Gravity-driven water exchange between streams and hyporheic zones

The exchange of water and nutrients between rivers and hyporheic zones has been recognized as a very important process for the stream ecosystem. This water exchange is generally explained by pressure gradients on the riverbed that are induced by fluvial geomorphological features or by turbulent coherent structures. In this work we discuss a different exchange mechanism due to density gradients between the in-stream and the hyporheic water. We present laboratory results that show how weak density gradients are able to induce significant hyporheic fluxes. This gravity-induced exchange is expected to play a major role in streams with low-permeability sediments or with small topographic features