As it appears recognized that the sewage channel of Chicago must be 15 ft. deep, and as provision is now being made all over the great lake system for vessels drawing 20 ft. of water, a comparatively small additional outlay would provide for a channel available for the largest lake vessels. It is claimed that by the co-operation of the Chicago municipality and the general government—the latter to advance a sum of not less than $50,000,000—a ship (and sanitary) canal 22 ft. deep could be made from the lake to Joliet, extended thence to Utica, 20 ft. deep, and from there to the Mississippi, 14 ft. deep.
That such a work would vastly enhance the commerce, not only of Chicago, but of the whole section of the country through which the canal would pass, admits of but little doubt, and probably the outlay would be justified by results similar to those achieved with other great canal works and rectified rivers in the United States.
The following figures, showing the tonnage carried in 1888-89, give some idea of the volumes of water-borne traffic in America:
| Tons. | |
| Detroit River | 19,099,060 |
| Erie Canal | 5,370,369 |
| Sault Ste. Marie | 7,516,022 |
| Welland Canal | 828,271 |
| St. Lawrence Canal | 1,500,096 |
| Mississippi to New Orleans | 3,177,000 |
| Mississippi below St. Louis | 845,000 |
| Ohio | 2,236,917 |
| Chicago Canal and lake | 11,029,575 |
Except on the Mississippi, it may be reckoned that navigation is closed by ice during five months a year. It may be mentioned, by way of comparison, that the traffic on the Suez Canal during the year 1888-89 was 6,640,834 tons.
One very interesting point in connection with this work is the effect that the diversion of so large a body of water from the lakes will have upon their regime. At least 10,000 cubic feet a second would be taken from Lake Michigan and find its way into the Mississippi; this is approximately 4½ per cent. of the total amount that now passes through the St. Clair River and thence over Niagara.
The following table gives some particulars of the great lakes and the discharge from them:
| Cubic Feet per Second | ||||||
| Lake. | Elevation above Mean Tide. | Area of Basin, Square Miles. | Area of Lake, Square Miles. | Rainfall. | Evaporation | Discharge. |
| Superior | 601.78 | 90,505 | 38,875 | 187,386 | 34,495 | 80,870 |
| Huron and Mich. | 581.28 | 121,941 | 50,400 | 262,964 | 66,754 | 216,435 |
| Erie | 572.86 | 40,298 | 10,000 | 96,654 | 13,870 | 235,578 |
| Ontario | 246.61 | 31,558 | 7,220 | 75,692 | 10,568 | 272,095 |
The average variation in level of the lakes is from 18 in. to 24 in. during the year, and the range in evaporation from year to year is also very considerable; thus the evaporation per second on Huron and Michigan, as given in the table above, is nearly 67,000 ft., but the figures for another year show nearly 89,000 ft. per second, which would represent a difference of 6½ in. in water level. As a discharge of 10,000 cubic feet a second into the new canal would lower the level of these two lakes by 2.87 in. in a year, it follows that the difference between a year of maximum and one of minimum evaporation is more than twice as great as would be required for the canal, and even under the most unfavorable conditions the volume taken from the whole chain of lakes would not lower them an inch.
When the variations in level due to different causes—rain, wind, and evaporation being the chief—are taken into consideration, the effect of 10,000 cubic feet a second abstracted would probably not be noticeable. That this would be so is the opinion, after careful investigation, of many eminent American engineers. On the other hand there is a similar unanimity of opinion as to the advantages that would be obtained in the condition of the Mississippi by adding to it a tributary of such importance as the proposed canal.—Engineering.