Recent Increases in Sediment and Nutrient Accumulation in Bear Lake, Utah/Idaho, USA

Recent increases in sediment and nutrient accumulation in Bear Lake,

Utah/Idaho, USA

Joseph M. Smoak1,* & Peter W. Swarzenski21Environmental Science, University of South Florida, 140 7th Avenue South, Davis Hall 258, St. Petersburg,

FL 33701, U.S.A.2US Geological Survey, 600 Fourth Street South, St. Petersburg, FL 33701, U.S.A.(*Author for correspondence: Tel.: +1-727-553-4078, Fax: +1-727-553-4526, E-mail: [email protected])

Received 15 October 2003; in revised form 28 January 2004; accepted 9 February 2004

Key words: paleolimnology, sediments, 210Pb dating, geochemistry, Utah, Idaho

Abstract

This study examines historical changes in sediment and nutrient accumulation rates in Bear Lake along thenortheastern Utah/Idaho border, USA. Two sediment cores were dated by measuring excess 210Pb activitiesand applying the constant rate of supply (CRS) dating model. Historical rates of bulk sediment accumu-lation were calculated based on the ages within the sediment cores. Bulk sediment accumulation ratesincreased throughout the last 100 years. According to the CRS model, bulk sediment accumulation rateswere <25 mg cm)2 year)1 prior to 1935. Between 1935 and 1980, bulk sediment accumulation rates in-creased to approximately 40 mg cm)2 year)1. This increase in sediment accumulation probably resultedfrom the re-connection of Bear River to Bear Lake. Bulk sediment accumulation rates accelerated againafter 1980. Accumulation rates of total phosphorus (TP), total nitrogen (TN), total inorganic carbon (TIC),and total organic carbon (TOC) were calculated by multiplying bulk sediment accumulation rates times theconcentrations of these nutrients in the sediment. Accumulation rates of TP, TN, TIC, and TOC increasedas a consequence of increased bulk sediment accumulation rates after the re-connection of Bear River withBear Lake.

Introduction

Human activities can accelerate sediment andnutrient loading to lakes and rivers. Alterations insediment and nutrient loading can result fromanthropogenic disturbances such as urban devel-opment, road and railroad construction, oil andgas exploration, agriculture, and hydrologicchanges. Limnological data rarely document thefull effect of human activities on water qualitybecause they are generally collected over a timespan that is too short to record pre-disturbanceconditions. The sediments of a lake, however canpreserve the environmental history of a drainagebasin, and provide valuable information about thelake’s response to external influences (O’Sullivan,

1979). The paleolimnologic record can documentpre-disturbance conditions and the influence ofexternal changes, and can be useful for makingpredictions about how future external changesmight alter the lake (Brenner et al., 1993).

Establishing a geochronology is essential toexamining the timing of changes recorded in sed-iment records. Because the past 100 years is theperiod of interest for this study, 210Pb was selectedas the most appropriate tracer for establishinggeochronology. Krishnaswami et al. (1971) firstapplied 210Pb to sediment chronologies in lakes,and the technique has been used widely in paleo-limnology to determine recent (i.e., approximately

Hydrobiologia 525: 175–184, 2004.� 2004 Kluwer Academic Publishers. Printed in the Netherlands. 175

100 years) age/depth relations (Appleby & Old-field, 1978; Robbins, 1978; Appleby et al., 1979;Appleby, 2001).

There are several models that can be applied tocalculate sediment ages. The constant rate ofsupply (CRS) model is the most appropriate modelfor calculating sediment ages when sedimentaccumulation rates vary over time (Appleby,2001). The basic assumption of this model is thatexcess 210Pb was supplied to the sediment core siteat a constant rate. The CRS model uses the 210Pbinventory of the entire core and the inventory be-low the interval being dated to calculate an age.

In this study, we used paleolimnologicalmethods to test the hypothesis that externalinfluences, such as land-use change and hydrologicmodification, altered the rate of sediment andnutrient supply to Bear Lake. Sediment andnutrient accumulation rates over the last 100 yearsin Bear Lake were examined. Sediment nutrientconcentration and total organic carbon/totalnitrogen (TOC/TN) atomic ratios also wereexamined.

