1 In hydrology, accumulations of sediment on the river bed, formed by variations in flow competence, and ranging in size from ripples in the sand, a few centimetres apart, to ‘dunes’ tens of metres in length.

River bed form is related to energy level; as flow energy and sediment transport competence increase, bedforms change from ripples to dunes, which alters the ratio of inertial to gravitational forces in the flow field. This form-flow feedback is a reflection of river energy reorganizing the sediment mass in the most effective configuration for that flow condition; see Nelson et al. (2011) ESPL 36, 14, 1928.

2 Migrating megaripples are bedforms that appear in the surf zone of sandy coasts; see Gallagher et al. (1998) Nature 394.

http://walrus.wr.usgs.gov/seds/ The excellent USGS bedform sedimentology site.