![]() ![]() The Paria helps change that, so much so in fact, that the high flow restoration events of the Grand Canyon depend almost entirely upon the amount of sediment that the Paria has moved that year. Before meeting the Paria, the Colorado River is emerald green and bracingly cold, which is great for nonnative species like rainbow and brown trout, but very hard on the native fish and amphibians of the region, not to mention the beaches and backwaters. “Paria Riffle,” where the Paria meets the Colorado near Lee Ferry, is the first rapid that Grand Canyon boaters encounter after they start their nearly 300-mile trip. While all tributaries play a critical role in the building and maintenance of a river’s ecological functions, the Paria plays a uniquely important role in the character of the Colorado, especially since Glen Canyon Dam. Although it doesn’t stand alone as a gifter of grit and sediment to the main channel, the Paria certainly reserves a seat near the top of tributaries credited with giving the Colorado River such a silty-legacy that early explorers and writers described it as “too thick to drink and too thin to plow”. Unlike its sparkling counterparts, the Paria is record-setting for its unique sediment transportation abilities and the quantities it delivers, and it performs this critical function for none other than the Colorado River. Mesozoic shale and sandstone shed like powdered sugar from the canyon walls, cascading to the canyon floor on the height of a breeze or on flood waters through the canyon. ![]() The Paria is celebrated as both a river and slot canyon. For the weight of its impact, the Paria is a relatively small stream that flows 95 miles south from its headwaters in the Grand Staircase National Monument of southern Utah to its terminus with the Colorado River, just downstream from Lee Ferry in the upper reaches of the Grand Canyon. The Paria is an expert in the delivery of sediment, which is critical for the health of all rivers, and for the habitat they provide. Velocity reversal at high flow stage maintains these features.While some rivers are celebrated for cobbled floors that flash through cool, clear water, the Paria River is best known for its turbid, muddy waters. Pools and riffles are genetically-linked in alluvial rivers. Pool-infilling subsequently occurs, as pools act as areas of deep, low flow velocity and near-standing water conditions. ![]() Subcritical flow occurs during divergent flow at low flow stage. Indeed, they may be removed and replaced during extreme floods, as they reform at lower flow stages (velocity reversal hypothesis).Īt high flow stage, when flow converges through pools, decreased roughness and greater bed shear stresses induce scour and flushing of sediment stored on the bed. Although very stable, with 5-10 % of the stream area in supercritical flow and some small hydraulic jumps over obstructions, riffles may be mobile at and above bankfull stage. Riffles are commonly dissected during the falling stage of floods, when the water surface is shallow and steep, and the stepped long profile is maintained. At high flow stage the water surface is smooth, as bed irregularities are smoothed out. In subsequent high discharges, deposition occurs as the resistance of these features induces a reduction in velocity over the riffle surface. Concentration of coarser fractions at high discharges (bankfull and above) produces incipient riffles, while lower flows (up to bankfull) may be sufficiently competent to amplify and maintain the initial undulations once they have reached a critical height. Riffles are zones of temporary sediment accumulation which increase roughness during high flow stage, inducing deposition. They form at characteristic locations, typically along the concave bank of bends in sinuous alluvial channels. Pools tend to be narrower than riffles and act as sediment storage zones. Alluvial pools are alternating deep areas of channel along an undulating reach-scale longitudinal bed profile. Pools may span the channel, hosting tranquil or standing flow at low flow stage. These sediment storage zones tend to comprise tightly imbricated bed materials, suggesting the action of local sorting mechanisms. Alluvial riffles are alternating shallow step-like forms that span the channel bed. Clusters of gravel (up to boulder size) are organised into ribs, typically with a rippled water surface at low-stage. They occur at characteristic locations, typically between bends (the inflection point) in sinuous alluvial channels. Topographic highs along an undulating reach-scale longitudinal profile. ![]()
0 Comments
Leave a Reply. |