14 
MUSEUM BULLETIN NO. 11. 
The valley of Okanagan lake and river is another example of 
a south-trending trench made up of short intersecting straight 
stretches. The Otter river in the Tulameen district occupies 
a remarkably straight valley and the streams in the Beaverdell 
quadrangle illustrate the rectilinearity in stream plan in great 
detail. This fact applies to streams upon upland surfaces as 
well as those occupying deep valleys. 
Straight stretches in adjacent valleys often show a marked 
parallelism which is only brought out in a topographic map. 
This parallel trend is the more striking when one of the valleys 
is deep and the others are on the high uplands on both sides of it. 
As an example from the Kamloops area one may cite the valley 
at the headwaters of Highland creek, which is paralleled by three 
troughs upon the upland north and south of it, and by a section 
of the South Thompson valley. A number of such parallel 
stream courses are cited in the detailed description of the Beaver- 
dell area, where they are found to be related to zones of weakness 
in the underlying rocks. Rectilinearity or parallelism of the 
valley courses is not the invariable rule for all streams or sets of 
streams within the plateaus, but the tendency is marked enough 
to make a noticeable feature of the drainage plan over at least 
the southern part of the plateaus. From a theoretical point of 
view, the parallelism of upland valleys and the troughs below 
the upland is of interest, because it suggests that the same 
conditions, namely zones of weakness in the underlying rocks, 
controlled the formation of both. 
Valley Bottoms . 
The valleys of the larger streams have flat bottoms which 
are sometimes from 1 to 2 miles wide. They are made up 
of series of flat terraces and often lie far below the upland sur- 
face. Stream grades are low, the fall of the Thompson for 
instance being about 10 feet to the mile at its lower end. Rail- 
ways use these broad valleys because they offer more constant 
and lower grades. Because of their comparatively low alti- 
tudes fruit ranching is becoming an important industry upon 
the larger valley floors, although upon the higher uplands fruit 