Study site

Bear Lake (Utah/Idaho) is located in the north-eastern margin of the Basin and Range tectonicprovince in a half-graben basin on the easternUtah/Idaho state line (Fig. 1). Bear Lake is about40 km long and 10 km across with a maximumwater depth of 55 m. While there are several smallcreeks draining into Bear Lake, Bear River is thedominant surface water supply. Because of tec-tonic processes, Bear River had been disconnectedfrom Bear Lake for thousands of years. Bear Riverwas re-connected with Bear Lake in a project thatbegan in 1908 and was completed in 1918 to createa water-storage reservoir. Since 1918, Bear Riverhas entered Bear Lake through a canal joining theriver to Mud Lake and another canal joining MudLake to Bear Lake. Water exits Bear Lake to thewest of Mud Lake via the Lifton Pumping Station.

Sampling and analytical methods

Two sediment/water interface cores (<1 m) werecollected in 1998 from Bear Lake (Fig. 1) using a

modified gravity corer. Core 98-10 surface samplewas from 0 to 2 cm while all other sections weresampled at 1-cm intervals at USGS laboratories inDenver. Samples were dried at 60 �C. Dry bulkdensity was measured by weighing wet sediment inplastic boxes of a known volume, then drying andre-weighing.

Lead-210 was determined by alpha counting itsgranddaughter, 210Po (half-life, t½ ¼ 138 days)(Flynn, 1968). Polonium-210 reaches secularequilibrium with 210Pb after approximately2 years. Five grams of dry sediment were leachedin a warm H2O2/HCl solution and a knownamount of 209Po was added to the pulverizedsample prior to digestion as a yield tracer. Polo-nium was spontaneously electrodeposited ontosilver planchets overnight on a warm stirring plate.The overall chemical yield was 83–95%, based onthe 209Po yield tracer. The planchets were thencounted under vacuum on a Si surface barrierdetector (300 mm2) with an alpha-energy resolu-

Figure 1. Bear Lake bathymetric map showing location of core

sites 98-06 and 98-10.

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tion of about 20 keV full-width-at-half-maximum(FWHM). The average efficiency for the 5.3 MeValpha-line of 210Po was estimated to be approxi-mately 20%.

Supported 210Pb activity was estimated fromthe mean 210Pb activity within a deeper region ofthe cores where total 210Pb activity was relativelyconstant and assumed to be in secular equilibriumwith 226Ra. This mean value (1.25 dpm g)1 forcore 98-06; 1.82 dpm g)1 for core 98-10) wassubtracted from the total 210Pb activities at eachlevel to calculate excess 210Pb activity. Supported210Pb activities for a subset of samples were com-pared with 226Ra activities assayed by gamma-raycounting 214Bi on a Ge well detector and werefound to be within 10% of the sample values.

Total phosphorus (TP) content was measuredby autoanalyzer after persulfate digestion of driedsediment samples (Schelske et al., 1986). Totalcarbon (TC) and nitrogen (TN) content weredetermined with a Carlo Erba NA1500 CNS ele-mental analyzer equipped with an autosampler(Verardo et al., 1990). Total inorganic (carbonate)carbon (TIC) in the sediments was measured co-ulometrically (Engleman et al., 1985) with a UIC/Coulometrics Model 5011 coulometer coupledwith an automated UIC model 5240 TIC prepa-ration device that used 2 N perchloric acid toevolve CO2. Organic carbon was calculated by thedifference between TC and TIC.

Bulk sediment accumulation rates are based onCRS modeled ages (Appleby, 2001). Calculatedsediment dates correspond to the base of eachsediment-core interval. Bulk sediment accumu-lation rates were calculated by dividing the massof sediment per area (cm2) in each sediment-core interval by the time represented in the inter-val. TP, TN, TIC, and TOC accumulation rateswere calculated for each interval by multiplying thebulk sediment accumulation rate by the corre-sponding concentration of each constituent in theinterval.

Results

Depth interval, cumulative mass, dry bulk density(g dry cm)3 wet), excess 210Pb activity, and sedi-ment date are shown for each core in Tables 1 and2. Tables 3 and 4 show sediment date, and accu-

mulation rates for bulk sediment, TP, TN, TIC,and TOC for each core. Sediment content of TP,TN, TIC, TOC, and TOC/TN are shown for eachcore in Tables 5 and 6.

