Relicensing Ferrand Dam: Sediment, Fish, and Cold Water

Ferrand Dam has stood across the Windemere River since 1926. The dam and the river are fictional, but the situation is common. Ferrand is small by the standards of its industry. Its powerhouse is rated at eleven megawatts, which is a fraction of a percent of what a project such as Hoover Dam is rated to produce, since the Bureau of Reclamation puts that plant's capacity at about 2,080 megawatts. Hoover's actual generation is lower today, because years of drought and low water in Lake Mead have cut it by about 30 percent. Ferrand's owner sells the power to a regional cooperative, and the reservoir behind the dam supports a marina, two campgrounds, and a town beach.

Dams like Ferrand operate under a federal license, and the license eventually expires. Renewal is not a formality. It reopens the operating conditions themselves, including how much water the dam must release, when it must release it, what it must do for migrating fish, and what it must monitor. Agencies, tribes, county governments, anglers, and the owner all file comments, and the record they build is the basis for the new license. For a dam approaching its hundredth year, renewal is the moment when a century of accumulated effects gets priced.

Three of those effects dominate the Ferrand record. The first is temperature. A reservoir separates into layers in summer. Sun warms the surface, the warm layer floats, and the deep water stays cold and still. Ferrand releases through a low-level outlet, so the water leaving the dam in August comes from the bottom of the reservoir. It arrives downstream far colder than the river would be without the dam. Cold water suits trout and suits the anglers who fish for them, and it suits native fish adapted to a warmer summer river much less well.

The second effect is oxygen. Water can dissolve only so much oxygen, and cold water can hold more of it than warm water can. That capacity is a ceiling on what the water can hold rather than a description of what it does hold. Deep reservoir water sits under the warm layer for months with no contact with the air, while bacteria on the bottom consume oxygen as they break down settled organic material. By late summer the deep layer can be well below the amount actually available at the surface. Most fish need dissolved oxygen above 5 mg/L, a benchmark agencies apply throughout the country. Water released from a low-level outlet in August can arrive under that benchmark even though it is the coldest water in the system.

The third effect is sediment. A river carries sand and gravel, and a reservoir stops that movement almost completely. The material settles into a growing delta where the river enters the reservoir, which slowly takes away storage the dam was built to provide. Below the dam the river runs clear and hungry. It picks up what it can from the bed, coarsens the channel, and receives nothing from upstream to replace the gravel that spawning fish require. A century of that arithmetic has left the reach below Ferrand paved with cobble.

The dam also changes when the river's water arrives. Snowmelt that would have run past in May is held and released through the late summer, which steadies flow for the powerhouse and for the marina. Storage of that kind is why dams are built. It comes with a loss that a free-flowing reach does not have, since a wide reservoir surface evaporates water all summer that the river would otherwise have carried downstream.

The conditions under discussion in the renewal follow from these three effects. A multi-level intake would let operators draw from a warmer layer when the river below needs warmth. Turbine venting would pull air into the water as it passes the runner and raise the oxygen of the release. A minimum flow requirement would set a floor under how low the river can drop when power is worth the most. Periodic sediment sluicing would move some gravel past the dam. Each condition costs the owner generation, capital, or both, and the owner has told the record that some combinations would make the project uneconomic to operate. The agencies have asked the owner to show that arithmetic rather than assert it.

The table gives four months of monitoring from two stations, one directly below the dam and one twelve miles downstream at Saltash Bridge.

1. According to the passage, what does renewal of the license reopen? Name three specific things.
2. Where in the reservoir does Ferrand's outlet draw its water, and what two properties does that water have by late summer?
3. Using the table, name the month in which oxygen below the dam fell under the benchmark the passage gives for most fish, and give both the measured value and the benchmark.
4. The passage states that cold water can hold more oxygen than warm water. The table shows the colder water below the dam carrying less oxygen than the warmer water at Saltash Bridge in every month. Explain why both can be true.
5. The passage says the river below the dam runs "clear and hungry." Explain what that phrase means in terms of sediment, and name two consequences the passage gives.
6. Explain how the dam changes the timing of the river's water, and name the loss the passage says a reservoir adds that a free-flowing reach does not have.
7. Choose one condition under discussion in the renewal. Explain what problem it addresses and what it costs the owner, using only what the passage states.