Isn't there a problem with the buffers ponding water as a result of levee formation along their edge and/or sediment trapping? Or how do you keep concentrated flow channels from developing through the buffer?

No, if the buffer and adjacent upland are managed properly.

Buffers, like any conservation practice, require management to maintain their functionality. There are two possible ways for ponding to develop in the buffer. The first is the development of a ridge along the field/native grass border as a result of cultivation. As this ridge develops, water may pond in the field at the edge of the buffer or run along the ridge before creating a breach of concentrated flow through the ridge and buffer. Proper management of the field-buffer edge is the key to minimizing the potential of concentrated flow across the buffer. A dense stand of switchgrass that is at least 5-7 m wide will halt the development of concentrated flow on most sites with slopes up to 14%. In areas with slopes of 0-2% switchgrass along the edge may be replaced by other native grasses and forbs. Switchgrass is used because it has less of a tendency to grow in bunches than most of the other native warm-season grasses that are used. Managing the edge to minimize the development of a ridge can be done using careful field cultivation. Should a ridge develop and lead to channelized flow, the edge of the buffer can be reworked by dragging the sediment back into the field and replanting the edge of the buffer.

The other cause of ponding may be the development of a levee along the streambank that results from deposition of sediment when the stream comes out of its banks. As flood water comes out of its banks its velocity slows quickly and the heavier suspended particles are dropped. As the levee builds water can be trapped in the buffer behind the levee and may have difficulty getting back into the stream. This is a natural phenomena and is most prevalent on outside bends.