How Rivers and Floodplains Work-and How They Work Together; Colin Thorne

The primary function of a river channel is to return water to the ocean after it has fallen as rain or snow. However, river channels do not operate alone in performing this function. While a great deal of the runoff from rain and snow quickly drains back to where it came from through the channel network, some of it flows through the hypo-rheos (literally, the ‘river below’), seeping more slowly through the porous alluvium below the river bed. Also, some runoff lingers on the river’s floodplains and in its wetlands: places where water is exchanged between the channel, the hyporhiec zone, and the regional groundwater. While rivers can, and often do, function independently, of their floodplains, wetlands and aquifers the hydro-system is more resilient when and where they are connected. This is because connected floodplains, wetlands and aquifers function as capacitors in the hydro-cycle by storing water during storms, then gradually releasing it during dry periods in ways that naturally modulate variability in river flow, soil moisture level and depth to groundwater. This presentation establishes that in the past most rivers and floodplains were connected, explains how and why many became disconnected, and evaluates the case for  reconnecting channel-floodplain-wetland systems where possible, to make rivers and their ecosystems resilient to future floods, droughts and wildfires.