Physicochemical and Biological Characteristics of Hills Creek Reservoir

by R. C. Scheidt

and ihl J. L. Nichols

Water Resources Research Institute Oregon State University Corvallis, Oregon WRRI-50 November 1976 PHYSICOCHEMICAL AND BIOLOGICA L

CHARACTERISTICS

HILLS CREEK RESERVOI R

A SUMMARY REPOR T

R . C . Scheidt Water Resources Research Institut e

an d

J . L . Nichol s Department of Botany

Water Resources Research Institut e Oregon State University Corvallis, Oregon 9733 1

October 1976 ABSTRACT

Physicochemical and biological variations in Hills Creek Reservoir ,

Oakridge, Oregon and physicochemical characteristics of influent tributarie s and effluent water were investigated weekly from mid-May 1975 to mid-Februar y

1976 . Lotic and limnetic parameters measured were ammonia, nitrite, nitrate , soluble orthophosphate, suspended solids, turbidity, conductivity, pH, an d water temperature . Insolation, tributary flows, reservoir algal populations , and reservoir dissolved oxygen were also measured .

In late spring and summer a strong, shallow metalimnion develops in th e reservoir between 12 m to 20 m water depth . Biological debris or potentia l

"recyclable" nutrients in the epilimnion are wind-rafted and mixed with down - welling Willamette River water . The result is that biological activity i s restricted to a shallow, nutrient poor epilimnion .

The disappearance of the metalimnion in late fall results in a uniforml y turbid reservoir and uniform soluble orthophosphate concentrations at al l depths . However, nitrogenous nutrient concentrations vary in an unpredictabl e manner due to different degrees of decay and oxidation . Also during this time clear, cold Willamette River water enters the main pool at all depths . The exception is during a major storm when the Willamette River increases it s suspended load and becomes very turbid . These waters enter the reservoir a s turbidity currents . The reservoir temperatures remain uniform at all depths , but a pycnocline develops which isolates the turbid mid- and lower waters fro m the surface waters .

Mass-balance calculations indicate that the reservoir retained 35 .2 kg/se c total suspended solids and 12 .2 g/sec soluble orthophosphate and released 24 .1 g/sec nitrogenous nutrients during the major storm interval betwee n

5 November 1975 and 17 January 1976 . A spring-summer model and a fall-winte r model are proposed to explain some observed physicochemical and biologica l behaviors of Hills Creek Reservoir . ACKNOWLEDGEMENT S

The authors wish to thank the Portland District Office, U . S . Army Corp s of Engineers, who supported this research under contract DACW57-75-C-0154, an d to Doug Larson, Hydrology Branch, for his keen interest in this research .

We give our heartfelt thanks to numerous individuals at Oregon State Uni- versity, who aided in establishing the field laboratory by loaning essentia l pieces of equipment .

We wish to thank Dave Curtis, Portland Office, U . S . Geological Survey , for furnishing flow data on Hills Creek, Sand Prairie and the Middle Fork o f the Willamette River .

And finally, we thank George Dittsworth, E . P . A., Corvallis, for hi s hours spent on determining the particle sizes of four turbid reservoir wate r samples .

TABLE OF CONTENTS

PART 1 Pag e

INTRODUCTIO N Previous Studie s Current Researc h 1 SAMPLING AND ANALYTICAL PROCEDURE S 2 Collection of Sample s 2 Analyses of Water Sample s 5 Dissolved Oxyge n 5 pH 5 Nutrient Analyse s 5 Suspended Solid s 6 Turbidity 8 Conductivity 8 Algal Enumeratio n 8 Summaries of Dat a 9 Insolatio n 9 RESULTS AND DISCUSSION . 1 0 Allochthonous Sources and Variations 1 0 Autochthonous Variation s 1 4 Incident Solar Radiatio n 33 Effluent Water Variation s 33 CONCLUSIONS 36 Descriptive Mode l 39 Spring-Summer Mode l 39 Fall-Winter Mode l 40 BIBLIOGRAPHY 42

PART 2 APPENDIX I : Lotic Data on Influent and Effluen t Waters of Hills Creek Reservoi r APPENDIX II : Limnological Data from Three Samplin g Stations on Hills Creek Reservoi r APPENDIX III : Percent Transmittance of Surface Ligh t •APPENDIX IV : Algal Species Enumeration from Thre e Sampling Stations on Hills Creek Reservoi r APPENDIX V : Total Daily Incident Solar Radiatio n

Appendices are contained in a separate volume . TABLES

No . Title Page

1 95% Confidence Level for Single Ammonia, Nitrite, Nitrat e 7 and Orthophosphate Determinations

2 Particle Size Analyses of Samples from 30 m, 40 m, 50 m , 23 and 60 m, Hills Creek Reservoir,-14 January 1976

3 Taxa List for Hills Creek Reservoir, 15 May 1975 t o 30 15 February 1976

FIGURES

No . Title Pag e

1 Reservoir Sampling Stations and Tributary Sampling Site s 3 at Hills Creek Reservoir

2 Time-Series of Physicochemical Characteristics of Hill s 1 1 Creek

3 . Time-Series of Physicochemical Characteristics of Middl e 1 2 Fork, Willamette River, at Sand Prairie

4 Pool Elevation and Depth-Time Temperature Isopleths , 1 5 Station 2, Hills Creek Reservoir

5 Depth-Time Series of pH at Station 2, Hills Creek Reservoir 1 6

6 Depth-Time Series of Dissolved Oxygen, Station 2, Hill s 1 8 Creek Reservoir

7 Depth-Time Series of Turbidity, Station 2, Hills Cree k 1 9 Reservoir

8 Depth-Time Series of Total Suspended Solids, Station 2 , 20 Hills Creek Reservoir

9 Vertical Profiles of Turbidity from 7 January to 11 Febru - 2 1 ary 1976, Station 2, Hills Creek Reservoir

10 Depth-Time Series of Total Nitrogen, Station 2, Hill s 23 Creek Reservoir

11 Depth-Time Series of Soluble Orthophosphate, Station 2 , 2 5 Hills Creek Reservoir

12 Time-Series of Five Most Abundant Species, Station 2 , 28 Hills Creek Reservoir

13 Weekly Mean Radiation in Langleys per Day 3 0

14 Time-Series of Physicochemical Characteristics of Effluen t 3 2 Water from Hills Creek Reservoir (Middle Fork, Willamett e River)

15 (a) : Proposed Spring-Summer Reservoir Model ;

(b) : Proposed Fall-Winter Reservoir Model INTRODUCTIO N

Previous Studie s

As a result of the large December 1964 flood, a particularly apparen t turbidity problem developed at Hills Creek Reservoir, in the Upper Willamett e

Basin . Causes and sources of the turbidity problem were uncertain and con- flicting opinions were expressed . When the pool and pool release waters i n

1965 failed to eliminate the existing turbidity, the Portland District, U . S .

