MEANDERING RIVERS AND THE DRAINING OF FLOODPLAINS: A PROPOSAL FOR THE GEOMORPHIC EFFECTS OF GROUNDWATER DISCHARGE TO LOW-GRADIENT RIVERS

The meandering of rivers within alluvial valleys creates alternating stream segments that increase or decrease river elevation relative to the surrounding regional groundwater potentiometric surface. This alternation results in alternating gaining and losing stretches similar to pool-and-riffle streams, with the greatest gains occurring at the lowest points relative to the regional GW surface, i.e. at transitions from transverse segments (perpendicular to regional GW gradient) to longitudinal segments (parallel to regional GW gradient). Because GW outflow tends to winnow fines from the riverbed whereas infiltration tends to plug pore throats, the discharge of the groundwater from the alluvial aquifer is enhanced by this alternation. Modeling of groundwater discharge from alluvial aquifers indicates that, even with very minor differential bed leakage changes, this alternation dramatically enhances GW discharge compared to straight channels with similar average differences between average river surface and GW potentiometric surface. In addition, greater sinuosities create greater discharge efficiency. The need for greater and more efficient discharge in areas with low alluvial plain gradients requires the increased sinuosity generally observed in these areas. Since raised GW outflow pressure gradients decrease sediment stability and efficient outflow results in lower average pressure gradients across the river bed, it is proposed that observed meander patterns represent a balance between available stream power (i.e. sediment transport capacity) and GW discharge requirements.