POSTGLACIAL DEVELOPMENT OF CONNECTING KIVEES OF GREAT LAKES. 501 



is too small to be noticed in the smaller streams, and is smaller than would be expected in most 

 of the larger streams, such as the Rouge, the Clinton, the Thames, and the Sydenham. Black 

 River alone shows a large amount of refilling, but apparently with more reason. The other 

 streams -flow through a gently sloping plain for a longer distance, and so have lower gradients 

 and gentler currents for longer distances above their mouths. Black River cuts through the 

 Port Huron morainic system at Wadhams, from near which the refilling extends to within 

 about 3 miles of the mouth of the river. The river flows in a valley trenched deeply in the drift 

 for several miles above and below Wadhams. Its valley is older than the time of Algonac 

 River, but it was deepened at that time up to a point near Wadhams. The deepened part is 

 now floored with a flat gravel deposit or an elongated estuarine delta, which constitutes the 

 present flood plain and is gradually extending downstream. The amount of this refilling 

 might be made the basis of a rough estimate of the time since the discharge of the upper lakes 

 returned to Port Huron. For this, however, other and much better data (Niagara Falls and 

 Gorge) are available, and the amount of refilling would be interesting chiefly in its relation to 

 that .better-deterrnmed estimate. 



A large part of the surface of the St. Clair delta is now above the water; its head near 

 Algonac is about 6 feet above. The water surface in the south end of Lake Huron and in the 

 head of St. Clair River has fallen about 15 feet since the end of the transitional stage from the 

 Nipissing Great Lakes ; at Algonac it has fallen slightly less, so that at the beginning of the present 

 stage the water was probably 6 or 7 feet deep over the ground at the head of the delta. Hence, 

 the emergence of the St. Clair delta in this last stage of the lake history is relatively recent, and 

 the development of the present distributary channels in it is nearly as recent, the necessity for 

 their existence arising only when the water began to grow thin over the surface of the delta. 



A large part of the westward crowding of the rapids at the head of St. Clair River, causing 

 the erosion of its western bank and the accretion of gravel on its eastern bank, is also relatively 

 recent, and has resulted in a lengthening of the rapids and the bend and a large increase in the 

 gravel deposit at the head of the river. 



Six principal changes are now going on: (1) The two great rivers are gradually deepening 

 their channels; (2) they are beginning to develop lateral swinging and bank cutting, as where 

 Detroit River is cutting into the Canadian bank at the Limekiln crossing; (3) the lowering of 

 the river surface is decreasing the area of Lake St. Clan and the effect of drowning in all the 

 tributaries; (4) the tributaries are filling and aggrading then- drowned parts with sediments; 

 (5) the St. Clair delta is growing and filling up Lake St. Clair; and (6) the spit gravels at the 

 head of St. Clair River are growing westward and pushing the river farther into the land on 

 the west side. 



RELATIVE AGES OF THE CONNECTING RIVERS. 



St. Clair River came into existence first and Detroit River almost at the same time; then 

 came the Niagara River; then, in all probabdity, the upper part of the St. Lawrence below Lake 

 Ontario, extending perhaps down to Cornwall; then the upper part of Nipigon River (the part 

 above the long rapids), and finally, about coincidently, the lower part of the Nipigon, the 

 lower part of the St. Lawrence, and the whole of the St. Marys between Lakes Superior and 

 Huron. The three last-named rivers are very new in then lower courses, and if differential 

 elevation is still in progress it is lifting their mouths out of the water and making their beds 

 shallower and longer. 