Lead-210 activity reached supported levels ator above 14 cm depth (mass depth of <4.6 gcm)2) in both cores. Bulk sediment and TP accu-mulation rates (Tables 3 and 4, Figs 2–5) are cal-culated only for sediments deposited after 1900because sediments deposited prior to 100 yearsago are assigned artificially ‘too old’ dates by theCRS model (Binford, 1990). In addition, errorterms are large on the older dates. Bulk sedimentaccumulation rates in cores 98-06 and 98-10 (Figs2 and 3) are <25 mg cm)2 year)1 prior to 1935.The sediment accumulation rate increased to36 mg cm)2 year)1 by 1935 and had a mean of40 mg cm)2 year)1 between 1935 and 1981 in core98-06. In core 98-10, bulk sediment accumulationrate increased to 38 mg cm)2 year)1 by 1952 andhad a mean of 45 mg cm)2 year)1 between 1952and 1990. The mean for both cores between 1935and 1980 was 40 mg cm)2 year)1. Cores 98-06 and98-10 exhibited another increase after 1981 and1991, respectively.

TP concentrations in core 98-06 ranged from0.180 to 0.422 mg g)1 to a depth of 17 cm (massdepth of 7.46 g cm)2). The mean TP concentrationin core 98-06 was 0.287 mg g)1 with a standarddeviation of 0.062. In core 98-10, TP ranged be-tween 0.282 and 0.499 mg g)1 to a depth of 14 cm(mass depth of 4.35 g cm)2). The mean and stan-dard deviation of TP was 0.433 mg g)1 and 0.071,respectively in core 98-10. TP concentrations var-ied within each core, but did not exhibit direc-tional trends of increase or decrease throughoutthe core. However, sediment greater than 100years old generally exhibited the highest TP con-centrations in core 98-06. Core 98-10 had fewerolder sediment intervals, and therefore it is un-known if the high TP concentration trend in oldersediments would exists at core site 98-10.

TN exhibited a similar concentration in thesurface sediments of both cores, but values de-creased in core 98-06 and increased in core 98-10with depth. The mean and standard deviationsof TN were 1.929 mg g)1 and 0.310, respectivelyfor core 98-06 and 2.531 mg g)1 and 0.322,respectively for core 98-10. TIC ranged from 69to 74 mg g)1 with a mean of 70 mg g)1 and a

177

standard deviation of 2.5 in core 98-06, and rangedfrom 65 to 79 mg g)1 with a mean of 74 mg g)1

and a standard deviation of 3.6 in core 98-10.Calcium carbonate content calculated from TICvalues was approximately 60% of dry mass in both

cores. TOC had a mean and standard deviation of19.9 mg g)1 and 3.2 in core 98-06. Core 98-10 hada mean TOC of 22.5 mg g)1 and a standarddeviation of 4.7. TOC/TN atomic ratios weregenerally less than 12 with a few exceptions.

Table 1. Depth interval, cumulative mass, dry bulk density, excess 210Pb activities, and sediment dates are shown for core 98-06

Depth interval Cumulative mass Dry bulk density Excess 210Pb Date

(cm) (g cm)2) (g cm)3) (dpm g)1)

0–1 0.58 0.580 9.05 ± 0.10 1990 ± 2

1–2 1.10 0.520 8.74 ± 0.10 1982 ± 2

2–3 1.61 0.513 9.76 ± 0.11 1968 ± 3

3–4 2.02 0.407 6.69 ± 0.58 1958 ± 4

4–5 2.45 0.443 4.55 ± 0.49 1947 ± 5

5–6 2.90 0.449 3.65 ± 0.55 1935 ± 7

6–7 3.34 0.443 4.28 ± 0.36 1910 ± 14

7–8 3.68 0.339 2.38 ± 0.24 1886 ± 25

8–9 3.96 0.289 1.13 ± 0.24 1869 ± 40

9–10 4.27 0.314 1.18 ± 0.15

10–11 4.61 0.345 0.22 ± 0.09

11–12 4.95 0.345

12–13 5.36 0.417

13–14 5.82 0.458

14–15 6.33 0.510

15–16 6.87 0.545

16–17 7.46 0.594

Errors in activities represent uncertainties in standards and counting statistics. Dates are calculated using the CRS model.

