Complex sentence: A misfit stream (or underfit stream) is a stream that is far too small to have eroded the valley which the stream occupies, nor is the shape of the valley consistent with a typical valley that has been eroded by water.
Split into simpler sentences: A misfit stream (or underfit stream) is a stream that is far too small to have eroded the valley which the stream occupies. The shape of the valley may also be inconsistent with a typical valley that has been eroded by water.