A few weeks ago, Dr. Mark Schmeeckle was sitting in the rain on the Colorado River, watching muddy water rush past him, hoping to help save the beaches of the Grand Canyon.

Schmeeckle, an Arizona State University scientist who specializes in the physics of sediments moving in rivers, was part of a team of researchers on the river that day. The flood that muddied the water was an experiment - a particularly timed rush of water meant to return sediment to the Canyon's sand-starved beaches. His job was to use his knowledge of fluvial geomorphology to know what was happening before his eyes.

 Since the closure of Glen Canyon Dam in 1963, the downstream areas of the Colorado have been denied the sediment needed to maintain sandy beaches on the banks. Scientists have noticed the decline of these natural sandbars, and the Grand Canyon Monitoring and Research Center sanctioned this project to help study the dynamics involved.

The Nov. 21 flood was designed to mimic a natural flood on the Colorado, one that would wash sand down from the mouth of the Paria River tributary downstream to the eroded stream banks. The researchers were looking for specific conditions - when the Paria had brought sediment down to the Colorado, but before it had been washed away. The flood was even scheduled on short notice because of the unexpected late fall rains.

During the flood, Schmeeckle was stationed at "30-Mile Beach," using a Doppler device to measure everything that was happening within the 45 feet of this small sandbar. Velocities of different parts of the streams, along with the amount and grain size of the sediment load were recorded. Also, a camera took pictures of the beach every hour.

What he saw was what he might have expected, based on the fine-grained sediment of the Paria River. The fine-grained silt was deposited early on, followed by the coarser sands. Natural canyon flooding would have deposited more of a mix of grain sizes, and the sediment laid down by this flood was different.

 Schmeeckle is investigating whether this different sandbar is a stable one. His job is "to assess how the beaches are going to change because they are muddier."

Fluctuating beaches are the result of a natural process in river dynamics. Upon widening after a narrow pass, and turning, the water velocity changes in different parts of the channel. If the velocity drops to zero on one side of the channel, water may deposit the suspended sediment being carried. Naturally, sediment erodes away, and is deposited further downstream, and the spring floods replenish the bank.

Sandbars along the Colorado are also the last safe haven for the native fish, the humpback chub. These fish used to thrive in the turbid (cloudy), warm water pools closed off from the main channel. Much research and restoration effort is being made for the now-endangered humpback chub, including this project.

Schmeeckle postulates that these different sandbars may actually be good for the fish - the sandbars that are muddier may erode more slowly, and more constantly over time. This would produce not only the warm backwater areas for the chub, but also constantly provide silt to the water.

The stability of these sandbars is in question, however. If water levels suddenly drop, after a flood, or a dam release, the water seeping back down out of the beach can push sand downward as well. Finer-grained sands hold more water, leaving the potential for increased erosion as the water leaks out of these beaches.

In his lab, Schmeeckle studies how sediments act in a controlled environment in order to predict what may happen naturally. In this case, he is testing out these conclusions on the natural canyon environment. This will help researchers know in the future just what it takes to build and maintain sandbars as they existed before the dam.

"What I want to do is eventually try to model what the turbulence and flow looks like, so we can model how the bar is going to erode over time," Schmeeckle said.

Overall, the experiment went as planned, however, the amount of silt and sand present may have been over-estimated. "The early story is that it looks like from 20 miles to 40 miles, where I was, right in the middle, this (experiment) did very well. Downstream from there, it didn't go as well. There were some beaches that gained, but some that were scoured."

From the preliminary data that he has reviewed, Schmeeckle is not encouraged. He concluded, "It's not a dead issue as to whether you can just use the Paria River to put sand back in the channel, but it doesn't look good."

However, the data collected will build on a body of knowledge ultimately aimed at solving the problem of sandbar beaches on the Colorado. What makes a good beach, and how do you keep it there? The study will continue to monitor the banks of the river for 18 months.

Sources

Dr. Mark Schmeeckle, ASU Assistant Professor of Geography

Resources

"The Grand (Canyon) Experiment" Science, Vol 306, Issue 5703, 1884-1886 , 10 December 2004