The construction of the Glen Canyon Dam disturbed the balance of many natural processes in the Grand Canyon stretch of the Colorado River. Science Moab spoke with geologist and fluvial geomorphologist Katie Chapman about the effect the dam has had on sandbars downstream. By understanding how the sand is (or is not) moving through the canyon, Chapman is working to try and restore these once plentiful sand deposits for the sake of recreationists as well as natural river ecosystems.
Science Moab: What has happened to sandbars in the Grand Canyon as a result of the Glen Canyon Dam?
Chapman: They did not fare well when the dam was initially put in. Our whole upper basin supply of sand was essentially cut off when the dam went in, and the transport abilities of the river were increased. Those two things working together flushed a lot of sand out of the Grand Canyon and the sandbars. The floods in 1983 sparked the idea for the current restoration scheme. Any sand that had built up in the canyon from tributaries left in the channel was distributed into these beautiful big sandbars, sparking the idea to maybe replicate those seasonal floods to use the sand left in the system to try to restore the sandbars.
Science Moab: Why should we care so much about sand in the Grand Canyon?
Chapman: They’re a durable, non-fragile surface that people can frolic around on and not damage any of the fragile pieces of the ecosystem around the river. They form critical habitats for a bunch of aquatic species. The backwaters in the return channels of the sandbars are critical to a bunch of native fish. The sand also serves as a vital resource for preserving fragile archaeological sites in the canyon. Some of them are along the channel margins, in places vulnerable to erosion, especially during monsoon season, because gullies erode away the areas surrounding these fragile archaeological sites. The wind blows sand from the sandbars onto further upslope areas and builds a sand dune around and over these fragile archaeological sites and essentially protects them from the elements.
Science Moab: Why was it important to distinguish between sand from the upper basin and sand from the Paria River?
Chapman: We’re trying to evaluate how well we’re actually using sand from the Paria River to rebuild the sandbars in Marble Canyon. In the pre-dam days, sand from the whole upper basin contributed to building these sandbars. Now, with the Paria River being the only reliable source of incoming sand, we wanted to see how well we’re actually using that sand specifically to rebuild sandbars. By measuring the composition of deposits created by the High Flow Experiments, or HFEs, we got a solid metric of how well we’re actually retaining Paria sand. The sandbars in Marble Canyon are about 76% Paria sand. That’s great news, considering that the pre-dam supply ratio of Paria-to-upper-basin-sand was only 6% Paria sand.
Science Moab: How did you use sediment fingerprinting to determine those percentages?
Chapman: Sediment fingerprinting is this technique where you find physical characteristics of the sand from two sand sources. In this case, the Paria River brings in over 90% of the sand that comes into Marble Canyon in post-dam days.
Ideally, I wanted to represent the entire upper basin of the Colorado River, but there’s no way that I could take samples of sand throughout the whole thing and have it be an accurate representation of what used to come through Glen Canyon. There are a whole bunch of pre-dam flood deposits below Glen Canyon Dam but above the Paria River confluence, so we sample those as a proxy for upper basin sand. We sift them down into different grain size fractions and measure their bulk elemental composition with an XRF.
XRFs are really cool devices that essentially target individual atoms within the sand and excite electrons within those atoms. Those electrons emit a little X-ray, which is specific to whatever element that atom is. You can look at the proportion of how much each of those different X-rays is emitted, which is how you measure the bulk elemental composition of the samples. Exposed rocks upstream have minerals in them that aren’t found in the rocks of the Paria watershed and the composition of that bedrock is reflected in the composition of the sand delivered by each watershed. And that’s where I got the total Paria contribution to those HFE deposits.
Science Moab: I’m sure there are a lot of different interests among scientists regarding High Flow Experiments. What sort of rules govern these HFEs?
Chapman: Everybody wants something a little different. We all want to recreate different aspects of the pre-dam annual floods and how the river was before, but there is a Long Term Experimental Monitoring Plan, which is essentially a set of rules of the conditions that could trigger an HFE, when those would be, how much sand was necessary to trigger it, how much water would be released for how long, etc. That is based on a flow routing model that basically describes what an ideal peak discharge would be for whatever given amount of sand has been delivered.
HFEs don’t change the overall volume of water released per year, so it was supposed to be impervious to other water conditions. But the drought conditions in the Colorado River Basin right now were part of what contributed to not having an HFE this year, even though we did have enough sand.
Science Moab is a nonprofit dedicated to engaging community members and visitors with the science happening in Southeast Utah and the Colorado Plateau. To learn more and listen to full interviews, visit www.sciencemoab.org. This interview has been edited for length and clarity.
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