Army Corps of Engineers and other Federal Agencies began limited sampling pro - grams in the reservoir and of effluent waters .

The Corps of Engineers reviewed turbidity problems with Oregon State Uni- versity personnel in 1970 and an investigation of Hills Creek Reservoir turbidity problem was initiated in May and terminated in September 1971 . The purpos e of the study was to (1) determine the causes and sources of turbidity at Hill s

Creek Reservoir, (2) compare turbidity at Hills Creek Reservoir with that a t other reservoirs in the Willamette River Basin, (3) examine, on the basis o f information obtained for Willamette Basin Reservoirs, the potential for turbidity problems at Lost Creek Reservoir, upper Rogue River, Oregon (then under con- struction), and (4) consider preventative and/or corrective measures whic h might be taken to minimize the turbidity problem at Hills Creek Reservoi r

(Youngberg, Klingeman, et al ., 1971) . The results of this investigation wer e reported by Youngberg, Klingeman, et al . (1971) .

Current Researc h

Sporadic investigation of the turbidity problem, by the Oregon Departmen t of Environmental Quality and certain Federal Agencies, was continued betwee n

1971 and 1974 . In November 1974, Oregon State University was contracted by the Portland District, Corps of Engineers to re-evaluate certain biologica l aspects and to determine physicochemical lotic and limnetic characteristics o f

Hills Creek Reservoir . The specific objectives of this study were to (1 ) initiate an uninterrupted sequence of data, concerning reservoir turbidity , reservoir algae and release water quality, (2) determine the allochthonou s and autochthonous sources of algal nutrients, (3) determine factors which ma y influence the vertical and horizontal distribution of algae in the reservoir ,

(4) locate point sources of material that enter the reservoir and alter nutri - ent concentrations and turbidity, (5) obtain information about Hills Creek

Reservoir during the period December through March when the pool is at a mini - mum, and (6) operate a laboratory facility at Hills Creek Reservoir on a per- manent basis for an entire year .

A field laboratory was established in the former Project Engineers offic e at Hills Creek Reservoir and became functional in May 1975 . Reservoir samplin g began at this time ; tributary sampling began in June and nutrient analyses o f reservoir and tributary waters began in July 1975 . This summary report dis- cusses the water quality research at Hills Creek Reservoir between May 197 5 and mid-February 1976, at which time the sampling ceased .

SAMPLING AND ANALYTICAL PROCEDURE S

Collection of Samples

Three sampling stations were established in the reservoir . Station 1 wa s located in the Hills Creek Arm ; station 2 was located in the widest reach o f the main pool and station 3 was at the log boom located in the main pool .

These stations are shown in Figure 1 . The reservoir stations were sample d

.[L

Figure 1 : Reservoi r Sampling Stations an d Tributary Samplin g Sites at Hills Cree k Reservoir .

Le i eYa el -Reservoir-4. Camp( Sta.t .on% . X -r-r4bufo.•e Sl'ies. a

3 weekly . Stations 1 and 2 were sampled from surface to near bottom with 62.,

Van Doren bottles . Three 1-Q samples were taken from each selected samplin g depth for algal enumeration, nutrient analyses, pH, turbidity, conductivity , and suspended solids . Light penetration was measured at these stations with a

Kahl, Model 268WA310, submarine photometer . The readings were converted to per - cent surface radiation available with depth . Dissolved oxygen was determine d with a portable Yellow Springs Instruments, Model 54, oxygen meter, immediately , on the samples taken for nutrient analyses . Station 3 was sampled from surfac e to 12 m for algal enumeration, dissolved oxygen, pH, conductivity an d turbidity . The 1-z sample bottles were amber Nalgene plastic . After sample s were obtained, the bottles were stored in an insulated cooler . It was fel t that using dark bottles and keeping the samples cool would minimize chemica l and biological changes between collection time and analysis .

Surface to bottom temperature profile data were obtained with a Montedoro -

Whitney, Model TC-5A, portable thermistor probe, at stations 1 and 2,and fro m surface to 12 m at station 3 .

All flowing tributaries entering Hills Creek Reservoir and effluent wate r from the reservoir were sampled weekly . Sampling sites were located out of th e reach of the reservoir and in flowing waters . These sites are shown in Figur e

1 . Temperatures were obtained with a cladded mercury thermometer . The dept h and width of each tributary (except Hills Creek, Sand Prairie and Middle Fork ) were measured and the water velocity determined with General Oceanics,Mode l

2030, Digital Flowmeter . Flow data on Hills Creek, Sand Prairie, and Middl e

Fork, Willamette River were obtained from the Geological Survey . A 1-Q sampl e was taken from all flowing influent waters and from effluent water for nutrien t

4 analyses, pH, turbidity, conductivity and suspended solids .

Analyses of Water Sample s

All physical and chemical analyses were performed as described in Standar d

Methods for the Examination of Water and Wastewater (1971) and/or in Method s

for Chemical Analysis of Water and Wastes (1974) . Because of personnel con- straints and the large number of samples, single nutrient analyses were per - formed . All samples were analyzed within 24 hours from collection .

Dissolved oxyge n

As mentioned, a Yellow Springs Instruments, Model 54, oxygen meter wa s

used to determine dissolved oxygen in reservoir water samples . No dissolve d

oxygens were determined on tributaries or release water . Prior to each sampl-

ing trip the oxygen meter was air calibrated and appropriate barometric an d

temperature correctionsmade . The probe was slowly agitated in a freshly drawn water sample until the system equilibrated . The water temperature and dissolve d

oxygen was then read from the meter . The dissolved oxygen values are reporte d

in ppm (mg/i) .

Immediately upon return to the laboratory, the water pH was determined wit h

an Orion 407A electrometer, interfaced with a Corning ruggedized Ag/AgCl-glas s

reference combination electrode . The system was calibrated with a pH 7 stand-

ard . Since natural waters are unbuffered, it was necessary to immerse the com-

bined electrode, without stirring, in the water for 5 minutes . This insured a n

adequate response time for the system .

Nutrient analyse s

A Bausch and Lomb Spectronic 700, U .V .-visible spectrophotometer, was use d

5 to determine nutrient concentrations . The spectrophotometer was calibrated for ammonia, nitrite, nitrate, and soluble orthophosphate . Calibration curve s were calculated over the range of 0-100 u9/Q for each nutrient (Imbrie, 1955) .

Each sample series for nutrient analyses was accompanied by reagent blank s

and two standards . The blanks and standards were treated in the same manne r

as the unknowns . Since replicas were not analyzed, there is some uncertaint y

in precision . Based upon instrumental calibration curves, Table 1 give s

tabulated 95% confidence levels for a single nutrient determination (Larse n

and Wagner, 1975) .

Matched 5 cm quartz cells were used in the nitrite, nitrate, and solubl e

orthophosphate determinations, while matched 1 cm quartz cells were used in th e

ammonia determination .

