348 E, M. KINDLE 
apart of the mainland. All stages of these islands may be observed, 
from the sandbar just emerging from the water, with no trace of 
vegetation, to the island with a mature forest of large spruce. 
As soon as a bar island is built sufficiently above low-water stages 
for any vegetation to survive on it, a dense growth of willows covers 
it. These for some years practically exclude other kinds of trees. 
Their enormously long roots form a network which protects the 
loose silty material of the young island from destruction by high 
water, while their twigs and stems greatly accelerate the accumula- 
tion of sediment by checking the velocity of the current around 
Fic. 4.—Constructive river work on the inside of a curve. Note three successive’ 
growths of willows in front of the tall spruce timber representing periodic increments 
of silt bands to the river bank. 
them. The growth of the silt island is therefore rapid after the 
first growth of willows has become well established. When the 
island has been built sufficiently high by the annual accretion of 
sediment, poplars and later spruce begin to displace the willows. 
Frequently three or more zones may be distinguished around these 
islands, each a year or more younger than the one inside it, by the 
height of the willows on them (Fig. 4). 
Above the mouth of Slave River for about 125 miles cut banks 
of yellow sand 15 to 18 feet high are common for long stretches. 
They terminate abruptly against the ordinary alluvial banks 
and evidently represent a different and earlier set of deposits which 
probably are of lacustrine origin. Immediately below the Grand 
