246 MODIFICATION OF THE GREAT LAKES 



points on this line, called the isobase of the outlet, are raised or 

 lowered equally by the tilting and are unchanged with reference 

 to one another. All points southwest of it are lowered, the 

 amount varying with their distances from the line, and all points 

 to the northeast are raised. The water, always holding its sur- 

 face level and always regulated in volume by the discharge at 

 the outlet, retreats from the rising northeast coasts and encroaches 

 on the sinking southjvest coasts. Assuming the rate of tilting 

 to be 0.42 foot per 100 miles per century, the mean lake level is 

 rising at Duluth 6 inches per century and falling at Heron bay 

 5 inches. Where the isobase intersects the northwestern shore, 

 which happens to be at the international boundary, there is no 

 change. 



Lake Ontario lies altogether southwest of the isobase of its 

 outlet, and the water is encroaching on all its shores. The same 

 tilting that enlarged it from the area markt by the dotted line of 

 figure 2 is still increasing its extent. The estimated vertical rise 

 at Hamilton is 6 inches per century. The whole coast of Lake 

 Erie also is being submerged, the estimated rate at Toledo and 

 Sandusky being 8 or 9 inches per century. 



The isobase of the double Lake Huron-Michigan passes south- 

 west of Lake Huron and crosses Lake Michigan. All coasts of 

 Lake Huron are therefore rising as compared to the outlet, and 

 the consequent apparent lowering of the mean water surface is 

 estimated at 6 inches per century for Mackinac and at 10 inches 

 for the mouth of the French river on Georgian bay. In Lake 

 Michigan the line of no change passes near Manistee, Michigan. 

 At Escanaba the estimated fall of the water is 4 inches per cen- 

 tury ; at Milwaukee the estimated rise is 5 or 6 inches, and at 

 Chicago between 9 and 10 inches. 



These slow changes of mean water level are concealed from 

 ordinary observation by the more rapid and impressive changes 

 due to variations of volume, but they are worthy of considera- 

 tion in the planning of engineering works of a permanent char- 

 acter, and there is at least one place where their influence is of 

 moment to a large community. The city of Chicago is built on 

 a smooth plain little above the high-water level of Lake Michigan. 

 Every decade the mean level of the water is an inch higher, and 

 the margin of safety is so narrow that inches are valuable. Al- 

 ready the older part of the cit}'' has lifted itself several feet to 

 secure better drainage, and the time wdll surely come when other 

 measures of protection are imperatively demanded. 



