How Glen Canyon Dam Cleared the Colorado River — and What That Means for Fish

By Nicholas Voichick, Theodore Kennedy, David Topping, Ronald Griffiths and Kyrie Fry. USGS Fact Sheet 2016–3053, October 2016.

Where the Little Colorado (right, full of sediment) meets the clearer Colorado (top). USGS photo, 2009 overflight of the river corridor.

The closure of Glen Canyon Dam in 1963 transformed the Colorado River through Glen, Marble and Grand Canyons:

• Clarity: the river is now much clearer.
• Temperature: less variable through the year, and much colder in summer.
• Flow: less variable from year to year, but with bigger daily swings.

Together these changed the fish community and its food. Before the dam, heavy sediment kept the water murky nearly all the time — conditions that suited native fish. Monitoring turbidity now lets scientists estimate what the pre-dam river was like and compare.

The humpback chub lives only in the Colorado River Basin and is federally endangered; the dam-regulated river does not suit it. USGS photo by David Ward.

Measuring clarity

Water clarity is how far light reaches, and how far things can be seen, underwater. It is usually tracked as turbidity: how much light the water scatters and absorbs (0 means clear). Readings from different instruments do not always agree, but turbidity is easy to measure continuously.

In the Colorado, clarity depends mostly on suspended sediment, and since the dam, on sediment washed in by floods on tributaries. USGS instruments along the river record turbidity, temperature, dissolved oxygen and specific conductance every 15 minutes (data).

The river, its tributaries and USGS monitoring stations. Map: USGS.

Grain size matters: sediment of fine silt and clay clouds the water far more than the same amount of coarser sand. So turbidity, which measures how much light gets through, tells biologists more than sediment concentration does.

Equal sediment, different grain sizes: silt and clay (left) make water far more turbid than sand (right). USGS photo by Jason Fobair.

Why clarity matters to fish

Light grows algae; algae feed invertebrates such as insects; many of those feed fish. And many fish hunt by sight, so clarity shapes competition and predation. In Grand Canyon the native humpback chub has declined partly because the river is now clearer and cooler — and nonnative rainbow trout and brown trout, better suited to it, compete with and eat them.

Before and after the dam

Before the dam, turbidity was not measured, but suspended sediment was, every day. Because pre-dam sediment was mostly silt and clay, scientists could estimate pre-dam turbidity from it.

• The dam traps sediment: it blocks all sediment from upstream reaching Glen Canyon and has cut suspended sediment in Marble Canyon by about 95 percent.
• Glen Canyon: the 15-mile stretch from the dam to Lees Ferry is now clear almost all year.
• Two tributaries now rule: the Paria and Little Colorado supply nearly all the turbidity below, so clarity drops in steps below each.
• How much clearer: pre-dam turbidity was on average 2,000 times today's at Lees Ferry, and 20 times today's at the 87-mile station.
• Seasons: the pre-dam river was clearest in winter at low flow and muddiest in spring runoff and the summer–fall thunderstorm season. Today Marble and Grand Canyons are usually clear (under 10 FNU) in spring and late fall, and turbid during winter storms and the thunderstorm season, when turbidity can approach pre-dam levels.

Pre-dam (red) and dam-regulated (black) daily turbidity at Lees Ferry (top) and 87 Mile (bottom). Each step on the y-axis is ten times the one below. Graph: USGS.

Before the dam, Lees Ferry and 87 Mile were about equally turbid: most sediment came from upstream of the future dam site, and the Paria and Little Colorado mattered little. Today those two tributaries supply the vast majority of the sediment below the dam.

Clear water, hungry trout

Beyond about 40 FNU, extra turbidity barely changes how well trout can see. In the pre-dam river at 87 Mile turbidity was always above that, so sight-feeding fish would have struggled; native fish use other senses. Today it is often below that level, and trout see their prey easily.

Five turbidity levels in formazin nephelometric units (FNU), measured with the same kind of instrument as the Grand Canyon study. USGS photo by Taylor Roe.

Clarity, temperature and biology

• A new food web: clearer water lets algae flourish, and algae now fuel more than 50 percent of the production of invertebrates and fish through Glen, Marble and Grand Canyons.
• Cold water: fish grow and breed best in particular temperature ranges. The cold river slows humpback chub growth, and small chub die more often and are easier prey for trout.
• The hot spot: chub are most plentiful near the Little Colorado confluence, their main spawning area. Trout hunt them there, and clarity shapes those encounters.

Sources

Based on Water Clarity of the Colorado River—Implications for Food Webs and Fish Communities, by Nicholas Voichick, Theodore Kennedy, David Topping, Ronald Griffiths and Kyrie Fry, USGS Fact Sheet 2016–3053, U.S. Geological Survey; a work of the United States government in the public domain. The map, graphs and USGS photographs are taken from the fact sheet's PDF; its underwater photograph of the Paria confluence (© 2013 Dave Herasimtschuk and Freshwaters Illustrated, used with permission) is not reproduced.