Evaluation of Historical Sediment Deposition 



for thousands of years. 



One of the more notable paleolimnological 

 studies, by Hutchinson and others ( 1 970), docu- 

 mented increased sedimentation rates in a lake 

 in Italy (Lago di Monterosi) over 2000 years 

 ago, coinciding with construction of the Roman 

 Road, the Via Cassia, in about 171 B.C. Numer- 

 ous studies report changes in sedimentation 

 rates in other lakes which correlate with various 

 land disturbance activities including timber 

 harvest, plowing of fields, and road building 

 (Davis 1975, Batterbee and others 1985, see 

 reviews in Berglund, 1986). Although there 

 have been previous paleolimnological studies 

 in the Flathead Basin (Moore et al. 1982), none 

 have quantified recent sedimentation rates. 



The present research was initiated to study 

 the historical record of sediment deposition 

 over the last 100-150 years in three lakes in the 

 Flathead Basin. Whitefish Lake and Swan Lake 

 are located in watersheds that have a history of 

 land disturbance activity (primarily road build- 

 ing and timber harvest) throughout much of this 

 century. Lake McDonald is located in Glacier 

 National Park, and its watershed has not been 

 logged. However, a road was constructed from 

 the bottom to the top of the catchment during 

 the middle part of this century. 



Methods 



Sediment cores were collected from the 

 deep central basin of each lake using a freeze- 

 coring technique modified from Shapiro (1958). 

 Sampling locations were located on flat areas of 

 the lake bottom away from steeper regions 

 which may be subject to underwater landslides 

 or slumps which would distort the sediment 

 record. The sediments deposited in the deep 

 central regions of the study lakes consist of fine 

 sediments composed largely of silt and clay 

 sized particles. The cores were sectioned in 



horizontal slices (approximately 1 cm thick), 

 and the date of deposition was estimated for 

 each section. Dates were established using a 

 naturally occurring radioisotope (^'°Pb) which 

 decays at a known rate (half life=23 years). By 

 measuring the activity of ^'°Pb in each section, 

 it was possible to establish a time line of sedi- 

 ment deposition dates along the length of each 

 core (Appleby and Oldfield 1983). In undis- 

 turbed sediments, this technique has been shown 

 to be useful in dating sediments deposited up to 

 150 years ago. 



An independent technique was used for an 

 alternate estimate of the location of the year 

 1963, along the sediment core. Atmospheric 

 testing of atomic weapons peaked in 1963, and 

 previous studies have documented a peak in 

 '"Cs activity in sediments deposited in 1963, 

 due to global fallout of the radioactive decay 

 particles (Pennington and others 1973). Com- 

 parison of dates indicated close agreement be- 

 tween the two dating methods in the study 

 cores. 



Once chronological ages were determined, 

 sediment accumulation rates (mg dry wt/cmV 

 yr) were estimated. These rates were calculated 

 by multiplying the width of the core section 

 (cm) by the dry weight density of the section 

 (mg/cm') and dividing by the length of time 

 (years) spanned by the section. These rates 

 represent mean sedimentation values which 

 occurred over the time span of each 1 cm thick 

 section of sediment. 



One core from each lake was chosen for de- 

 tailed analysis. Ideally, several cores would 

 have been analyzed from each lake; however, 

 the available budget was not sufficient for 

 multiple core analysis. Although only one core 

 was analyzed in detail from each lake, the 

 selected cores appeared to be representative of 

 whole-lake conditions in Whitefish and Lake 

 McDonald. For example, distinct patterns of 

 horizontal banding noted in a particular lake 



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Flathead Basin Cooperative Program Final Report 



