The more it snows over the winter the more water is available over the summer. However, the weather in the spring greatly influences the water flow through the river season.  If the spring is relatively cool, the snow melts slowly and as it gradually warms the snow melts more and the river rises evenly.  Years like this are ideal as more snow is left in the mountains to feed the flow all summer long and the river stays higher longer into the summer. If the spring and early summer weather is very warm or even hot, then more snow melts and disappears from the mountains.  This makes river levels higher for a short time in the spring, but leaves less snow available to melt later in the summer.  On years like this the river will reach lower levels earlier in the season.

Link for Idaho Snow Survey: National Resources Conservation Service