65 
latter stream, with Revillon Coupe and Riviere des Rochers, are the only 
communications now maintained with lake Athabaska and the Claire- 
Mamawi basin, and as above stated when the Peace is not high the water in 
these channels flows aw r ay from the lake. At present nearly all the detritus 
carried by the Peace continues on down the Slave, where it still continues 
the filling of the southern part of Great Slave lake (33). The Quatre 
Fourches maintains at present two connexions with lake Mamawi, and two 
with lake Athabaska. Lake Claire, carrying the waters of Birch river, nc 
longer has any connexion with the Peace except by the channels of Hay 
(Prairie) river and lake Mamawi, 
The topography of the delta plains show r s a gentle rise away from the 
lake shores, broken only by the low ridges that have been formed as 
natural levees along the water courses, and by the granite hills. When 
channels are abandoned, the ridges remain to separate the sloughs and wei 
meadows thus formed. By the interaction of neighbouring channel deposits 
large lagoons are cut off from the main lake. 
The granite hills stood out as islands in the former lake and are now 
in many cases completely encompassed by land. Their presence has been 
of considerable significance in determining the contours of the plain in the 
region where they are common, since they have diverted the streams and 
caused eddy currents which have facilitated deposit. The vicinity of the 
Government Dog Camp on lake Mamawi, to be discussed later, will serve 
to illustrate this point. 
Peace and upper Slave rivers have meandered extensively through 
their deposits in the old lake bed. They are actively undercutting their 
banks in places, and in others forming great island bars and shore deposits. 
As a rule the inside shore on curves has the deposits, while the outside one 
is being torn away rapidly during flood times. Such shore deposits have 
a characteristic topography consisting of a series of successively higher 
terraces whose tops slope dow r nw T ard away from the river. The soil in these 
deposits seems to depend upon the relative activity of the currents that 
produced them. If the eddy currents made by the curve of the river are 
swift and active the deposits are coarser, consisting mainly of sand. A 
gentle curve, with slower currents, usually produces less prominent terraces 
with a larger amount of clayey materials in the soils. These differences 
are reflected, in turn, in the manner in which the plant cover develops. 
The w T ay in which the great rivers are diverted, and islands formed 
in them, has been admirably described by E. M. Kindle. Rock outcrops, 
or the lodgment of drift timber on shallow bottoms, may serve to start the 
formation of bars and islands. Eddy currents thus formed, or arising 
from cross-currents in the streams, are the scenes of further accumula- 
tions. Once an island is started it acts upon the current of the river much 
as does a meander curve, tending to divert and retard the speed of the 
water along its flanks. Thus its growth usually takes the form of lateral 
bars which cut off long narrow lagoons from the main stream. These 
lagoons soon Jose the connexion they have maintained wuth the river for a 
while at their dowmstream ends, and become sloughs, eventually filled with 
characteristic slough vegetation. A section through the large island in 
Slave river at the 30th base line (Figures 11, D, and 34) show r s this forma- 
tion clearly. Further changes in the currents frequently start the under- 
cutting of the banks of these islands and eventually cause their destruction. 
