Wet or saturated soil. Saturated soil can't absorb rain and water from melting snow. The excess water becomes runoff that rapidly flows into rivers and streams. Unsaturated soil acts like a sponge, absorbing some of the water from rain or snow melt. Saturated soil by itself does not cause flooding.
Reservoirs are large, mostly man-made basins that hold water for irrigation and drinking.
Reservoirs can alleviate river flooding by absorbing and spreading out flood crests flowing down the river. This would reduce the height in which the water rises downstream of the reservoir. If the reservoir is already full, then it can not absorb any water from swollen rivers. High river and stream levels. Streams or rivers that are already at bankfull can be a precursor to major flooding.