Wednesday, June 17, 2009
A trip down the Grand Canyon will force me to rethink how I use the term "beach." Here, they refer to sand bars along the river bank, deposited when the river is running higher and thicker with sediment. And then gradually eroded by currents and small waves until a sufficient flow comes along to rebuild them. Or at least that's how I understand them to work. Beaches are an important element in river travel, as they are almost always where folks pull over to lunch or to camp. Where the winds are right and there is enough sand, dunes can form, so some of these features get pretty high.
The beaches in the canyon are also a big problem. Their maintenance requires sand and big flows. Since Glen Canyon dam was built, sediment from the upper Colorado has been preoccupied filling up the upper end of Lake Powell. And with the exception of 1983, when spring floods overwhelmed the storage capacity of the reservoir and huge volumes were released, flows in the now-tamed river aren't enough to rebuild the beaches. So for the last decade, managers and scientists have experimented with designer floods, specifically intended to restore beaches. The flows aren't of the scale of larger historic floods and the lack of sediment is still a big problem, but apparently the effort is having some positive results. The last big release was in early March, 2008.