In the unseasonably warm March of 1936, the Northeast’s snowpack melted quickly and joined with a heavy rain to send water rushing down the Allegheny and Monongahela rivers toward Pittsburgh. The rivers rose 21 feet above flood stage in what became known the Great St. Patrick’s Day Flood.
In the city, the flood killed 45 and caused $250 million in damage (equivalent to $4 billion today). Throughout the Ohio River Valley, the totals were even higher. Across the nation, the disaster brought a big change to the way governments protect people from floods. It helped motivate Congress to pass the Flood Control Act of 1936, which made corralling rising rivers the job of the federal government and assigned the task to the U.S. Army Corps of Engineers.
Fast forward 75 years and millions of dollars in construction projects later, the Army Corps’ Pittsburgh District manages 16 dams and reservoirs throughout Pennsylvania, Ohio, West Virginia and New York. All told, the projects cost around $500 million to build and are estimated to have prevented as much as $10 billion in flood damage since the first dam was completed on West Virginia’s Tygart River in 1938.
The dams are still serving their original purpose: A 2004 flood would have risen nearly eight feet higher if it weren’t for the runoff-storing capacity of the reservoirs. But the Congressional authorization under which the dams are built and operated gives the Corps responsibility for more than just controlling floods. Depending on location, the reservoirs and downstream waters are also managed for water quality, water supply, conservation of fish and wildlife, and recreation.
The Corps’ dams in the Mahoning River watershed were built as much to help water quality as they were for curbing floods, said Rose Reilly, a biologist with the Pittsburgh District.
“Steel manufacturing facilities and support industries would reuse the river water three or four times before it reached the mouth of the Mahoning,” Reilly said. “Before we built our headwater reservoirs, river water temperatures during the summer could reach as high as 130 degrees Fahrenheit.”
The Corps built dams in the Mahoning River’s upper reaches and tributaries to keep downstream water temperatures below 100 degrees. Water stored in the reservoirs and released during the summer helped bolster the season’s low flows and buffer the effects of industrial discharges.
Though water quality in the watershed has improved dramatically since the heyday of steel manufacturing and the passage of the Clean Water Act, the additional flow from the reservoirs continues to help streams meet cleanliness standards by diluting pollution.
Though some of the dams were built to address World War II-era problems, today the District is taking a 21st century approach to its water-quality mission. A modern monitoring system is an important part of that. At 12 of their 16 reservoirs, a buoy-supported sensor string is continuously measuring temperature at every three feet in the water column and relaying the data back to their offices. Dissolved oxygen, specific conductivity and pH are measured at three buoys.
Temperature data collected at multiple depths shows water managers a picture of reservoir stratification patterns. Monitoring is particularly important at the district’s selective withdrawal dams, which have intake gates for drawing reservoir water at multiple depths. As stratification progresses throughout the summer, metals, nutrients and other pollutants concentrate in the reservoir’s cooler bottom layer. Water managers can choose to release higher quality from the reservoir surface water or to blend releases from multiple gates so that highly anoxic and reduced waters with high concentrations of dissolved metals and nutrients are not being released downstream.
Selective withdrawal intakes also allow water managers to regulate downstream water temperatures to support fisheries or improve water quality. “In essence, the upper and lower intakes are used like cold and warm water spigots that allow blending to meet downstream temperature objectives,” Reilly said.
Though water managers follow prescribed schedules which dictate how much and when water passes through each dam, they can make subtle adjustments based on environmental and hydrologic conditions, using real-time data. “So water managers follow schedules but have some flexibility to modify operations to optimize water quality and aquatic life benefits. For example, the amount of flow released or timing of the release can be modified slightly to better support a fish spawn.”
The Pittsburgh District’s 16 dams will be doing that job for the foreseeable future, despite a nationwide trend that has seen such structures increasingly targeted for removal. More than half of the dam removals in U.S. history have occurred since 1999, according to data from American Rivers, a non-profit group that tracks and advocates for removals. Work on the world’s biggest-ever dam removal project started in 2011 when crews dismantled the Elwha Dam on the Elwha River, which flows from Olympic National Park in Washington out to Puget Sound. A few miles upstream, demolition of the Glines Canyon Dam is underway. It will be the tallest dam ever removed.
The science that describes the environmental degradation associated with dams is relatively new, Reilly said. It appears that the Corps is unlikely to turn to new dams to solve water resource problems; the Pittsburgh District’s most recent dam was finished in 1988. Meanwhile, the districts dams aren’t on the table for removal.
“Our projects have Congressional authorizations for flood control, water quality, navigation, water supply and water quality,” Reilly said. “If the dams were removed, these benefits would be lost and there would be a significant change in the quality of life for the folks who live in the watersheds.”