The freshet is basically a flood that occurs because of heavy rainfall or because of the melting of a snow.
This type of flooding is most common in the valleys of the larger rivers, especially the ones that start their flow high in the mountains.
The large rivers have much more water, and they also have lot of tributaries, so when there's a heavy rainfall they receive enormous amounts of water, which easily causes a flood.
Also, when the snow melts, lot of water is released from it, and most of that water gets into the river, or first in its tributaries and that in it, so there's a high possibility of a big flood occurring suddenly.