Highly turbid samples were filtered through Millipore, binder free, boro-

silicate prefilters, backed with Millipore BD, polyvinyl chloride filters (0 .6p) .

The filtrates were analyzed for nutrients and reagent blank corrections made o n

the gross data . Nutrients in lightly turbid samples (<4 NTU) were determine d

without filtering and reagent blank and turbidity corrections applied to th e

gross data . All nutrient data are reported as ug/R of the element .

Ammonia/ammonium was always the first nutrient analyzed . This insured

minimal loss of ammonia gas . It was determined by the phenate method . Nitrite

was determined by the diazotization method, soluble orthophosphate by the com -

bined reagent technique (ascorbic acid reduction) and nitrate by Cu-Cd reduc-

tion, followed by diazotization,and by the ultraviolet method .

Suspended solid s

Total, fixed and volatile suspended solids were determined by filterin g

Table 1 . 95% Confidence Level for Single Ammonia, Nitrite, Nitrate, an d

Orthophosphate Determinations .

Determined 95% Confidence Level, Single Determination Concentration NH3-N> -`NO2-N ~ N03-N P04 -P (ug/ 2, ) (u g /Q) (ug/Q) (ug/Q ) (u9/L )

0 +20 .81 +0 .31 +20 .51 +6 .1 8

10 +20 .34 +0 .27 +20 .11 +6 .1 4

20 +19 .93 +0 .25 +20 .09 +6 .1 3

30 +19 .58 +0 .28 +20 .31 +6 .1 2

40 +19 .28 +0 .33 +20 .78 +6 .1 1

50 +19 .05 +0 .40 +21 .51 +6 .1 2

60 +18 .89 +0 .47 +22 .46 +6 .1 4

70 +18 .78 +0 .56 +23 .612 +6 .1 6

80 +18 .76 +0 .64 +24 .94 +6 .2 0

90 +18 .79 +0 .73 +26 .41 +6 .24

100 +19 .39 +0 .83 +27 .99 +6 .29

the water through prewashed, fired and preweighed Millipore AP-40, binderfree ,

borosilicate filters . One liter samples for reservoir analyses and 500 m z

aliquotes from the tributary and release waters were filtered . The filter s

were dried for one hour at 103-105°C, cooled and weighed . The net weight wa s

taken as total suspended solids . The filters were then fired at 550 0 for 1 5

minutes, cooled and again weighed . This net weight was taken as fixed sus-

pended solids . The difference between the net total suspended solids and ne t

fixed suspended solids was considered suspended volatile solids (and some water s

of hydration) . Suspended solids are reported in mg/z and the standard deviatio n

in the determinations was found to be +0 .1 mg/R .

Turbidity

Turbidity was determined with a Hach, Model 2100A, Turbidity Meter, cali-

brated against secondary standards . These standards were originally calibrate d

against primary formazin standards . The turbidity is reported in Nephilometri c

Turbidity Units (NTU) and is assumed to be equivalent to Jackson Turbidit y

Units (JTU) and Formazin Turbidity Units (FTU) .

Conductivity

Conductivity was measured using a Hach, Model 2511, Conductivity Meter .

The conductivity meter was temperature compensated to 25°C and was calibrate d

against a NaCl standard . The conductivities are reported as umhos/cm .

Algal enumeratio n

A 1-z water sample from the surface, 4 m, 8 m and 12 m from each reservoi r

station was returned to the laboratory . Two 75 mQ subsamples from each

sample m+re drawn off and preserved with Lugols solution . An appropriate por -

tion of the subsample was then filtered on a 25 mm Millipore RA filter (1 .8p) ,

8 cleared with immersion oil and mounted on microscope slides for countin g

(Millipore Bulletin AB310) . From one to four passes across the filter wer e counted and all counts converted to cells or colonies/L .

Summaries of dat a

Weekly summaries of influent and effluent water temperature, pH, conductivity, suspended solids, nutrients and flows have been tabulated in Ap- pendix I . Similarly, temperature profile data, pH, conductivity, dissolve d oxygen, turbidity, suspended solids and nutrients for reservoir stations 1 an d

2 are tabulated in Appendix II . Limited data on station 3 are also in thi s appendix .

The percentage transmittance of surface light at stations 1 and 2 ar e tabulated in Appendix III .

The algal species enumeration for stations 1, 2, and 3, at the surface ,

4 m, 8 m, and 12 m, are given in Appendix IV .

Insolatio n

A solar radiation pyrheliograph recorder, Belfort Instrument Company ,

Catalog Number 5-38504, was mounted on the laboratory roof . The instrumen t recorded total daily incident solar radiation for the duration of the study .

A Dietzgen polar planimeter was used to measure the area under each day s inked record ; the chart readings were thereby converted to Langleys/day (gra m calories/cm 2/day) . The insolation records are tabulated and reported a s

Langleys/day in Appendix V .

RESULTS AND DISCUSSION

Results and discussion of allochthonous sources and variations ar e

restricted to the two major tributaries entering Hills Creek Reservoir . These

tributaries are Hills Creek and Middle Fork Willamette River (referred to i n

the text as Sand Prairie) . Reduced data on all tributaries entering Hill s

Creek Reservoir are found in Appendix I .

Discussion of autochthonous variations are confined to reservoir statio n

2, located in the center of the main pool (Figure 1) . Reduced data on reser-

voir stations 1, 2, and 3 are found in Appendix II .

Allochthonous Sources and Variation s

Time-series physicochemical variations in Hills Creek are shown in Figur e

2 and variations in Middle Fork Willamette River, at Sand Prairie, are show n

in Figure 3 .

Temperatures increased in both tributaries through the summer months .

Maximum temperatures occurred in mid-August . The temperatures then graduall y

decreased and finally reached a relative constant 5°C . Sand Prairie, a t

maximum temperature, was slightly higher than Hills Creek . This may hav e

been due to surface water temperatures at Timpanogas Lake, the headwaters o f

the Middle Fork .

The pH remained relatively constant in both streams during the summe r

months . The pH began to decrease in mid-October ; this was due to high rain -

fall . Sand Prairie had a higher pH than Hills Creek . In August the p H

reached 8 .

As streamflows decreased and groundwater sources dominated, the conduct-

ivities increased . Conductivity in Hills Creek gradually increased and reache d

HILLS CREEK

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. SAND PRAIRI E

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12 a maximum of 72 umhos/cm in mid-September ; whereas, Sand Prairie rapidly in -

creased in conductivity, and remained between 60-64 umhos/cm from mid-Augus t

to late September . When the rains began in October and stream discharge in -

creased, conductivities decreased . This was obviously due to dilution of the . harder groundwaters .

Turbidities in both tributaries increased as stream flows increased . Hill s

Creek became Mery turbid, compared to Sand Prairie, during the storm in October .

