Evaluation of Historical Sediment Deposition 



Related to Land Use 



Through Analysis of Lake Sediments 



By Craig N. Spencer' 



Introduction 



One of the biggest environmental concerns 

 with regard to timber harvest is the potential for 

 enhanced erosion and transport of sediment to 

 surface waters. Increased sediment loadings are 

 considered undesirable as they may degrade 

 gravel spawning habitat used by stream fish 

 (see Weaver and Fraley, Module D). Further- 

 more, since sediments represent a major source 

 of water-borne nutrients (Mortimer 1941; Perry 

 and Stanford 1982) increased erosion and sedi- 

 ment transport may accelerate the eutrophica- 

 tion process in surface waters, especially in 

 lakes and reservoirs. 



Current debate over the impact of logging 

 activities on water quality in the Flathead Basin 

 is hampered by a scarcity of quantitative data on 

 conditions prior to the commencement of tim- 

 ber harvest. Early logging activities began 50 or 

 more years ago in many parts of the Basin. 

 However, most water quality monitoring ef- 

 forts in the Basin were initiated only within the 

 last 10 to 20 years or less. Without pre-logging 

 water quality data, it is difficult to assess the 

 impact of harvest activity on water quality in the 

 Flathead Basin. 



Previous studies in other areas have docu- 

 mented increased sediment loadings to surface 

 waters resulting from timber harvest activities 

 (Likens et al. 1970, Lowe et al. 1986). Never- 

 theless, it may not be appropriate to extrapolate 

 findings from other areas to the Flathead Basin. 

 A number of streams in the B asin are flanked by 



steep, naturally occurring, unstable stream banks 

 composed of sand, silt and clay. The presence of 

 these erosive deposits may result in naturally 

 elevated sedimentation rates in the Basin. Thus 

 it is possible that erosion of sediments associ- 

 ated with logging activities in the Flathead 

 Basin may be minor in comparison to natural 

 sediment loadings. 



The vast majority of suspended stream 

 sediments carried into large deep lakes are 

 deposited within the quiescent lake environ- 

 ment. Significant changes in surface erosion 

 and sediment transport within the catchment of 

 a lake should be reflected in changes in the 

 sediment character and the rate of sediment 

 accumulation in the lake (Berglund 1 986). Thus, 

 undisturbed sediments deposited on the bottom 

 of lakes contain a record of the past history of 

 sedimentation from their respectivecatchments. 

 Modem paleolimnological techniques are avail- 

 able which allow estimation of past sedimenta- 

 tion rates through detailed analysis of the "paleo" 

 record preserved in the lake sediments (Ber- 

 glund 1986). A record of past changes is most 

 evident in sediments deposited in the deep- 

 water (profundal) region of the lake. This envi- 

 ronment is much more stable than near-shore, 

 shallow lake, or stream environments; deep 

 lake sediments may remain largely undisturbed 



'Dr. Craig N. Spencer is a Research Assistant 

 Professor with the University of Montana, Flathead 

 Lake Biological Station in Poison, Montana. 



V J 



Flathead Basin Cooperative Program Final Report 



Page 19 



