The dominant processes of sediment distribution and focusing in a small, eutrophic, monomictic lake

There is evidence for 10 mechanisms of sediment distribution in small lakes. The sediment accumulation rates measured from a common horizon in cores from 64 sites on Esthwaite Water, U.K., were used to determine the relative importance of these mechanisms on the distribution of sediment in the lake. River inflows generally produced localized effects but there were indications of periodic turbidity flows to the deepest point of the lake from the major inflow. Active sediment focusing processes were the dominant distribution mechanisms, a major resuspension of sediment at autumn overturn probably being the most important. The remaining variance was much greater than observed in similar work elsewhere, suggesting that redistribution of sediment by direct and indirect wave action may be taking placeall over the lakebcd during isothermal periods. The relative contributions of the different processes were corroborated by analysis of previously published sediment trap data. It has long been recognized that sediment is not distributed evenly over the bed of most lakes. Many early workers (Wilson 1938; Wilson and Opdyke 1941; Deevey 1955; Tutin 1955) commented on the increased rate of accumulation in the deeper parts of some lakes compared to the shallower areas and suggested mechanisms by which this could be achieved. Likens and Davis (1975) coined the term “sediment focusing” to describe the effect. More recently studies of the transport and accumulation of sediment have been carried out with either sediment traps (e.g. Davis 1968; Pennington 1974) or multiple sediment cores (e.g. Kimmel 1978). These techniques give complementary, but different, information: sediment traps record short term, subannual events while cores give integrated long term records. Most work has been done with sediment traps. We compile here a list of the mechanisms of sediment distribution in small lakes from the evidence available in the literature and use a large set of sediment core data to assess the importance of each mechanism in a small, eutrophic, monomictic lake. Sediment distribution processes in small lakes Positive identification of the processes involved in sediment distribution is not easy. Different processes will be dominant at different times of the year and several may occur at the same time. Many workers have ignored the problem and assumed, with little or no evidence, that certain dominant mechanisms define the sediment distribution in their lake. This has resulted in a lack of clarity in published work. In an attempt to improve the situation, we have reviewed the literature. The resulting list contains the minimum number of processes that can explain all the reported data. In general we have simply reiterated the “best” description in the literature and drawn attention to the implicit, but often unstated, consequences of each mechanism. Inevitably the results are somewhat subjective, but we feel that this is justified by the present, confused state of the art. There is a good evidence for at least 10 distribution mechanisms: riverine delta formation; riverine plume sedimentation; continuous complete mixing; intermittent complete mixing; epilimnetic intermittent complete mixing; peripheral wave attack; random redistribution of sediment; current erosion/redeposition; slumping and sliding on slopes; and organic degradation. Riverine delta formation-When a river enters a lake, coarser particles which were carried along in the river channel are deposited as the energy of flow is dissipated into the lake water. Classical, fan-shaped deltas are formed from this deposited ma-