About one-half of the northwestern portion of the State is drained through the Chippewa River, while the remainder of this region drains directly into the Mississippi or St. Croix and into Lake Superior. The Wisconsin River has its source at a small lake nearly 1,600 feet above mean sea level on the Upper Michigan boundary and drains most of central Wisconsin. Most of its tributaries also spring from the many lakes in the north. Except for the Rock River, a Mississippi River tributary which flows through northern Illinois, eastern Wisconsin drains into Lake Michigan. Most of the streams and lakes in the State are ice-covered from late November to late March.
Snow covers the ground in practically all the winter months, except in extreme southern areas.
Flooding is most frequent and most serious in April, due to the melting of snow and spring rains. During this period, flood conditions are often aggravated by ice jams which back up the flood waters. Excessive rains of the thunderstorm type sometimes produce tributary flooding or flash flooding along the smaller streams and creeks.