Table 2. Depth interval, cumulative mass, dry bulk density, excess 210Pb activities, and sediment dates are shown for core 98-10

Depth interval Cumulative mass Dry bulk density Excess 210Pb Date

(cm) (g cm)2) (g cm)3) (dpm g)1)

0–2 0.31 0.156 8.98 ± 0.11 1995 ± 1

2–3 0.64 0.337 8.77 ± 0.11 1991 ± 1

3–4 0.97 0.336 13.00 ± 0.15 1985 ± 1

4–5 1.31 0.351 11.80 ± 0.14 1977 ± 2

5–6 1.66 0.349 8.62 ± 0.10 1970 ± 2

6–7 2.03 0.382 7.94 ± 0.98 1962 ± 2

7–8 2.41 0.386 6.68 ± 0.85 1952 ± 3

8–9 2.78 0.380 7.33 ± 0.92 1936 ± 4

9–10 3.17 0.385 4.39 ± 0.62 1920 ± 4

10–11 3.55 0.368 3.95 ± 0.58 1892 ± 8

11–12 3.84 0.292 2.12 ± 0.39 1866 ± 12

12–13 4.09 0.248 1.43 ± 0.33

13–14 4.35 0.243 0.48 ± 0.23

Errors in activities represent uncertainties in standards and counting statistics. Dates are calculated using the CRS model.

178

Nutrient accumulation rates had approxi-mately the same trend as bulk sediment accumu-lation rates because the nutrient concentrationschanged little as compared to the bulk sedimentaccumulation rates. In core 98-06, the nutrientaccumulation rates increased four to six fold be-tween 1910 and 1998. Nutrient accumulation ratesincreased approximately four fold between 1920and 1998 in core 98-10.

Discussion

Sediment accumulation rates generally increasethroughout the last 100 years. Bulk sedimentaccumulation rates were <25 mg cm)2 year)1

prior to 1935 in both cores. Mean bulk sedimentaccumulation rates in both cores were 40 mg

cm)2 year)1 during the period from 1935 to 1980.However, in core 98-10, which was retrievedapproximately 4 km from core 98-06, the increasein bulk sediment accumulation occurred later thanit did in core 98-06 and with greater magnitude.The observed increase in sedimentation rates mighthave resulted from re-connection of Bear River toBear Lake in 1918, even though 210Pb dates for thisincrease post-date the time of re-connection. There-connection of Bear River might have suppliedmore sediment from the river or flushed sedimentout of Mud Lake through which the river has flo-wed to Bear Lake. There also might have been in-creased sediment loading in the river as a result ofundocumented land-use changes in the drainagebasin after the re-connection of the river.

The total excess 210Pb inventory in a sedimentcore can be used to calculate the excess 210Pb flux

Table 3. Date and accumulation rates for bulk sediment, TP, TN, TIC, and TOC for core 98-06

Date Bulk Sediment TP TN TIC TOC

(mg cm)2 year)1) (mg cm)2 year)1) (mg cm)2 year)1) (mg cm)2 year)1) (mg cm)2 year)1)

1990 75.14 0.0174 0.18 5.20 1.47

1982 60.37 0.0122 0.13 4.18 1.27

1968 38.65 0.0105 0.08 2.68 0.78

1958 38.85 0.0090 0.08 2.69 0.75

1947 41.19 0.0125 0.09 2.66 0.99

1935 35.99 0.0073 0.07 2.59 0.64

1910 17.36 0.0031 0.03 1.20 0.34

1886 14.57 0.0061 0.03 1.07 0.27

1869 16.11 0.0049 0.03 1.15 0.34

Table 4. Date and accumulation rates for bulk sediment, TP, TN, TIC, and TOC for core 98-10

Date Bulk Sediment TP TN TIC TOC

(mg cm)2 year)1) (mg cm)2 year)1) (mg cm)2 year)1) (mg cm)2 year)1) (mg cm)2 year)1)

1995 97.39 0.0465 0.24 7.29 1.91

1991 89.63 0.0432 0.21 6.59 1.82

1985 51.75 0.0255 0.12 3.73 1.15

1977 46.05 0.0221 0.11 3.20 1.15

1970 50.41 0.0239 0.12 3.78 1.00

1962 43.01 0.0121 0.09 2.80 1.25

1952 38.36 0.0149 0.10 2.90 0.78

1936 23.61 0.0071 0.06 1.76 0.45

1920 24.00 0.0120 0.06 1.78 0.44

1892 13.73 0.0065 0.03 1.02 0.25

1866 10.99 0.0050 0.03 0.87 0.21

179

to the sediment. This value can be compared withatmospheric flux to evaluate sediment focusing.Excess 210Pb inventory was 24.56 dpm cm)2 forcore 98-06 and 29.5 dpm cm)2 for core 98-10. Theestimated mean atmospheric flux is 24 dpm cm)2

based on the Turekian et al. (1977) model. If theTurekian et al. (1977) model is accurate for thislocation, it would indicate that there has been

sediment focusing to site 98-10. Excess 210Pbinventories in Bear Lake, however indicate thatsediment focusing has not been significant, andwould produce only a small dating error if sedi-ment focusing has changed over the dating period.

