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In the middle course, the sedimentary deposits are not too thick because the quantity of sediment transported in is about equal to the quantity leaving. The deposits are mainly sandy gravel and have a high permeability.
During highwater, the water can overlap the natural levees. The coarsest sediments (sand and gravel) are dropped closer to the stream channel with a decrease in grain size away from the channel. The silt and clay deposits are often found near the valley sides, and although these sediments are deposited a considerable distance from the channel, they may not be permanent as the river meanders back and forth across the valley, and the meanders continually migrate downstream. This erosional action removes the silt and clay on the surface and deposits sand and gravel. Because the velocity distribution across the stream tends to be erratic, the deposits are usually inhomogeneous and often are lenticular. There are wide differences in the grain size and variations of gradation from uniform to well-graded.
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