An Ecosystem Perspective of Alluvial Rivers: Connectivity and the Hyporheic Corridor

Abstract

Floodplains of large alluvial rivers are often expansive and characterized by high volume hyporheic flow through lattice-like substrata, probably formed by glacial outwash or lateral migration of the river channel over long time periods. River water downwells into the floodplain at the upstream end; and, depending on bedrock geomorphology and other factors, groundwater from the unconfined aquifer upwells directly into the channel or into floodplain springbrooks at rates determined by head pressure of the water mass moving through the floodplain hydrologic system. These large scale (km3) hyporheic zones contain speciose food webs, including specialized insects with hypogean and epigean life history stages (amphibionts) and obligate groundwater species (stygobionts). Biogeochemical processes in the hyporheic zone may naturally load groundwaters with bioavailable solutes that appear to exert proximal controls on production and biodiversity of surface benthos and riparian vegetation. The effect is especially evident in floodplain springbrooks. Dynamic convergence of aquifer-riverine components adds physical heterogeneity and functional complexity to floodplain landscapes. Because reaches of aggraded alluvium and attendant ecotonal processes occur serially, like beads on a string, along the river continuum, we propose the concept of a hyporheic corridor in alluvial rivers. We expect predictable zonation of groundwater communities and other aquifer-riverine convergence properties within the corridor from headwaters to river mouth. The landscape-level significance and connectivity of processes along the hyporheic corridor must be better understood if river ecosystems, especially those involving large floodplain components, are to be protected and/or rehabilitated.