Six flume runs using a substrate of 30% bentonite and 70% fine sand were used to examine channel incision into a cohesive, homogeneous substrate. Gradients for the various runs were 1%, 2%, 5%, 10%, and 20%; other variables remained constant (discharge = 2 L/s, sediment discharge averages 3 g/s, run time = 15). With increasing flume gradient, bed forms change from parallel longitudinal grooves to a broad, shallow channel with weakly undulating bed and walls and then to a progressively deeper and more undulating inner channel. Downstream changes in reach-scale (several channel widths) gradient may thus be responsible for downstream variability of erosional bed forms in the absence of changes in other controlling variables. For bedrock channels, it is not necessary to invoke substrate variability to explain the presence of erosional bed forms and associated variations in cross-sectional area. Erosional bed forms may approximate an equilibrium state in which negligible change occurs in bed-form dimensions or shape with time. Downstream alternations between solely erosion and deposition of an alluvial veneer over incisional features occur along channel reaches of constant gradient; the downstream length of these alternations is inversely proportional to gradient.