Study: Grand Canyon Is Losing Bare Sand
Sand is an important resource in the Grand Canyon. It creates campsites for river runners, protects archeological sites, and provides wildlife habitat. But stretches of bare sand have vanished since the construction of Glen Canyon Dam. A new study published last week looks at how the Grand Canyon has changed because of the dam, and what it will look like twenty years from now. KNAU’s Melissa Sevigny spoke with lead author Alan Kasprak, a geoscientist at Fort Lewis College in Colorado.
So why are worried about the bare sand, what’s happening to it?
The obvious one is really that when Glen Canyon Dam was completed in 1963 it formed a 700-foot high wall that creates Lake Powell upstream, and all of the sediment which would otherwise be transported through the Grand Canyon… is now sitting in Lake Powell. So Grand Canyon is really starved of sediment…. And the other reason that Glen Canyon Dam affects bare sediment is a more subtle story. That’s how flows have changed in the post dam period. In the pre-dam period, the river system is marked by really, really big spring and summer floods. … Following those floods the river would essentially go dry… It’ a desert river. That’s what we would expect. But today the flows through Grand Canyon are steady year round, so you’ve lost those large floods but importantly for bare sand you’ve lost those really low flows as well throughout the summer and fall months.
Tell me briefly how you went about answering this question, about what was going to happen with the sand.
So we compiled a map of all the bare sand that existed from the foot of Glen Canyon Dam downstream about 115 miles, to Phantom Ranch… and so what we did was we combined that with some flow modeling, where we could look at, for any amount of water that was coming down the river, how much of that bare sand would be underwater, verses how much would be exposed.
So what did you find out?
I think the first and most important finding is right now in the post-dam period there’s about half as much bare sand at the bottom of Grand Canyon, as there was during the pre-dam period. That’s due largely to the fact that flows in Glen Canyon Dam are elevated throughout the year compared to what they were in the pre-dam period, you’ve lost those really low flows that would expose a lot of sand. But it’s also due to the fact that, given those steady flows, there is essentially the perfect place for plants to make a home… Grand Canyon has become greener place and those plants tend to colonize bare sand.
What does that mean ecologically for the river?
It’s such an altered ecosystem that most of the riparian vegetation we see, they tend to be invasive plant species… There’s a lot of tamarisk… We actually as part of the study did a bit of vegetation modeling as well. What we found is over the next 20 years or so, the small amount of bare sand that remains, almost all of it is really good habitat for tamarisk. We expect to see more and more tamarisk come into the river corridor over the next couple of decades. So as part of this study we asked, one of the important questions we asked is what does the future hold for bare sand in the canyon? ... Unfortunately the management plan over the next 20 years is business as usual in terms of flows. It does not allow low flows from the dam. That’s going to have the effect of continuing to inundate a lot of the bare sand along the river corridor and also make great habitat for plants. Over the next two decades we predict that there’s going to be progressively less and less bare sand throughout at least the upper half of Grand Canyon and likely throughout the entire system.
So in other words, the current management plan isn’t really doing anything to reverse these changes?
That’s correct… There’s two ways we could think about increasing the extent of bare sand…. Neither way is necessarily easy, but some combination of a reintroduction of extreme low flows that were seen in the pre-dam period, along with widespread vegetation removal is the key to re-exposing a lot of the bare sediment throughout Grand Canyon.
Alan Kasprak, thank you so much for speaking with me.
Thank you, Melissa.