Predicting the spatial mud energy and mud deposition boundary depth in a small boreal reservoir before and after draw down

We predicted the distribution of coarse- and fine-grained substrata in a small boreal lake at natural lake level and then assessed if the extents of sediment focusing due to water level manipulation could be predicted. The littoral substratum and upper limit to the distribution of mud was mapped completely prior to experimental draw down and the upper depth limit of mud was mapped in each of the first 2 years of a new water level regime. Six published equations for estimating the position of the mud boundary and the lower limit of surface wave energy were applied using maps of fetch, slope, and depth. At natural lake level, the agreement between observed and estimated mud boundaries in deep water was remarkable (<5 m horizontal). Agreement between the depth of mixing by surface waves and mud boundaries in shallow water was closest, at times exact, for equations with estimates similar to maximum wave height. However, the size of waves responsible for the shallow sediment boundaries remains unclear due to natural variation of lake level. All models overestimated energy in shallow depositional settings where exposure and slope was low. Refocusing of sediment due to maximum winter draw down of 3 m resulted in contraction and expansion of the profundal zone; a net decrease in area of 3% was evident after 2 years. Our results demonstrate that mud deposition models can be used in deep lake basins to map the littoral and profundal zones. Sediment refocusing in the first few years of winter draw down is forced mainly by falling lake levels. The interpretation of littoral habitat complexity can be simplified by understanding the lake-wide spatial pattern of erosion, transport, and deposition of sediments.