Floods, channel change, and the hyporheic zone

We investigated the influence of flood-induced channel changes on the hypor heic zone of 4th- and 5th-order reaches of a mountain stream network. Prefl ood versus postflood comparisons were made in three study reaches from well networks established before and reestablished after a major flood. Flood e ffects were scale dependent and varied with channel constraint and the domi nant channel forming process. Large changes were observed in unconstrained stream reaches where channel incision drove large changes in subsurface flo w paths and the extent of the hyporheic zone. However, subreach scale diffe rences were apparent. In the lower portion of the studied reach, channel in cision lowered the water table, leading to abandonment of secondary channel s, and decreased the extent of the hyporheic zone that previously extended more than 30 m into the floodplain. In contrast, the extent of the hyporhei c zone increased at the head of the studied reach where channel incision st eepened head gradients through a meander bend. In another unconstrained rea ch, lateral channel jumps dramatically altered exchange flow paths. However , the extensive hyporheic zone was maintained throughout the reach. Less ch ange was observed in the constrained stream reach where both the depth and area of sediment available to be reworked by the flood were limited by bedr ock constraining the width of the valley floor. This flood dramatically cha nged the hyporheic zone at the three study sites and these physical changes are expected to be biologically important, considering the role of the hyp orheic zone in stream ecosystem processes.