Accumulation rates of TP, TN, TIC, and TOCincreased between four and six fold from the oldestdated interval in the 1900s to the most recent

Table 5. Depth interval, TP, TN, TIC, TOC concentrations, and TOC/TN atomic ratio for core 98-06

Depth interval TP TN TIC TOC TOC/TN

(cm) (mg g)1) (mg g)1) (mg g)1) (mg g)1) atomic ratio

0–1 0.231 2.40 69.15 19.55 9.5

1–2 0.202 2.20 69.21 20.99 11.1

2–3 0.271 2.10 69.33 20.27 11.3

3–4 0.231 2.10 69.21 19.29 10.7

4–5 0.303 2.20 64.49 24.01 12.7

5–6 0.203 2.00 71.95 17.65 10.3

6–7 0.180 2.00 69.24 19.46 11.4

7–8 0.422 2.00 73.19 18.51 10.8

8–9 0.306 2.00 71.06 20.94 12.2

9–10 0.327 2.20 67.79 26.61 14.1

10–11 0.324 2.00 70.43 22.07 12.9

11–12 0.312 1.90 74.57 18.33 11.3

12–13 0.305 1.70 68.20 21.20 14.6

13–14 0.297 1.40 68.29 20.51 17.1

14–15 0.322 1.90 69.05 20.75 12.7

15–16 0.291 1.30 74.52 11.68 10.5

16–17 0.355 1.40 70.72 15.98 13.3

Table 6. Depth interval, TP, TN, TIC, TOC concentrations, and TOC/TN atomic ratio for core 98-10

Depth interval TP TN TIC TOC TOC/TN

(cm) (mg g)1) (mg g)1) (mg g)1) (mg g)1) atomic ratio

0–2 0.477 2.50 74.83 19.57 9.13

2–3 0.482 2.30 73.58 20.32 10.31

3–4 0.493 2.40 72.03 22.17 10.78

4–5 0.48 2.40 69.39 24.91 12.11

5–6 0.475 2.30 74.99 19.81 10.05

6–7 0.282 2.20 65.05 29.05 15.41

7–8 0.388 2.50 75.62 20.38 9.51

8–9 0.301 2.40 74.51 18.89 9.19

9–10 0.499 2.30 74.21 18.19 9.23

10–11 0.47 2.50 74.40 18.50 8.63

11–12 0.452 2.70 78.92 19.38 8.37

12–13 0.416 3.20 77.61 30.69 11.19

13–14 0.411 3.20 77.18 30.62 11.17

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interval in each core based on model dates. In-creases in bulk sediment accumulation rates areresponsible for increased rates of nutrient accu-mulation. Nutrient concentrations do not increaseup-core consistently or by a magnitude sufficientto produce the observed nutrient accumulationrate increases. It is possible that the enhancednutrient input was in a particulate form that wasbiologically unavailable for uptake. Therefore, theincrease in nutrient input did not contribute to aproductivity increase within the lake. The possi-bility also exists that the input of inorganic mate-

rial has increased along with available nutrientinput, which would have diluted the nutrientconcentrations in the sediments. However, thelikelihood of such equivalent increases seems re-mote, and it seems more probable based uponsedimentary evidence that Bear Lake has notundergone significant increases in available nutri-ents during the past 100 years.

TP concentration in sediments should be thebest indicator of nutrient removal from the watercolumn in this carbonate-rich system. Surveystudies have demonstrated a positive correlation

Bear Lake 98-06Mass Sedimentation Rate mg cm-2 yr-1

0 20 40 60 80

Cum

ulat

ive

Dep

th g

cm

-2

0

1

2

3

4

5

1990-1998

1982-1989

1968-1981

1958-1967

1947-1957

1935-1946

1910-19341886-19091869-1885

Figure 2. Mass sedimentation rate vs. mass depth; age intervals are shown for each mass depth.

Bear Lake 98-10Mass Sedimentation Rate mg cm-2 yr-1

0 20 40 60 80 100 120

Cum

ulat

ive

Mas

s g

cm-2

0

1

2

3

4

5

1995-19981991-1994

1985-1991

1977-1984

1970-1976

1962-1969

1952-1961

1936-1951

1920-1935

1892-19191866-1891

Figure 3. Mass sedimentation rate vs. mass depth; age intervals are shown for each mass depth.