Undoubtedly, there was a local slumping of slope-material within the Hills Cree k drainage . Both tributaries recovered rapidly from turbid conditions . Turbidity appears to be related to total suspended solid concentratins in the tributaries ; however, no correlation could be formed between total suspended solid concentrations and turbidities . This is understandable since turbidities depend not onl y upon suspensoid concentrations, but also upon particle sizes and nature of th e suspensoids (organic and/or inorganic) . Sand Prairie is more competent fo r sediment transport than Hills Creek . This is particularly evident ip December .

Nutrient concentrations were somewhat related to stream flows . Increased flows were accompanied by increased nutrient concentrations . The concentrations , of course, would depend upon (1) what degree of litter had decayed in the water - shed and was carried by runoff, (2) what amounts of nutrients have percolate d downward through soil horizons to reappear through " pulsed " groundwaters an d

(3) the amounts of nutrients "recycled" through rainfall . Ammonia-nitroge n concentrations slowly increased from September to mid-February . This increas e is more evident in Hills Creek than in Sand Prairie . The average ammonia - nitrogen concentration in Hills Creek was calculated as 34 .1±-A-3 .4 vg/t„ . war e the average concentration in Sand Prairie water was 32 .4 ± 13 .9 ug/a .

2< Nitrate-nitrogen concentrations showed a more cyclic trend (time-series dat a averaged) . Maximum concentrations in the cycle occurred in late October , early November and early January in Hills Creek . Maximum concentrations i n the cycle for Sand Prairie occurred in early October, late October and earl y

December . Average nitrate-nitrogen concentrations were calculated to b e

27 .7 ± 24 .1 ug/x for Hills Creek and 12 .9 ± 13 .3 ug/Q for Sand Prairie water .

Soluble orthophosphate-phosphorous concentrations were associated with high strea m flows, turbid conditions and high total suspended solid concentrations .

Calculated average soluble orthophosphate-phosphorous concentrations wer e

26 .6 ± 7 .7 ug/t in Hills Creek and 34 .5 ± 9 .9 ug/Q in Sand Prairie water .

Autochthonous Variation s

Pool elevation and depth-time variations in temperature are shown i n

Figure 4 . As pool elevation decreased, the thermocline was eventuall y drawn off by the regulating outlet and penstock .

Depth-time series of pH are given in Figure 5 . The pH in the uppe r layers (0 - 12m), during development of the thermocline in May and early June , reflected control by snowmelt . The pH then reached maximum pH durin g maximum biological activity from mid-June to late July . The pH then decrease d gradually at all depths and pH values were similar from surface to botto m depth . This was probably due to vertical mixing as the thermocline was destroyed . During the summer months the deeper water layers were not as basic as th e upper layers . This was probably due to organic debris being moved from th e surface layers by wind,mixed with water from Sand Prairie and carried belo w the thermocline, where decay occurred .

14 O)

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Figure 4 . Pool Elevation and Depth-Time Temperature Isopleths, Statio n 2, Hills Creek Reservoir .

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Figure 5 . Depth-Time Series of pH at Station 2, Hills Creek Reservoir .

1 6 Variations in dissolved oxygen, with time and depth, are shown in Figure

6 . The general trend was for dissolved oxygen to decrease as biological activity and water temperature increased . As the thermocline developed, lower wate r

layers were isolated from surface aeration . Wind rafted, surface and near - surface, organic debris was mixed with downwelling Sand Prairie water . The decay of this debris and oxidation of ammonia to nitrate would also explain th e low dissolved oxygen below the thermocline . Lowest dissolved oxygens were observed at 12 m and 20m where some decaying detritus came to rest on th e thermocline .

Time-depth variations in turbidity are given in Figure 7, and in tota l suspended solid concentrations in Figure 8 . During the bloom, in early June ,

the surface water increased in turbidity and total suspended solid concentration .

However, turbidity and total suspended solid concentrations remained low abov e

the thermocline during the remainder of the summer and fall months . As expected ,

turbidity increased with depth during these months, but total suspended soli d

concentrations did not increase . This can be explained by the insensitivity o f

the analytical methods to determine small concentrations (<0 .1 mg/0 of suspende d

clay minerals . Beginning in November, turbidities increased in the surfac e

layers (0 - 20 m) and the reservoir became uniformly turbid (Appendix III) .

Not until heavy storms occurred in December and January did depths below 20 m

increase in turbidity and total suspended solid concentrations . This increas e

was due to allochthonous sources (see Figure 3) . The storm in January will b e

considered in further detail . On 7 January 1976, reservoir and Sand Prairi e

waters had similar temperatures . Reservoir turbidity was near uniform as show n

in Figure 9 . Sand Prairie water increased its total suspended solid concentra-

tion and turbidity . This water did not enter the reservoir uniformly throughou t

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BAT JUME Jut. T AUGUST 7LTTENIER OCTONER NOVENBEN OECEMBER S tat s tu n 5 10 11110 15 sl0152011 3 10Rf2o E! 3101R10JO smIslnl■ esne• ! ~~■a~aaa a - i I Q

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0 0 0 0 0 0 0 0 Q 0 0

Figure 8 . Depth-Time Series of Total Suspended Solids , Station 2, Hills Creek Reservoir .

20 all depths, but entered as a density current (turbidity current), due to ad- ditional suspended load . This effect can be seen in Figures 7, 8, and 9 . After this initial pulse of material into the reservoir, between 30 m and 60 m, th e reservoir waters decreased in total suspended solid concentrations and in turbidity . The sequence of this event is seen in the vertical turbidity profile s shown in Figure 9 . The decrease in turbidity can be explained by (1) depositio n of suspensoids, (2) dilution by less turbid waters proceeding the turbid pulse , and/or (3) displacement of the turbid water by clearer water . Particle siz e analysis of suspended solids from 30 in, 40 m, 50 m, and 60 m, taken on 14 Januar y

1975, showed little variation in either mean or median particle size diameters .

Median and mean diameters were around 5 .0 u and 4 .4 u, respectively . Further data reduction revealed that the particles were fairly uniformly sorted, tende d toward finer particle size diameters, and near normally distributed at 40 m an d less normally distributed (tails of distribution have less spread than norma l distribution) at 30 m, 50 m, and 60 m . Detailed analyses are given in Table 2 .

Time-depth variations in total nitrogen concentrations (E NH3-N, N02-N ,

N03-N) are shown in Figure 10 . During August, September, and October, tota l nitrogen concentrations were lower in surface, 4 m, 8 m, 12 m than at 20 m an d below . Some organic material rested on the thermocline (ca . 12 - 20 m) and wa s decaying . Wind action had rafted organic material toward Sand Prairie water .

This material mixed with and downwelled with Sand Prairie water . The eventuall y released nutrients were no longer available for biological activity above th e biocompensation zone . When the thermocline was destroyed, the water column be - came nearly uniform in its total nitrogen concentration . In December, a rapi d decrease in nitrogen concentration occurred . Much of this nitrogen was remove d during reservoir drawdown (Figure 14) .

