Before the vast
surface changes produced during the last glacial period, three of the
great lakes--Michigan, Huron and Superior--discharged their waters
southward into the Gulf of Mexico by a broad river. The accumulation
of glacial debris changed all this; the southern outlet was cut off,
and a new one to the north was opened near where Detroit stands,
making a channel to Lake Erie, which then became the outlet for the
whole chain by way of Niagara. A very slight change in levels would
serve to restore the present _regime_. Around Lake Michigan the land
has been slightly raised, the summit above mean water level being only
about 8 ft. Thirty miles from the south shore the lake level is again
reached at a point near Lockport (see Fig. 2); the fall then becomes
more marked. At Lake Joliet, 10 miles further, the fall is 77 ft.; and
at La Salle, 100 miles from Chicago, the total fall reaches 146 feet.
At La Salle the Illinois River is met, and this stream, after a course
of 225 miles, enters the Missouri. In the whole distance the Illinois
River has a fall of 29 ft. "It has a sluggish current; an oozy bed and
bars, formed chiefly by tributaries, with natural depths of 2 ft. to 4
ft.; banks half way to high waters, and low bottoms, one to six miles
wide, bounded by terraces, overflowed during high water from 4 ft. to
12 ft. deep, and intersected in dry seasons by lake, bayou, lagoon,
and marsh, the wreck of a mighty past.
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