A warm spell can accelerate melt beyond what the drainage basin can absorb. Runoff, and hence the height and speed of the river, will vary over the course of the day. As the day progresses the amount of heat accumulates which increases the runoff, creating higher water in the river. The same docile river that you paddled last Thursday may be a wild coursing rapid on Saturday after two days of torrential downpours. The drainage basin can't handle the excess water and there is a dramatic increase in runoff.
Flash floods can occur when there is a sudden thaw after a long cold winter.
This doesn't allow for the normal softening and breaking of surface ice. Instead the sheer force of the swelling river ruptures the surface ice. Sheets of ice become lodged, forming dams. The dams cause flooding upstream. Eventually the force of the flood water ruptures the ice dam and a flood wave careens downstream wreaking havoc in its path.