Controls on Pool Characteristics along a Resistant-Boundary Channel Available to Purchase

The North Fork Poudre River in Phantom Canyon has a pool-riffle sequence formed in granitic bedrock. We address two questions: What controls downstream pool spacing? Within a pool, what factors control pool dimensions? We hypothesize that both pool spacing and pool geometry could be governed by available flow energy or by substrate characteristics. These hypotheses were addressed using channel-bed and water-surface gradients surveyed under moderate flow, Selby rock-mass strength at vertical outcrops forming lateral pool constrictions and at other vertical outcrops, and joint density measured from high-resolution digital images of the bedrock outcrop along each of 10 pools. The downstream length between pools is quite variable (⁠$mean[m]=117$ m, standard deviation $[SD]=106$ m), as is the cumulative drop between pools (⁠$mean=1.3$ m, $SD=0.8$ m). Total upstream bed gradient and approach bed gradient vary by an order of magnitude. Thus, pools do not have a uniform longitudinal or gradient distribution along the study reach, indicating that the downstream spacing of pools does not reflect systemwide available flow energy. The mean of 10 rock-mass strength measurements at pool constrictions (⁠$m=73$⁠), and 10 measurements elsewhere (⁠$m=72$⁠), indicate no significant differences in rock strength at pool locations. However, pools are almost always associated with lateral constrictions where the active channel intersects a bedrock ridge. The downstream spacing of these ridges reflects regional patterns of structure and weathering, and the local jointing of each outcrop influences the formation of constrictions. The downstream spacing of pools thus appears to reflect substrate controls. Both flow energy and substrate appear to exert an influence on individual pool geometry. Pools with stronger lateral constrictions are deeper and have a shallower approach gradient. More-constricted pools decrease upstream sediment transport capability through a backwater effect. This decreases approach gradient by promoting coarse-sediment deposition. The stronger constrictions maximize stream power downstream and promote flow separation and bed scour. This leads to longer and deeper pools. Increased joint density on the bedrock walls corresponds to deeper pools. These results indicate that substrate may influence pool geometry via constriction ratio, which in turn influences approach gradient, pool length, and pool depth.