The influence of lake morphometry on sediment focusing

Abstract

Sediment focusing is a process whereby water turbulence moves sedimented material from shallower to deeper zones of a lake. Sediment focusing occurs in lakes in both the erosional (coarse-grained sediments) and the transportational zone with the latter characterized by discontinuous sedimentation and resuspension. The zone of accumulation is diefined as the zone where sedimentation is final (i.e. no resuspension) and where there is no further sediment focusing. A geochemical tracer (Pb) was used to trace sediment focusing patterns in 12 lakes of different morphomctries. The area occupied by the zone of accumulation is predicted from the mean basin slope (a',) with 86% of the variance explained. Only in large, exposed lakes in which turbulence from waves is more severe are the resulting sediment distributions more erratic. This study is the first to provide a general model of sc:diment focusing patterns among lakes.The study of lake-bottom dynamics has received attention in the contexts of sediment, contaminant, and eutrophication studies. The fact that sediments are major depositories for contaminants and nutrients has made them a focus for research in limnology. Also, because sediments provide a record of past events in lakes, an understanding of the manner in whllch sediments accumulate is essential for hindcasting. Interpretations of the historical record in sediments will be confounded if the record has been obscured by sediment resuspension.The term "sediment focusing" was coined by Likens and Davis ( 1975) to describe the resuspension of sediments in shallower zones by waves and water currents with subsequent transport to and settling in the deeper zones of lakes. Much of the work on sediment focusing has examined sedimentation patterns over glacial time scales (Likens and Davis 1975;Lehman 1975;Davis and Ford 1982). This emphasis leaves unstated the relation between sediment distribution and mere dynamic lake properties such as mixing depth in the water column, wave and current shear stress, and sediment cohesiveness. The purpose of our study is to examine patterns of recent sedimentation (< 120 yr) in relation to physical variables such as exposure (circular integral of fetch), slope, and sediment texture (water content, organic content).Models relating sediment resuspension to surface waves and mean water velocity have been generated for selected lakes (Luettich et al. 1990; Hawley ard Lesht 1992; Aalderink et al. 1984). In general, they predict resuspension where wave shear stress exceeds current shear stress and implicate waves as the main determinant of sediment resuspension. In all cases, the study lakes were large and shallow, thereby emphasizing the importance of surface AcknowledgmentsWe thank Joseph Rasmussen for helpful suggestions. Claude Jean and Douglas Craig helped with held and lab work. The manuscript was improved by suggestion:; from an anonymous reviewer.This project was funded by an NSERC PGS scholarship to J.M.B. and an FCAR Grant to Yves Prairie and J.K.Contribution 345 of th...

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