Abundant precipitation makes water an icon of the Pacific Northwest, especially on the west side of the Cascades. Well watered landscapes result in heavy lush growth of trees and other vegetation. Though western Washington is well known for its rainfall, ironically, in typical years, there is a summer drought. The western slopes of the Cascades receive only 15 to 25 per cent of their annual rainfall from April to September with the remaining 75 to 85 per cent falling from October to March. During the dry summer months stream flow is maintained by water stored in the ground, in the snowpack and in glaciers.
Waterfalls are created by a sudden break in the elevation of a stream bed. Waterfalls can be formed by a number of processes. Earthquake faults can cause a stream bed to lift vertically, creating a cliff that the stream flows over. In glacier carved valleys, the valley walls of the main, or bigger valleys are often very steep or even vertical and the valleys of smaller side streams flowing into the main valley are often not eroded to the same depth as the main valley. This leaves a steep hanging valley and waterfall at the lower end of the smaller stream where it enters the main valley. Waterfalls are also formed where a stream flows over a more “competent” erosion resistant type of rock onto a softer more easily eroded rock or other material such as unconsolidated soil or gravel. Niagara Falls is a famous example of water flowing over a section of hard rock onto softer rock. Temporary falls may also be formed by large logs falling across a stream channel.
Waterfalls are important biologically speaking because they present barriers to the upstream migration of fish and other completely aquatic organisms. However, most waterfalls are easily bypassed by organisms such as aquatic insects and amphibians which, in their adult life stages, can fly, hop or crawl around them.
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