinitum. 

The scope of this report does not permit 

analysis of all reasonable alternatives. However, 

to illustrate this type of model application, three 

additional simulations were performed. Each of 

these three simulations were identical to the previous 48-year simulation, except that the flow diversions used were those estimated for the years 

1980, 2000, and 2020. Thus, including the previous simulation, 1926--73 data sequences of 

monthly streamflows reflecting four levels of estimated flow diversions (table 4) are available for 

comparison. Frequency-discharge relations of 

annual minimum monthly flows for all four simulations are presented in figure 11. These relations 

may be used to determine the annual minimum 

monthly low flow for any recurrence interval 

(hereafter denoted by tQ m , where t is the recurrence interval and Qm denotes average monthly 

flow rate). 

Also, as mentioned above, estimates of annual 

minimum 7-day low flows for any recurrence 

interval (tQ 7, where t indicates recurrence interval and Q7 denotes average 7 -day flow rate) may 

be obtained using the t Q m data. The relation of 

annual minimum 7 -day to annual minimum 

monthly low flows (fig. 5) may be expressed as 

(6) 

Table 10 summarizes estimates of annual minimum monthly and annual minimum 7 -day flows 

for the 1970 and 2020 levels of flow diversions. 

Also shown is the percent reduction in these flows 

resulting from the increased flow diversions projected for 2020 relative to the estimated flow diversions of 1970. A DO model could be used to 

assess the impact that the increased flow diversions (reduced minimum flows) would have on DO 

concentrations. 

Other low-flow periods may also be useful for 

assessing potential river-quality problems. For 

example, the temperature, sunlight, and flow 

conditions that combine for critical algal growth 

conditions may not coincide with the annual 

minimum 7 -day or annual minimum n1onthly 

low-flow period. Frequency-discharge relation by 

calendar months might be more applicable. Figure 12 illustrates such relations for the individual 

months of July, August, and September b!lsed on 

the simulated streamflows reflecting the 1970 diversions. Another potentially useful tool for 

river-quality assessn1ent might be the annual 

minun1um flow for multimonth periods. Figure 13 

shows frequency-discharge relations for the annual minimum flow for consecutive 2- and 3-

month periods based on the simulated streamflows reflecting the 1970 diversions. 

Only the Willamette River streamflow at 

Salem is utilized in the above examplee., However, model results may be obtained at any control point (fig. 1 ). Therefore, similar evaluations 

could be made for the streamflow at several points 

for basinwide river-quality assessment. 

SUMMARY AND DISCUSSION 

A reservoir-system model provides a useful tool 

for improving an available streamflow data base 

which consists of streamflow data that have been 

observed during a period of changing conditions. 

Applicability of the U.S. Army Corps of Engineers' HEC-3 reservoir-system model to the 

W illamette River basin reservoir system is verified by comparing model results with ol~~erved 

stream-flow at Salem for a 4-year period (1970--

73. In general, modeling errors for this verification period are within reasonable limits. Predictive errors could possibly be reduced by (1) revising model input data to better define actual 

reservoir-system operation; (2) refining unregulated streamflow estimates by using waterbudget computations along with corr~lation 

analyses; and ( 3) using more refined evaporation 

estimates and including precipitation data in the 

analysis. This last source of possible error applies 

to the entire basin. The first two, however, could 

possibly be isolated to one or more segm~nts of 

the basin by comparing model results with observed streamflow at other control points in the 

basin. Also, a longer verification period could add 

increased confidence in model applicabilit:r. 

Despite the above limitations, the HEC-3 

reservoir-system model is used for four ::-imulations of monthly streamflow data to demonstrate 

H19 
1- r-----..... - 1- ~ 

~ 0" ~ 

1- ~ v - 1-

I-

··~ 

~ ~---~ t-- 0 -

f-

(C) 

1- r-- - 1- v v 

1- VVv ~ 

1-

1-

·"'vo 

~ 

516/ ~ - 1-

396t ~ - 1-

318~ f'-

1-

- 50 

(D) 

1. 01 1. 05 1.11 1. 25 2 5 10 25 50 100 

RECURRENCE INTERVAL, IN YEARS 

FIGURE H.-Frequency-discharge relations at Salem, 1926---73, computed from simulated streamflows reflecting the 

existing reservoir system subjected to different estimated flow diversions. A, 1970 flow diversions. B, 1980 flow 

diversions. C, 2000 flow diversions. D, 2020 flow diversions. 

TABLE 10.-Determination of tQm and tQ7 at Salem for the 

simulated streamflou•s reflecting the 1970 and 2020 flow 

diversions 

Simulatton tQm 1 

using- lyearsl 1tP/s1 

1970 10 6,1.'\0 

diYersions 25 5,880 

50 5,700 

1From figures 11 tor 1970 and 2020 flow dlVersions. 2Computed by equatiOn 6, tQ7=0.87tQ 111 • 

Reductwn 

tQ7" in flow 1lt"/sl I percent 1

3 

5.380 

.5,120 

4.960 

TABLE 10.-Determination of tQm and tQ7 at Salem for the 

simulated streamflows reflecting the 1970 and 2020 flou.• 

dit~ersions-Continued 

Reduction 

S1mulatwn 1Qm 1 tQ7" m flow 

using- I years I ltt"/sl lft"/si I percent is 

2020 10 5,160 4,490 17 

diYersions 25 3,960 3,450 33 

50 3,180 2,770 44 

"Percent reduction oflow flows at Salem reflecting 2020 diverswns as compared 

to those reflectmg 1970 di verswns. 

