Drainage basins.—The physical features of Illinois con sist of a plain whose surface is varied somewhat by glacial moraines and stream valleys. Differences in altitude are not sufficiently marked to divide the state into distinct physical regions on the basis of elevation. It is possible, however, to divide the state into definite drainage areas. While divides between these basins may not be conspicuous, detailed maps make it possible to mark out their limits with accuracy. The entire state belongs to two large drainage regions: the Lake Michigan Basin and the Mississippi River Basin. The Mississippi Basin in Illinois may be further divided into a number of subordinate basins. The state may be divided as indicated in the accompanying table into eight regions.
Lake Michigan Basin.—The Lake Michigan Basin has the smallest area and the largest population of the eight drainage regions into which the state has been divided. It is narrowest in Lake County where the divide between Lake Michigan and the Des Plaines River is within 4 to 6 miles of the lake shore. It widens southward to a width of S to 12 miles in the Chi cago region, increas ing in southern Cook County and north eastern Will County to a width of about 20 miles. The divide lies on the Valpa raiso moraine near its inner margin, thus leaving the greater part of this moraine in the Illinois River Basin. The length of the divide from the Wisconsin bound ary to the Indiana boundary is about 100 miles.
The area assigned to the Lake Michigan Basin, 722 square miles, is equal to a square whose sides are 27 miles in length, or to a circle whose diameter is 30 miles. The city of Chicago with an area of nearly 200 square miles occupies 27 per cent of that part of the Lake Michigan Basin in Illinois. More than 40 per cent of the population of Illinois live on the 1.3 per cent of the area of the state included in the Lake Michigan Basin.
The chief topographic features of this drainage basin are the Chicago Plain and the inner portion of the Valparaiso moraine.
The Chicago Plain extends from Winnetka southward to the Indiana boundary and eastward into Indiana around the head of Lake Michigan. It is the bottom of the ancient glacial lake, Lake Chicago, the ancestor of Lake Michigan, which formed to the southward of the great ice sheet and discharged its waters across the Valparaiso moraine through the Chicago Outlet, the present valley of the Des Plaines River. The Chicago Plain is flat with occasional loW sand dunes and a few rem nants of the Valparaiso moraine.
The natural drainage of the Chicago Plain is through the Chicago and Calumet rivers and their tributaries. The North Branch of the Chicago River and the South Branch of the Chicago River unite near AIarket and South Water streets in the city of Chicago, forming the Chicago River which extends eastward 11 miles to Lake Michigan. Under natural conditions the waters of
these streams flowed with sluggish current into the lake. With the opening of the Chicago Drainage Canal in 1900 the currents of the Chicago River and of the South Branch were reversed, and sufficient water from Lake Michigan has since been flowing westward and southwestward across the natural divide at Summit into the Des Plaines River at Lockport to give the city of Chicago proper drainage, and to insure a supply of good water from the lake.
The Grand Calumet flows with sluggish current from the sand dunes of Indiana through Gary and Hammond, Indiana, and through South Chicago to Lake Michigan. The Little Calumet enters Illinois from Indiana flowing northwestward; it makes a sharp bend to the east and joins the Grand Calumet at the southern edge of Chicago. Lake Calumet, Hyde Lake, and Wolf Lake are shallow lakes connected with the Grand Calumet River.
The indefiniteness of the divide between the Lake Michigan and Illinois River basins is strikingly shown in various ways. In pioneer days a continuous passage for boats was found at times of high water along the Chicago portage between the Des Plaines River and the South Branch of the Chicago River. No great difficulty was experienced in digging the Illinois and Michigan Canal across the divide at Summit; nor in furnishing the canal with a water supply through the "Canal Feeder" constructed across the low divide in the "Sag." The Sag is a broad valley once occupied by water flowing from Lake Chicago and separated from the Chicago Outlet, or Des Plaines Valley, by Mount Forest Island. The observant traveler can sec and appreciate these topographic relationships today as he is carried swiftly on the railroad along the tedious water route followed by Joliet and Marquette in 1073.
That portion of the Valparaiso moraine which lies in the Lake Michigan Basin is divided into two parts by the Chicago Outlet. The narrow belt to the north, between the divide and the lake, consists of a series of morainic ridges drained by short, wet-weather streams which have cut deep V-shaped gullies into the lake bluff north of Winnetka, and by the upper course of the North Branch of the Chicago River. The lake shore from the Wisconsin boundary line to the city limits of Chicago, a distance of 36 miles, is occupied by cities and villages at intervals of about two miles. Excellent railroad service makes possible this remarkable series of residential suburbs along the lake. Here many thousands of people have homes situated on beautiful sites, with healthful sur roundings, and with easy access to the great city.