181

between TP content of surficial lake sediments andwater-column phosphorus concentration (Brenner& Binford, 1988) or phosphorus loading (Son-dergaard et al., 1996). Phosphate can co-precipi-tate with calcium carbonate (Pettersson &Bostrom, 1986; Kleiner, 1988). The results ofphosphorus analyses in some paleolimnologicalstudies have been of limited use in assessingeutrophication because phosphorus can be re-leased during diagenesis. However, calcium-boundphosphorus is less likely to be released than otherforms of phosphorus, so TP in Bear Lake sedi-ments should provide reasonable indication ofpast limnetic phosphorus concentrations. The lack

of an increase in TP concentration in Bear Lakesediments suggests nutrient input alone was notresponsible for the increase in sediment accumu-lation in Bear Lake. The enhanced nutrient inputto the system could have been in the unavailableparticulate form or available form. If in theavailable form, the nutrients entered the systemalong with inorganic material that diluted theconcentration in the sediments. The effects ofavailable nutrient loading to Bear River mighthave been diminished by the river passing througha wetland upstream of Mud Lake and Bear Lakebecause freshwater wetlands sequester nutrients(C, N, P) (Craft & Casey, 2000).

Bear Lake 98-06TP mg cm-2 yr-1

0.000 0.002 0.004 0.006 0.008 0.010 0.012 0.014 0.016 0.018 0.020

Cum

ulat

ive

Mas

s g

cm-2

0

1

2

3

4

1990-1998

1982-1989

1968-1981

1958-1967

1947-1957

1935-1946

1910-1934

Figure 4. Total phosphorus accumulation vs. mass depth.

Bear Lake 98-10TP mg cm-2 yr-1

0.00 0.01 0.02 0.03 0.04 0.05

Cum

ulat

ive

Mas

s g

cm-2

0

1

2

3

4

1995-1998

1991-1994

1985-1991

1977-1984

1970-1976

1962-1969

1952-1961

1936-1951

1920-1935

Figure 5. Total phosphorus accumulation vs. mass depth.

182

TOC/TN can be used to distinguish sedimen-tary organic matter from aquatic as opposed toland sources (Meyer & Teranes, 2001). Low TOC/TN values in both cores indicate dominance ofaquatic-derived organic matter in sediments. Core98-06, which is closer to shore, has TOC/TN val-ues that are generally greater than values in core98-10. This indicates that core 98-06 received aslightly greater influence from land-plant organicmatter than did core 98-10. Core 98-06 hassomewhat higher TOC/TN values in sedimentsolder than 100 years, which suggests that land-derived organic matter had a greater influenceprior to the re-connection to the river. Core 98-10has fewer older sediment intervals in which toexamine these possible changes. Core 98-10 hasgreater sediment accumulation rates, which alsoincrease the temporal resolution of the profile. The1962–1969 interval in core 98-10 has an elevatedTOC/TN ratio, which presumably represents agreater input of eroded material from the sur-rounding land. This increased input might haveresulted from increased precipitation during thisperiod. This increase in TOC/TN was not ob-served in core 98-06 perhaps because of the lowerresolution in the sediment profile. The longer timeintervals in core 98-06 might obscure recognitionof short-term events.

Conclusion

Bulk sediment accumulation rates in Bear Lakeincreased approximately twofold following the re-connection of Bear River in 1918. Althoughnutrient accumulation rates increased, nutrientconcentrations within the sediments did notincrease in recent times as might be expectedhad bulk sediment accumulation rates increasedmainly because of an increase in availablenutrients supplied to the lake. The cause ofincreased bulk sediment and nutrient accumula-tion rates appears to have been increased supplyof sediment washed in from Bear River orMud Lake. Although re-connection of the BearRiver with Bear Lake was the likely cause of in-creased sediment loading, there was a time lag ordelay between the re-connection (1918) and theincrease in sediment accumulation observed in thestudy cores.

Acknowledegements

We thank W. Dean, T. Whitmore, J. Rosenbaum,and W. Kenney as well as the many participants ofthe USGS LACS project. Reviews by M. Brennerand an anonymous reviewer were most helpful inimproving the manuscript. Funding for PWS wasprovided in part by the USGS Coastal and MarineGeology Program and Earth Surface Processes.

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