STEP-POOL STREAMS - ADJUSTMENT TO MAXIMUM FLOW RESISTANCE

Steep headwater streams are often characterized by alternating steps a nd pools, which may be described by mean step height (H) over bar and mean step length (L) over bar. A conceptual model is developed based o n the notion that the largest floods are just capable of moving the la rgest debris in the channel. The model suggests that step pools evolve toward a condition of maximum flow resistance because maximum resista nce implies maximum stability and that this condition is achieved when steps are regularly spaced and the mean step steepness (H/L) over bar is slightly greater than the channel slope S. To test this conceptual model, four series of flume experiments were performed. These experim ents show that the relation between resistance to flow and (L) over ba r is convex upward with maximum flow resistance occurring when steps a re regularly spaced and have (H/L) over bar/S values between 1 and 2. Field measurements reveal that 18 natural step-pool streams also satis fy the inequality 1 less than or equal to (H/L) over bar/S less than o r equal to 2, strongly suggesting that the form of such streams is adj usted to maximize resistance to flow. The results of the flume experim ents are inconsistent with the proposition that step pools form as ant idunes, as Froude numbers for the flume step pools at which flow resis tance was maximized fall well below those values usually associated wi th these bed forms.