Abstract
Riverine water flows both above the streambed and within the porous sediment surrounding streams and rivers, ubiquitously entering and exiting the permeable bed and banks as the flow moves downstream. This interstitial movement of riverine water is called hyporheic flow and forms a saturated volume of sediment surrounding alluvial rivers that is termed the hyporheic zone. The extent of the hyporheic zone can be defined through biological, geochemical, or hydraulic means and is a transitional area between surface and ground waters. Movement of water into and out of the alluvium (hyporheic exchange) can be predicted for different channel morphologies and across a range of spatial scales, each of which is characterized by different rates and magnitudes of exchange. As water-borne reactive solutes move through the sediment, they undergo biogeochemical reactions, which form strong physical gradients supporting a unique ecotone. The products generated by these transformations depend on the flux of hyporheic water, its residence time within the sediment, and biogeochemical reaction rates, all of which are stream-size dependent. The hyporheic zone is the main biogeochemical reactor in small streams, while processes at the streambed surface (benthic zone) and in the water column chiefly control biogeochemical reactions in intermediate and large rivers. This is because small streams have a higher density and diversity of microbial communities within the hyporheic zone than in the surface stream and exhibit high hyporheic connectivity (high hydraulic conductivity and strong physicochemical gradients), both of which decrease as river size becomes larger.