A riffle is a shallow landform in a flowing channel, and it has specific topographic, sedimentary, and hydraulic indicators. These are almost always assessed at a very low discharge compared to the flow that fills the channel, and as a result the water moving over a riffle appears shallow and fast, with a wavy, disturbed water surface. The water's surface over a riffle at low flow also has a much steeper slope than that over other in-channel landforms. Channel sections with a mean water surface slope of roughly 0.1 to 0.5% exhibit riffles, though they can occur in steeper or gentler sloping channels with coarser or finer bed materials, respectively. Except in the period after a flood, the sediment on the riverbed in a riffle is usually much coarser than on that in any other in-channel landform.
