The percentage of the total bank storage that has drained at various points in time from floodplains composed of gravel, sand, silt and clay in valleys of varying width is shown in Figure 3. Gravel banks have such high hydraulic conductivity that they drain completely in a few days. Water is retained in floodplains composed of silt or clay far longer than floodplains composed of sand or gravel. These wider valleys would continue to drain for more than 50 years after the water level dropped. Summary The amount of water stored in the banks and thus potentially available for release is greater in valleys that are wide, with channels that are deep, and where the material is coarse.
Wide valleys have more area available for storage.
Valleys with deep channels have a greater thickness of floodplain material in which storage can occur. The time frame over which water in bank storage contributes to baseflow depends on hydraulic conductivity, specific yield, width of the valley, and water table drop. The conditions likely to sustain baseflows involve a balancing between yield and duration of flow. There must be sufficient discharge from bank storage to maintain a free-flowing stream during drier seasons but the drainage must not be so rapid so as to disappear before precipitation replenishes the stream and bank storage. Conditions sustaining riparian vegetation largely require sustaining the moisture regime in riparian soils.