In Chugach the dominant rock is graywacke which colors the streams a muddy gray-brown. These opaque waters hide the river bottom, making crossing on foot uncertain and navigating a boat difficult. Hot Most rivers drop during long hot summers. Glaciers melt more on hot, sunny days. Close to a glacier, river levels rise dramatically on sunny afternoons and drop during cool early morning hours.
Rainstorms also increase glacial melt, as well as add their own runoff to a river.
Occasionally, large rainstorms will swell glacial rivers and fill all channels from shore to shore. Evidence of these floods is seen in the piles of driftwood left high and dry on gravel islands. When days shorten and temperatures drop, so do the rivers. Visitors will hardly recognize these rivers in the winter. Many trails and routes in Alaska will lead you to unbridged streams and rivers.