It is no coincidence that many of the rapids in the Grand Canyon are named after tributary streams. Soap Creek, House Rock, Hermit, Crystal. Virtually all of the Canyon's 160 or so rapids are products of flash floods and debris flows - even very small drainages can deliver more than enough boulders to partially block the Colorado. The result is a long series of riffles and intervening pools where the debris has dammed the river. Although many of the rapids are hairy, few are very long.
None of the rapids in the Canyon result from nick points (where a river flows over a particularly resistant geologic unit) and even deep in the metamorphic gorges the river runs smoothly except where blocked by debris. I guess the river has had more than enough time and power to smooth out those bumps and now just puts its effort into redistributing the stuff the tributaries pump out. This has become much more difficult with the Glen Canyon Dam upstream preventing large floods from periodically flushing out the debris, so over time the rapids are expected to get bigger. Someday a big flash flood will create a rapid so nasty it throws a real monkey wrench into the rafting scene. This almost happened at Crystal in 1966.
The House Rock Valley drains a large area northwest of Marble Canyon and forms a nice rapid at its mouth - pushing the Colorado sharply against the left bank. As always, click on the title of the post to see the bird's eye view.
Grand Canyon 2009: June 17th
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