EU . : .

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Table 2 . Particle Size Analyses of Samples from 30 m, 40 m, 50 m, and 60 m ,

Hills Creek Reservoir, 14 January 1976 .

Inman Indices, Phi-Units a Mac aid e f Depth Turbidity Total Suspended Md b d 4) Solid s (m) (NTU) (mg/1) (median) (mean) (sorting) (skew- (peaked - ness) ness )

30 32 18 .6 7 .450 •7 .730 1 .050 0 .27 0 .45 (5 .72u) (4 .71p )

40 41 19 .4 7 .604) 7 .75(p 1 .034) 0 .15 0 .60 (5 .15u) (4 .65p )

50 54 41 .4 7 .80(p 7 .904> 0 .950 0 .12 0 .56 (4 .49u) (4 .19u )

60 76 87 .4 7 .70(p 7 .90(p 1 .104 0 .18 0 .52 (4 .81p) (4 .19u)

a - 4 4)=log 2 (d (mm) = 2 ) 10g d (mm) bM~s 2016 + 064 ) .

CMd(i) 450 .

Q4 2(9584 416) . Standard deviation in term of Wentworth Scale .

ed( = M4 - Md . .t = o, particle size distribution is symmetrical ;d < o, distri - bution tends towards coarser particles (smaller phi-units) ; °(p J> o, distribution tends towards smaller particles (larger phi-units) .

fa y = 2095 - (p5 ) - 6 . s = 0 .65, particle size distribution is normal ; slp < 0 .65 , 64) normal distribution ; s o > 0 .65, tails of distribution have more spread than normal distribution .

al , MAT JUN1 JULY AUGUST UPTtMBER OCTOBE R NOVENIER DECEMBE R IANUAR Y F[1II II ART 110 +.Y 5 .0102025 510152025 5 10 15 20 25 510152025 310157.025 510152025 510152025 115201 : 1 10 15 20 2 5

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Figure 10 . Depth-Time Series of Total Nitrogen ; Station 2, Hill s Creek Reservoir .

b . MAY JUNK JOLT RUBURT StPT!MRt OCTOBER NOVEMBE R DECEMBt K i IO 1B 2a 35 1015x02 5 510112023 9,0132025 s 1013 x025 1110,52013 5101/12025 5101310x5 t ., A xc is 510,5x02 5 ,111111 . ~ ■ ~I=1= -~ _~ • 1 410 t CZC C : - n1 --~ -Cit = r `I= k 1 : .: -- f GO le" I - TT --i- BIES MIMLILIIIL -- CC■■ ~C ■d° _ 1 - A . .. ■■ 1 i. . ■ i I - r- a - s 60~ .--_ - , ._ -.. 4 f 3- 1 Ma a 11 al A l- a. 4 I k

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2 5

c. MAY JUBE JOLT AUGUST SEPTEMBER OCTOBER MUTINEER DECEMBER Fie BRUAR 72vi5 SIP IS 02S 3101!x027 T 3101 ;0x5 510132023 SIOIS2023 510132025 510152025 ~1,I . :C2S 51013203S TEE. nn nnn f 1a5 _-- ■M= I 19■Y MIME 46 E1 1U r 2o°, Lim NU in Eur

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2 6

Variations in soluble orthophosphate concentrations with time and dept h

are given in Figure 11 . Much that has been mentioned about total nitroge n

concentrations in surface waters holds for soluble orthophosphate concentra-

tions . However, there is a complication associated with phosphates . They ar e

predominately adsorbed by inorganic hydrous oxides and clay minerals . Assuming

a constant equilibrium coefficient between soluble and sorbed phosphates, th e

more clayey suspended solids found in the water, the more soluble orthophos-

phate would be expected . This phenomenon was observed from the surface to

20m depths from mid-September to mid-February . Below 20m the soluble ortho-

phosphate concentrations gradually increased with depth . When the thermoclin e

disappeared, surface to bottom soluble orthophosphate concentrations becom e

near uniform . During the January storm, soluble orthophosphate concentration s

increased at 4m, 8m, 20m, and 50m .The increased concentrations in upper water s

was probably due to an increase in suspended material, "recycled" solubl e

orthophosphate-phosphorous in rainfall and soluble orthophosphate-phosphorou s

carried by surface runoff waters into the reservoir . The increase at 50m ca n

be attributed to Sand Prairie water and its subsequent occurrence as a densit y

current within the reservoir .

Light penetration measurements were made from 28 August until the end o f

the study . The reduced data are in Appendix IV . The lower limit of the photi c

zone (compensation depth) (Parsons, 1973) was about 12 m during Sep -

tember and came up to a minimum of 4 m in December, January and Febru-

ary . The photic zone remained above the thermocline all summer . The viabl e

algae were effectively limited to the area above the thermocline and were iso-

lated from the lower waters which were relatively rich in nutrients .

27 SAT JUNE ,ut., ADOVE1 SEPTEMBER xTOreR NOVEMBER DECEMBER ilOI1161f 50 ME=rInEarMarnaaalsII fl IO15 SiOlsxo ES SIOtS1011.am%Tm51YISTOUS 51OTI2OIS 51615x6}5 SIOl A1o x 5 mmmu rinul 2o ;,a.~=■~ ..~ a 1a ::: :~. :~~~ . ..; - L:_~::m■:::~L:h.: : .: ::_:: In 4 :- - so 40 rii■■■ia■i■■i■■■■1Cr■■■■■■■ii---r■r■■ : ::1 : .wo ■■u■aii■■■■■■■i■■■ ii ■■ ■■■ 20 ■ i ■ ■ i •■Clm■aaaa~■■.■ . ■F - ►1■iIM■ ■ E lo ■ ■- Ut:: ::: .r - a so UUaa~■taaa■-~ 4o w■ ■ ■~■■■i■aaaanf: i o ■i■■UII~ u■ ■•■■U- :x11■a~taC.~■ir. . Ni■■rCA .i..iirIri ~•. : to . • ~::I ~■mN:IR::I i 6 wC IIMI INFEE011■°~i•Ii•Laaa~ t°•■A IL ■aIrC~■MILI ■■■~:iC:■■EMIMIl idoCsC i C a)

n i■■ ■■■i■~iN■~~i~■iii -ii • ■rte■ ~ ~ z■■■■■aaa~■■■■■■■ .:■■■ " I■■i■■■iail :::•U~:11UMI El Ci11MI rr■•■ia~•i:r~•iIrr : i ~ ..i. :_ ■taaaa 111MIa•yi■•i~ialleC■■ g E - "■ lIaMI•aa•■■••rtaC salm ::gu:: ■■C : :MRIll"aM oul■o::,°■_P:: !:~=CR- ■., _ a) •~N -i Ii!■ii` ■il~■■■ M■ ill. ■ • ■1 iii:rt■i■~ri■■ .1■ i ■ ■ ~ ■■pN1 - ■ `ri■■MA■i■■■.■■if ~r%nir ~ mimm.. .s. UM O 111.11rn.E.1111111f.prn N ae•■•■■•rai{.■Ga■vik■■■■ .#■ ■ 1 -1 ~! ■■~i.EMMUNiMiainusuaralmin p EMU

