Scaling of confluence dynamics in river systems: Some general considerations

The arrangement of streams in networks is a fundamental spatial property of fluvial systems. As water moves through a drainage network, it is forced to converge at confluences. Over the past twenty years a salient body of theoretical, experimental, and field research has emerged on the fluvial dynamics of stream confluences. This work has begun to reveal the complex three-dimensionality of flow at confluences and to connect flow structure to the morphodynamics dynamics of confluences. It has also shown that coherent flow structures of various scales can exist at confluences and that the presence or absence of particular structures can vary both from confluence to confluence and from time to time at a specific confluence. Despite these advances, to date most studies have focused either on laboratory confluences or on small natural confluences. "Small" as used here implies that field measurements in the confluence can be obtained by wading into the flow (depth<1.0-1.5 m) or by spanning the confluence with a small bridge (width<12-18 meters). Few, if any, process-based field investigations of the fluvial dynamics of confluences of large rivers have been conducted and data do not exist to evaluate the relevance of conclusions derived from small-scale confluence studies for understanding the dynamics of large-river confluences. The extent to which findings for small confluences provide insight into the fluvial dynamics of large-river confluences is a scaling issue. Past work on the scaling of channel form and fluvial processes suggests that scaling relations for confluences are likely to be fundamentally nonlinear and therefore complex. This paper discusses potential differences between the dynamics of large-river confluences and the confluences of laboratory channels or small streams. The discussion draws on research results for small confluences and on general principles regarding scaling relations in river systems. This synthesis of extant knowledge provides the basis for identifying hypotheses regarding possible similarities and differences between the dynamics of small stream confluences and large river confluences. Issues that are addressed include three-dimensional flow structure, turbulence, mixing, and patterns of erosion and deposition within confluences. A research project on large-river confluence dynamics that seeks to evaluate the hypotheses is briefly described.