H20 
15.---r---------.-----~-----r------------~----------~------~------r---~--~400 

10 

(A) 

350 

300 

250 

200 

150 

0 

z 

0 

u 

"" rt:J 

4----~--------~----~----~------------~----------~------~------~--~--~ 0:: 

"" 0.. 

-

-

- ff- ~ --a---e-~ .... - r- .~ 

r- .... ,..,.., 

f- 0 u r-- - f- (C) 

1.01 1. 05 1.11 1. 25 2 5 10 25 50 100 

RECURRENCE INTERVAL, IN YEARS 

FIGURE 12.-Frequency-discharge relations at Salem, 1926--73, computed from discharges for calendar months. A, 

July. B, August. C, September. 

1. 01 1. 05 1.11 1. 25 2 5 10 25 50 100 

RECURRENCE INTERVAL, IN YEARS 

FIGURE 13.-Frequency-discharge relations at Salem, 1926-73, computed from mm1mum multimonthly discharge. A, 

2-month minimum discharge. B, 3-month minimum discharge. 

potential applications. Each simulation utilizes 

identical input data for (1) defining system inputs 

and losses with unregulated monthly streamflow 

and evaporation data for a 48-year period (192~ 

73) and (2) defining the operation and configuration of the existing reservoir system. Different diversion data, reflecting estimated flow diversions 

for the years 1970, 1980, 2000, and 2020, are used 

for each simulation. 

Frequency-discharge relations of low flow at 

H21 
Salem based on these simulated streamflows for 

regulated conditions and on the estimated 

streamflow for natural conditions are used to 

demonstrate potential applications of simulated 

streamflow data. Detailed examples illustrate (1) 

use of low-flow characteristics reflecting existing 

conditions and those reflecting natural conditions 

as a basis for assessing the total impact that the 

existing reservoir system has had on DO concentrations at Salem and (2) use of low-flow characteristics reflecting existing conditions and those 

reflecting estimated 2020 flow diversions as a 

basis for assessing the impact that the higher flow 

diversions projected for the future will have on 

DO concentrations at Salem. Other potential applications of the reservoir-system model are 

briefly discussed. 

Each reader, of course, is free to pass independent judgment as to the applicability of the simulated streamflow data. The author feels that the 

discharge-frequency relation based on the 

streamflow data simulated using estimated 1970 

flow diversions (fig. 11A) is a more reliable estimate of long-term basin response to existing conditions that can be obtained by using the limited 

observed data that are applicable (fig. 3). Also, 

comparison of figures 11A-D provides a sound estimate as to the relative decrease in low flows as a 

result of increased future flow diversions. 

Assuming that either (1) the model is adequate 

as described herein or (2) it can easily be altered 

to yield more acceptable results, the potential 

benefits of using it are obvious. It is hard to en vision any other tool that could provide such flexibility for analyzing planning or managementalternatives in the Willamette River basin. 

REFERENCES CITED 

Draper, N. R., and Smith, H., 1966, Ap":llied regression 

analysis: New York, John Wiley and So:'l.s, Inc., 407 p. 

Gleeson, G. W., 1972, The return of a river, the Willamette 

River, Oregon: Advisory Comm. Environmental Sci. and 

Technology and Water Resources Inst., Oregon State 

Univ., Corvallis, 103 p. 

Hardison, C. H., 1969, Accuracy of streamflovr characteristics, 

in Geological Survey research 1969: U.S. Geol. Survey 

Prof. Paper 650-D, p. D210-D214. 

Hines, W. G., Rickert, D. A., McKenzie, S. W., and Bennett, J. 

P., 1975, Formulation and use of practical models for 

river-quality assessment: U.S. Geol. Survey Circ. 715-B, 

p. 13. 

Jennings, M. E., Shearman, J. 0., and Bauer, D. P., 1976, 

Selection of streamflow and reservoir-rdease models for 

river-quality assessment: U.S. Geol. Survey Circ. 715-E, 

12 p. 

Rickert, D. A., and Hines, W. G., 1975, A pra-:tical framework 

for river-quality assessment: U.S. Geol. Survey Circ. 

715-A, 17 p. 

Texas Water Development Board, 1972, Economic optimization and simulation techniques for management of regional water resource systems, river h'lsin simulation 

model, SIMYLD-II program descriptio'l: Texas Water 

Devel. Board, 106 p. 

U.S. Army Corps of Engineers, 1968, HEC-3, reservoir systems analysis: Hydrol. Eng. Center U ~ers Manual No. 

23-53, 86 p. 

U.S. Geological Survey, 1973, Water res0urces data for 

Oregon-part 1, surface water records: 409 p. 

Willamette Basin Task Force, 1969, Appendix M, Willamette 

basin comprehensive study: Pacific Northwest River Basins Comm. Rept., p. II-1-II-34. 

H22