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24 MM. UMW.. ~-•I is k--. ■■■■■■Li ■ Qi ■ i.lslll■ - o ■■■■■M■i■■~■~.mki-■ a■Ii■Il l- Tr 1 ` .~.- . s o . 14 _ 40 17 1 Li 1 30 7 T .- ,- i 20 --r tt-T } - -- I 1 k J } )o f T i f 0 E I6 ISBC21 S1U 55:033 S1515:031 S pA to,atS NAT N11B 2w.y 0.00. 444444444 OCTGBEE NOVEMBER DECEMBER JANUARY IFFORUAR101570T:.. 1975 1976 28 Algal samples were collected from 15 May 1975 to 11 February, 1576 :.

Samples were taken at three stations (shown as Stations one, two, three ,

Figure 1) . Samples from four depths at each station were taken (surface, 4 m, •

8 m, and 12 m) . A taxa list of the algal species observed during the stwdy i s

given in Table 3 . Weekly counts of the algae in cells per liter are given i n

Appendix V . The dynamics of the populations can be characterized very gener-

ally by looking at the growth patterns of the five major taxa observed (Fig-

ure 12) . This figure shows the population counts for the five taxa plotted 1 • weekly for the surface sample at station 2 . . The counts were transformed to

Log l0 since the magnitude of the differences were so great .

By 22 May the water temperature had warmed considerably ;(12 .85°C) and th e reservoir was receiving an average of 511 Langleys/day (gram calories/cm 2 ) .

Larson reported a Spring Asterionella formosa Hassall growth in Hills Cree k

Reservoir during the summer of 1971 (Youngberg, Klingeman, et al ., 1971) . I f such a Spring pulse occurred in 1975, it was prior to the beginning of th e sampling period . The initial phytoplankton populations sampled were dominate d by Anabena circinalis Rabenhorst . Hutchinson suggests that an Anabena circ.inali s

Rabenhorst population would lead one to suspect that "the water has a pH greate r than seven, that nitrogen is being fixed, that there may well be an obviou s water bloom and that the ratio of plant to animal plankton may be quite large "

(Hutchinson, 1967) . All the conditions mentioned, except nitrogen fixation , were observed . Anabena circinalis Rabenhorst dominated the phytoplankton unti l about the first of July, growing almost uhialgally throughout early-June . I t was during this time that the algae were most noticeable marcos :copically . The

Anabena colored the water a brighter green and gave it an opaque appearance .

Larson reported mats of decaying algae during the summer of 1971 (Youngberg ,

29 Table 3 . Taxa List for Hills Creek Reservoir, 15 May 1975 to 15 February 1976 .

:, k

Chlorophyt a Ankistrodesmus falcatus (Corda) Ralf s - Botryococcus Braunii KUtzing Eudorina elegans Ehrenber g Pandorina morum (Mueller) Bor y Staurastrum paradoxum Mayen Tetraedron quadratum (Reinsch) Hansgir g Volvox aureus Ehrenber g Chrysophyta Mallarionas sp . Bascillariophyt a Asterionella formosa Hassall Fragilaria capucina Desmaziere s Fragilaria crotonensis Kitton Melosira granulata (Ehrenberg) Ralfs Melosira italica (Ehrenberg) Kitzing Stephanodiscus astrea var . minutula (KUtzing) Grua-o w Pyrrophyt a Ceratium hirundinella (Mueller) Dujardin Cyanophyta Anabena circinalis Rabenhora t Anabena sp . Cryptophyt a Cryptomonas obovoidea Pasche r Cryptomonas ovata Ehrenber g

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Figure 12 . Time-Series of Five Most Abundant Species at th e Surface, Station 2, Hills Creek Reservoir .

31 Klingeman, et al ., 1971) . Floating mats of algae were not found in 1975 .

Cryptomonas ovata Ehrenberg appeared in the initial sample of 15 May and wa s found in most samples throughout the summer . Cryptomonas ovata Ehrenberg i s a flaggelated unicellular species with a gullet . It may be thatCryptomona s ovata Ehrenberg is more heterotrophic than the other algal species found an d can-maintain a viable population all year since it responds less critically t o the same environmental changes that most algae do . On 9 July, Fragilari a crotonensis Kitton, Asterionella formosa Hassall, and Melosira qranulata(Eh- renberg) Ralfs all appeared for the first time . These three algae are all mem- bers of the Bascillariophyta . Anabena circinalis Rabenhorst. was virtually gon e from the samples on this date . Melosira granulata (Ehrenberg) Ralfs quickl y disappeared and did not reappear at station 2 until the second of October .

Fragilaria crotonensis Kitton and, to a lesser extent, Asterionella formosa

Hassall bloomed throughout July and into August . There was a short . reappearanc e of Anabena circinalis Rabenhorst in September . As previously mentioned ,

Melosira granulata (Ehrenberg) Ralfs reappeared on 2 October and bloomed through - out October and into November . Light levels (Figure 13 and Appendix VI) droppe d dramatically at the beginning of October . Temperature began to drop in Octobe r and through November . These two factors undoubtedly led to the overall declin e in cell numbers in November and December . The great,influx-of water during . storms and subsequent flushing during post-storm drawdoww caused the winte r populations to be so diluted, that on 18 December 1975, no viable cells wer e found at station 3 . From 21 January until 11 February 1976, when the study ended, very few cells were found at any of the stations .

32 Hutchinson lists the dominant genera found in Hills Creek Reservoir a s indicators of eutrophic waters, i .e ., Anabena, Asterionella, Fragilaria ,

Melosira and Stepanaodiscus . (Hutchinson, 1967 )

Incident Solar Radiatio n

Daily radiation values in Langleys/day are listed in Appendix V . The daily radiation values were averaged for the period from one sampling data to th e next (approximately one week) and plotted in Figure 13 . There was a dramati c decrease in radiation during the first week in October . Hutchinson presents mean monthly estimates of daily direct radiation for an altitude of five hundre d meters and a latitude of 47° (Hutchinson, 1957) . The values given are in close agreement with those found for Hills Creek Reservoir (ca . 4704m elevation and

44° N) .

Effluent Water Variation s

Variations in water leaving Hills Creek Reservoir are shown in Figure 14 .

Temperature maximum was not reached until early October, when the upper, warme r waters were being removed .

There was a gradual decrease in pH, with a minimum occurring in lat e

December . This trend was also seen at all depths in the reservoir (Figure 4) .

