Saturday, May 23, 2015

Pools and Riffles

In gravel bed rivers, alternating sequences of pools and riffles develop as the cross-section of flow alternates between shallow and deep terrain.

Pool-riffle sequences commonly occurs at intervals of 5 to 7 stream widths.

Pools and riffles are prime habitat for trout. The wavy surface and turbulence of riffles hide trout from predators like raptors and humans.

photo: Bob MacNeal

Communities of insects grow to maturity in riffles providing the primary food source for trout. River current dislodges nymphs from rocks. Nymphs are the immature form of invertebrate insects. Drifting nymphs provide an intermittent stream of sustenance for trout who face upstream and maintain a stationary position relative to direction of flow to minimize the energy they must expend to feed.

Trout prefer the higher dissolved oxygen content found in riffles. Riffles tend to be 1-3 feet deep, while pools are often much deeper.
“If you have not touched the rocky wall of a canyon. If you have not heard a rushing river pound over cobblestones. If you have not seen a native trout rise in a crystalline pool beneath a shattering riffle, or a golden eagle spread its wings and cover you in shadow. If you have not seen the tree line recede to the top of a bare crested mountain. If you have not looked into a pair of wild eyes and seen your own reflection. Please, for the good of your soul, travel west.”
― Daniel J. Rice, This Side of a Wilderness