SEDIMENTATION IN MACKENZIE RIVER BASIN 347 
the river. The spruce timber is found caving into the river along 
considerable stretches. Where a strong current sets against the 
bank caving proceeds as rapidly as the face of the cut bank thaws. 
The heavy mat of moss and vegetable matter prevents thawing 
downward beyond a couple of feet, or less in most sections. The 
lateral thawing on the face of cut banks results in overhanging 
masses of silt covered with forest trees (Fig. 3). These finally 
break off from the bank as thawing and undercutting proceeds, and 
the slumping frequently splits the trunks of trees, leaving half of 
Fic. 3.—Destructive river work on the outside of a curve, lower Slave River 
stump on shore. Islands are formed quickly, and many of them 
disappear quickly. A sandbar first appears; then a multitude of 
willows spring up. If ice and floods are not too devastating during 
the next seasons, the small willows persist and materially aid in 
adding more sediment to the bar. 
The building of silt islands below places of maximum cutting, 
or opposite cut banks on the inside of the curve which results from 
their development, is seen throughout the course of the Slave 
River below Fort Smith. These islands, if near one bank, are likely 
to have successive zones of sediment added to them on the sid2 next 
the channel with the lesser current, until it is closed and they become 
