How much they store, and how quickly the underground reservoirs empty depends on how we treat riparian areas. Alteration and loss of vegetative cover can cause runoff to occur over a shorter time period as well as produce higher peak flows. That translates into more stream horsepower to do damage plus less retention time for water to soak into the soil and underlying substrate. Flooding is one way to putting water in the bank, figuratively and literally. Flooding allows water to saturate the floodplain and raise the water table.
Most floods occur during spring and early summer.
In years without overbank flooding, water enters the floodplain through the bottom and sides of the streams channel, adding to ground water. Stream channel meanders and a well-vegetated riparian area slow water down allowing it to seep into storage underground.