Many of these side affluents also have the advantage of access to the main lateral moraines of the vanished glacier that occupied the canyon, and upon these they draw for lake-filling material, while the main trunk stream flows mostly over clean glacier pavements, where but little moraine matter is ever left for them to carry.

Thus a small rapid stream with abundance of loose transportable material within its reach may fill up an extensive basin in a few centuries, while a large perennial trunk stream, flowing over clean, enduring pavements, though ordinarily a hundred times larger, may not fill a smaller basin in thousands of years.
[Illustration: Tamarack and Echo Lakes] [Illustration: Cascade Lake, Near the Automobile Bouldvard, Lake Tahoe] [Illustration: Memorial Cross at Donner Lake] Many striking examples of these successive processes may be seen in the Tahoe region, as, for instance, Squaw Valley, which lies between the spurs of Squaw Peak and Granite Chief.

This was undoubtedly scooped out by a glacier that came down from Squaw Peak and Granite Chief.

The course of the ice-sheet was down to the Truckee River.
When the glacier began to shrink it left its terminal moraine as a dam between the basin above and the river below.

In due time, as the glacier finally receded to a mere bank of half-glacierized snow on the upper portions of the two peaks, the basin filled up with water and thus formed a lake.