Controls on the size and occurrence of pools in coarse‐grained forest rivers

Abstract

Controls on pool formation are examined in gravel-and cobble-bed rivers in forest mountain drainage basins of northern California, southern Oregon, and southeastern Alaska. We demonstrate that the majority of pools at our study sites are formed by flow obstructions and that pool geometry and frequency largely depend on obstruction characteristics (size, type, and frequency). However, the effectiveness of obstructions to induce scour also depends on channel characteristics, such as channel gradient, width:depth ratio, relative submergence (ratio of flow depth to grain size), and the calibre and rate of bed material supply. Moreover, different reach-scale channel types impose different characteristic physical processes and boundary conditions that further control the occurrence of pools within a catchment. Our findings indicate that effective management of pools and associated aquatic habitat requires consideration of a variety of factors, each of which may be more or less important depending on channel type and location within a catchment. Consequently, strategies for managing pools that are based solely on single-factor, regional target values (e.g. a certain number of wood pieces or pools per stream length) are likely to be ineffective because they do not account for the variety of local and catchment controls on pool scour and, therefore, may be of limited value for proactive management of complex ecosystems.

A flume experiment on the effect of constriction shape on the formation of forced pools

“…The shape and orientation of constrictions also influences pool characteristics. Buffington et al (2002) found that the largest, best developed pools were created by vertical versus horizontal constrictions. Lisle (1986) found that deflection angle of the flow at an obstruction influenced pool development.…”

“…Channel obstructions generate flow convergence and subsequent forced pools (Lisle, 1986;Montgomery et al, 1995;Thompson et al, 1999;Buffington et al, 2002;MacWilliams et al, 2006). In channels with forced pools, over 80% of pools are associated with structural controls and large obstructions that include boulders, bedrock outcrops and large woody debris (Dolan et al, 1978;Lisle, 1986;Montgomery et al, 1995;Thompson, 2001).…”

“…In channels with forced pools, over 80% of pools are associated with structural controls and large obstructions that include boulders, bedrock outcrops and large woody debris (Dolan et al, 1978;Lisle, 1986;Montgomery et al, 1995;Thompson, 2001). For example, Buffington et al (2002) list obstructions as the primary control on pool formation in 32 Pacific Northwest study reaches.…”

“…Based on conservation of mass, more severe restrictions create higher velocities under steady flow conditions (Kieffer, 1985). Several field studies show the connection between degree of constrictions and pool volume (Kieffer, 1985;Thompson and Hoffman, 2001;Buffington et al, 2002;Wohl and Legleiter, 2003). Lisle (1986) even estimated the minimum obstruction size needed to create a pool.…”

Debris dams and the relief of headwater streams

“…These impoundments typically fill with sediment to form steps and force nominally bedrock channels to be alluvial (Montgomery et al, 1996(Montgomery et al, , 2003bMassong and Montgomery, 2000; figure 2). Wood thus affects amounts and patterns of sediment storage in drainage networks (Swanson and Lienkaemper, 1978;Marston, 1982;Nakamura and Swanson, 1993;Lancaster et al, 2001;May and Gresswell, 2003) and also affects smaller features such as the locations and morphologies of pools (Buffington et al, 2002). Montgomery et al (2003a) surmised that the relative immobility of large wood, its impounding of sediment, and the concomitant shielding of the underlying bedrock decrease the effective bedrock erodibility and, hence, steepen the longitudinal channel profile.…”