Conductivity did not show trends observed in Hills Creek and Sand Prairi e waters . Biological activity, physicochemical reactions and, to some extent , mixing above and below the thermocline in the reservoir may have mediate d trends in conductivity of the effluent water .

Turbidity and total suspended solid concentrations were very low durin g summer months and into mid-November . From November until the end of th e study, turbidity occurred as pulses, but total suspended solid concentration s increased only slightly .

0 a

=MI G IE t X ERIE 1111m Pa WI =IMMIX= " CM EA @I 2 .1 MO Ni MINEII MU MIMR=

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1111 1111 .11 1.1111WEEIAEM111t IMIIMEIIMIBIMIMMIIIMZEEWMI 11111111M1iml.=11 M limmunsummmimmuniummm N MOM MEW 1111MINIM11 1MINININIPIAIIIIIEr 0 o 11MUMNAMMIM .1 1IMMINIIIIIIIII15WIdIE51111ME 111IM11111M U11111111111•11RE N 211111IMMM MEIN N NB ME INN Mil11111NIMINI EIUMSIUIIMNIIIIIMII No mm wnmila momloom=m w 0 WMEIIII.IKE .Ellill.lMIMMEEMMIN u UM ERNE E11.115IMMililE WM =AIMMEE wa 111MINEINIHIN101IMMMEM11=M121111111ENIIIIIE1.11111 OM ER .11 WI NE WE ME MIE IN II NMI=MIEN =IES IIEIOIIMIIEN NM EN IN • 111111111MIMMIMIMMIMIEHIMIMMIMEW11 M 2BIZIBUI T MISNEEME111111MILEIIIIIIN 11IMERMIUME 111=1111MIgMNIIIIIIIIIEIMIll.inilWI11111 MIME Mi MOM= MI illNM EIIMIIIEM 11MAIIIIIEUMBEEIMIUM11.1111■111 BMI= 111115MMINIMI N Mil BE N Ul II MIMI 111111W RIM 1111I•EMIIIIIRIIU if 0 Ii. MIME 10flIMIMUMBUIU11111MINEIREEIIEt=MEE.IX MIME 1M I-I 1111 WE MU MIN Oki WJM111.11111MIMIIIIIII■IIIIE ■■ Ina-lGI-onlillItEMEMEE.W OE ME IMEMIMIUMER11.11111111FAIMEIN.11. KIIIERMI11111NEMENNIIIIEMMIl OM r 11IMEIERMIll 1111111IIIIETMIM11111 =11111WEll MEMiK IIIIIIM1111NEIMMIMIUMMil MUNNIOW O 111111MIMEIRiMEME 1111EINIMMMIEURE =WM =Ell M RIMEllMINIES N NE E IIII111MIME111111hMIMIEWE N EN=ER.111•111I.N. 0 MN WM 11111111IIIIIIIERILI 31IMI MIlEMNIMEMEENEERIIIIIIIII 1111111111111111IMINWIRR =111111111=111•11MIMI 0 1HIMIEE1111IIIERIMIEW11111111MBEE.W1111111111 MI EN WENIMMI111M 1111EEIIIII EBBW.

o 0 0 0 o 0 ,z) o 0 rt.% r.1

. SA 3 -1 N

Figure 13 . Weekly Mean Radiation in Langleys per Day .

MIDDLE FOR K

JURI JULY AUUUIT 11!!1111- OCYOgI 1101[RI[I 01C11111R JANUARY FT OVARY 11011 ;011 110150015 S10 I510 ]s 51015101/ 5101110 II 11015101S 510151011 010111015 51015x001

4- • o •r U, O E r 0 s_ a)4- I s- r-+ a a) s- E 4-) a) •r ro > F- 3 •r c • C a) • a) 4 . 3 5- a) a)4- E S.- 4- (0 W - a) 1- •r 4-

10 I 5 10 15 51015x015 112151015 110151013 51015301 510152023 51015001 5 JU RI JIM JUIURS 1IPTUURR 0410111 R0Y4U1R MINSIR JANUARY 11 ROAS T 1975 1976

.35 Ammonia-nitrogen and nitrate-nitrogen concentrations showed inverse re- lations in the effluent water . This relation might be due to inhibite d

Nitrosomonas and Nitrobacter activities . The general trend was for ammonia - nitrogen concentrations to increase and nitrate-nitrogen concentrations t o decrease . Average ammonia-nitrogen was 52 .5 ± 29 .9 ug/t and the averag e nitrate-nitrogen was 40 .3 ± 18 .0 ug/2 . Total nitrogen concentrationsshowe d spureous variations, but the general trend was a gradual increase from August to November, and then a rapid increase to maximum concentrations betwee n

December and January, followed by a rapid decrease through January and Feb- ruary .

Soluble orthophosphate-phosphorous concentration gradually decreased fro m

August through September . The concentration then increased and remained constant during October . It sharply increased through November and into earl y

December . Beyond December the concentration remained constant at 30 ug/Q . Th e average concentration was found to be 20 .9 ± 9 .0 ug/Q .

CONCLUSIONS

Uninterrupted data sequence on tributaries entering Hills Creek Reservoi r

and on Hills Creek Reservoir, from 15 May 1975 to 11 February 1976, has give n

insight into the dynamics of a mid-water withdrawal reservoir and the influenc e

inflowing tributary waters exert upon turbidity and nutrient characteristic s

within the reservoir .

Tributaries may be classified by their competence to transport suspende d sediments and dissolved nutrients into the reservoir . Primary sources ar e

Hills Creek and Middle Fork, Willamette River (Sand Prairie) . Secondary source s are Packard Creek, Coffee Pot Creek and, during storms, Larison Creek . Al l other tributaries may be considered tertiary and/or quaternary in importance .

36 The two primary tributaries are free flowing into the reservoir . Al l other tributaries flow into backwater emboyments, connected to the reservoi r by culverts . This physical manipulation of inflowing water premixes tributary and reservoir waters . These embayments also act as stilling basins in whic h suspended sediments transported by tributaries, are deposited .

Hills Creek and Sand Prairie waters could not be identified in th e reservoir by conductivity, pH, or nutrient concentrations . Biological an d physicochemical processes, such as absorption and desorption of C0 2 , precipi- tation and particulate sorption, and wind mixing in the epilimnion and curren t mixing in the hypolimnion, alter the individual features of each water mas s in the reservoir .

Turbidity within the reservoir can be associated with water condition s in Hills Creek and Sand Prairie . Tributary turbidities occur as pulses , with rapid recovery times, and are due to local and temporal unstable soi l conditions in each watershed drained .

Turbid tributary waters enter the reservoir as turbidity currents (densit y currents) which increase the reservoir turbidity from mid-depth to the bottom .

No statistical correlation could be established between turbidity an d suspended solids, either in the main tributaries or in the reservoir . Thi s is to be expected since turbidity is wavelength dependent and shape and particl e size dependent . Also certain particles are anisotropic in their nephliometri c characteristics .

