Steep headwater streams are often characterized by alternating steps and pools, which may be described by mean step height H¿ and mean step length L¿. A conceptual model is developed based on the notion that the largest floods are just capable of moving the largest debris in the channel. The model suggests that step pools evolve toward a condition of maximum flow resistance because maximum resistance implies maximum stability and that this condition is achieved when steps are regularly spaced and the mean step steepness H¿/¿L¿ is slightly greater than the channel slope S. To test this conceptual model, four series of flume experiments were performed. These experiments show that the relation between resistance to flow and L¿ is convex upward with maximum flow resistance occurring when steps are regularly spaced and have H¿/¿L¿/S values between 1 and 2. Field measurements reveal that 18 natural step-pool streams also satisfy the inequality 1≤H¿/¿L¿/S≤2, strongly suggesting that the form of such streams is adjusted to maximize resistance to flow. The results of the flume experiments are inconsistent with the proposition that step pools form as antidunes, as Froude numbers for the flume step pools at which flow resistance was maximized fall well below those values usually associated with these bed forms. ¿ American Geophysical Union 1995.