Time variations in soluble ammonia/ammonium, nitrite and nitrate concentrations were observed in all tributaries . Ammonia tended to increase in concentration from mid-August, whereas nitrate concentrations were variable and associated ,with tributary flows . These variations are due to the stages of leaf-litte r

37 decay in the watersheds, oxidation (nitrification) and/or reduction (denitri- fication) by soil bacteria . Similar variations occur within the reservoi r and are due to oxidation/reduction and bacterial action in the sediment . To some extent, volatile suspended solids represent particulate organic debri s and potential nitrogenous and phosphate nutrient sources . After each majo r storm, reservoir verticle profiles show systematic increases in volatil e suspended solids, from mid-water to bottom water, and pronounced, but random , increases in total nitrogen .

Soluble orthophosphate concentrations in all tributaries increase a s major storms move through the area and as tributary flows increase . However , the long-term trend was for soluble orthophosphate concentrations to remai n near constant . The increase in concentrations are associated, but not corre- lated, with tributary turbidities and total suspended solids . Equilibrium exists between water soluble orthophosphate and clay sorbed phosphate . An increase in suspended solids can contribute to an increase in soluble orthophosphate concentrations in the tributaries and within the reservoir .

Mass balances of total nitrogen, soluble orthophosphate and tota l suspended solids were calcuated for the major storm interval between 5 Novembe r

1975 to 17 January 1976 . Total suspended solids entering the reservoir wer e

3 .85 x 10 7 mg/sec and solids leaving the reservoir were 3 .29 x 10 6 mg/sec .

Total nitrogen was 4 .79 x 10 7 ug/sec entering and 7 .20 x 10 7 lug/sec leavin g the reservoir . Soluble orthophosphate was 3 .07 x 10 7 pg/sec entering and

1 .79 x 10 7 pg/sec leaving the reservoir . During this interval, the reservoi r retained 35 .2 kg/sec total suspended solids and 12 .2 g/sec soluble orthophosphate and released 24 .1 g/sec total nitrogen .

Phytoplankton activity is restricted to a very shallow, nutrient poo r

38 epilimnion during the spring and summer months . Once the metalimnion dis- appears andthe reservoir overturns in late fall and winter, the insolatio n and water temperatures are so low that no appreciable algal blooms can occur .

.Nutrient concentrations available for the spring algal blooms are probabl y

60 14/1 total nitrogen and 30 pg/l soluble orthophosphate .

Descriptive Mode l

From the preceding results and discussion and conclusions, a propose d descriptive model for Hills Creek Reservoir has been developed for spring - summer behavior and fall-winter behavior .

Spring-summer mode l

The metalimnion (thermocline) is well established between 10 m•to 20 m in late spring and early summer . This isolates the epilimnion from the hypolimnion and prevents replenishment of nutrients in the epilimnion . Cooler , nutrient-rich tributary waters enter the reservoir as interflow or underflo w in the hyperlimnion . The development of the shallow epilimnion and the :12 m biocompensation depth throughout the summer, restrict algae to a shallo w nutrient pool . Some detrital material from each successive bloom settles upon the metalimnion, where it decays and some of the solubilized nutrients becom e available for the next bloom . Upward mixing of these "recycled" nutrient s is by wind-generated wave action . The summer winds are also responsible fo r the nutrient-poor epilimnion . Under wind stress, algae and algal debris . are . rafted from the main pool epilimnion and mixed with colder, downplunging San d

Prairie water . Some nutrients are returned to the epilimnion by upwelling near the dam . However, most of the upwelling nutrients are withdrawn from the reservoir by the penstock . This spring-summer modelis diagrammatically presented

3.9

r in Figure 15 (a) .

Fall-winter mode l

The metalimnion is rapidly destroyed in late fall by drawoff throug h

the regulating outlet and penstock, and by epilimnion cooling due to low in-

solation . This results in an isothermal reservoir, with uniform turbidity

and soluble orthophosphate concentrations . Tributary water temperatures are

near that of the reservoir and tributary waters inflow at all depths .

During major storms, turbid tributary waters enter the isothermal reser-

voir as turbidity currents (density currents) . These turbidity currents resul t when tributary suspended loads increase, which increase the water bulk density .

Theydownplunge into the reservoir and develop very turbid conditions from mid -

to bottom water depths . The turbidity currents induce surface currents whic h

converge with the turbidity currents . These surface waters downwell and over -

ride the turbidity currents . The reservoir remains isothermal but a pycnoclin e

develops, which isolates the turbid waters from clearer surface waters . As

these turbidity currents slowly move through the reservoir, their particle siz e

distributions change and their suspended loads decrease and, of course, turbidi-

ties decrease . The results are a decrease in bulk density, disappearance of the pycnocline, and eventual mixing with upper waters . The proposed fall-winte r model is shown in Figure 15 (b) . Various mechanisms contributing to the de - crease in suspended solids and turbities are Stokes settling, flocculation , dilution by overriding water, and displacement by clearer inflowing waters .

40 (a-) SPRING--SUMMER CoNDMONS

W IN D

(b) FALL.-LJINTG R COPTAITtO1S

de ter %tihe Q r ~-- -.r- t- , -

STORM .f- Maws l

///

-• .

rte/ C )

"0" ._ _ ///

Figure 15 (a) : Proposed Spring-Summer Reservoir Model ; (b) : Proposed Fall-Winter Reservoir Model .

41 BIBLIOGRAPH Y

Hutchinson, G . E ., 1957, A Treatise on Limnology . Vol . 1, John Wiley and Sons, Inc . New York . p 371 .

, 1967, A Treatise on Limnology . Vol . 2, John Wile y and Sons, Inc . New York . pp 377 and 387-88 .

Imbrie, J . 1955, Survey of some basic statistical concepts . Bull . Am . Mus . Nat . Hist . 108, 211-252 .

Larsen, I . L . and J . J . Wagner . 1975 . Linear instrument calibration wit h statistical application . J . Chem . Ed . 32, 215-218 .

Methods for Chemical Analyses of Water and Wastes . 1974 . U . S . E . P . A . , Office of Technical Transfer . Washington, D .C . EPA-625/6-74-003 . 298 pp . Corrected 1976 .

Parsons, T . and M . Takahashi . 1973 . Biological Oceanographic Processes . Pergaman Press . .New York . p . 83 .

Standard Methods for the Examination .of Water and Wastewater . 1971 . 13th Ed . American Public Health Association . Washington, D .C .

Youngberg, C . T ., P . C : Klingeman, M . E . Harward, D . W . Larson, G . H . Simonson , R. K . Phinney ; D . Rai, and J . R . Bell . 1971 . Hills Creek Reservoi r Turbidity Study . Water-Resources .Research Institute, Oregon Stat e University . Corvallis, OR ., WRRI-14 .