There’s a large body of scientific work going back several decades on the impacts of dams to rivers and streams. Scientists have looked at dams all over the United States to determine what the impacts are both downstream and upstream of the dam. Upstream effects seem pretty similar, but downstream impacts are variable depending on what the dam is operated for and how the dam is managed. For example if the dam is used for hydroelectric generation there is a regular release of water depending on when electric power is needed to feed the overall electric grid. Oftentimes that electric generation is needed during peak daytime hours and not needed so much at night. Sometimes the power is needed more in the summer to supply electricity for all the air conditioners running out there and needed less in the winter. If the dam is being operated for water supply, flood control or recreation, then there may be state to state contracts or other management controls on how much water has to be released on a daily basis to provide water for downstream users. The water releases in these circumstances can be more controlled by the dam managers. They can release it continuously or they can release it intermittently. It depends on what the management controls are. Do they need to provide water for downstream fish habitat? Do they need to supply a certain number of acre/feet of water downstream for irrigation? Do they need to provide freeboard in the reservoir to avoid destructive flooding? Controlled releases are kind of a new game in dam management. In the heyday of dam building in the early 20th century, there certainly was no thought of controlled releases or providing sufficient water downstream. It took the Endangered Species Act and a bunch of ticked off water users with an army of lawyers to put management controls in place.
It has been a long time since a new major dam was built in the United States. I suspect any new dam wouldn’t make it through a review under the National Environmental Policy Act (NEPA). But when most dams were built, there was no NEPA. There are a lot of dams. A statistic from the U.S. Geological Survey claims there are 75,000 dams over six feet high in the United States. The high dams like Glen Canyon Dam and Hoover Dam on the Colorado River have big reservoirs behind them. When the dams were first built, it took a while for those reservoirs of water to fill up and form the giant artificial lakes which exist today. What happens to those reservoirs is kind of interesting and is a big impact to both the upstream reservoir and the downstream river. The reservoir starts filling up with sediment. Essentially a river gets to the reservoir and drops its load of sand, silt and clay. After so many years, the reservoir fills up. There are a couple of problems with this. Oh for heaven’s sake, there are tons of problems with this.
Number one, the sediment accumulating in the reservoirs is not always very nice. Some of it is contaminated from past industrial practices which released water with contaminants upstream of the dam. Sometimes the sediment entering the reservoir is full of naturally eroded minerals from upstream areas with lots of metals and this results in an accumulation of toxic metals in the reservoir. And reservoirs can be full of agricultural runoff with loads of phosphate and nitrogen. The reservoir accumulates all these contaminants and it results in a great big mess of nasty sediment at the bottom. If you wanted to remove a dam as many environmental advocates propose, you might have to deal with the release of reservoir sediment as a potential source of toxins to fish and other wildlife downstream. Good grief.
Water released out of a dam and flowing downstream is free of sediment. It’s nice clear water. It comes right out of the gates or spillways on a dam like spring water; sometimes just as cold. The big problem with water rushing out of a dam clear and free of sediment and with big gaps between flows is that it is unnatural and what happens as a result is unnatural. A lot depends of course on how the dam is managed but typically the released water starts undercutting rivers banks immediately downstream and scouring the riverbed.
In some southern rivers the river downstream of a dam begins to look like a big muddy mess with highly eroded concave banks where trees actually are perched out on a shelf over the river. Once the river reaches the coastal areas and sea level, the velocity of the water flow slows down and it spreads out into the wetlands where all that new sediment drops out, essentially choking up the marshland.
If it’s a western river, water released from a dam starts scouring the downstream river bed and removing sediment in the form of sandbars. Some people describe it as armoring the river bed. Essentially the released water removes all the sand and silt downstream, leaving nothing but gravel, cobbles and bedrock. The only thing that can move the gravel and cobbles is a good flood and guess what? A dam controlled river rarely floods.
In the north and Midwest, rivers are so frequently dammed in series with numerous reservoirs joined by short stretches of river channel that practically no sediment makes it down the river. As a result, downstream beaches on shorelines become starved of new sand. Or in the overpopulated and overdeveloped east coast, the dam retention of sediment offsets the huge amount of eroded material coming in downstream from cities and agricultural lands. On some rivers in the east if the dams weren’t there, all the extra sediment now in reservoirs would be clogging up estuaries and creating excessive need for dredging in ports and navigable waterways.
Probably one of the more interesting questions about the sediment in the reservoirs behind dams is: What happens when the reservoir fills up with sediment? A lot of dams on rivers in the United States were built in the early 1900s. They have been accumulating sediment for a long time. You’d think some of them would be almost full by now, wouldn’t you? Well it just happens some of them are. Take for example the dams on the Susquehanna River. The Susquehanna River is a huge river. It is one of the largest rivers in the United States and flows about 450 miles through New York, Pennsylvania, and into Maryland’s Chesapeake Bay. There are three dams on the lower Susquehanna River. The last dam on the Susquehanna River is the Conowingo Dam about 10 miles upstream from the River’s entry into the Chesapeake Bay. It was commissioned in 1928. This dam is almost 90 years old. It has been collecting sediment for a long time.
Back in 2011, a very big storm passed over this part of the country. It was called Tropical Storm Lee. If you passed over the Chesapeake Bay Bridge anytime in the immediate weeks following that storm, you would have seen a big muddy mess with trees and garbage floating down the bay. A lot of the muddy mess was caused by sediments out of the Conowingo Dam. The Geological Survey did a great study after the storm. They have had a stream gauge at the Conowingo Dam for almost 50 years. The gauge measures water volume flowing through the Susquehanna at the dam. In the last 30 years measurements for nitrogen, phosphorous and suspended sediment also have been taken. The Geological Survey said the discharge at their stream gauge after Tropical Storm Lee was the second largest maximum daily flow in the history of monitoring at the station. Flow is measured in cubic feet per second. So the stream flow on September 9, 2011 was 709,000 cubic feet per second. The only flow higher was during Hurricane Agnes in 1972. Hurricane Agnes is kind of legendary in the Mid-Atlantic States. If you visit Maryland, Virginia and Pennsylvania you will see commemorative signs everywhere about the impacts of the floods during Hurricane Agnes. The National Oceanic and Atmospheric Administration reports 122 people were killed during the Hurricane.
The Geological Surveys report says the reservoirs behind the three dams on the lower Susquehanna are almost full of sediment; they give a calculation of 80 percent. They further say that flood events are creating sediment scour within the reservoir and the sediment now flows downstream when floodwaters are released. This is what happened after Tropical Storm Lee. They also have calculated what flow levels will scour the sediment and send it down the river and into the Chesapeake Bay – 390,000 cubic feet per second. The Geological Survey calls this the “scour threshold”. But they also note as the reservoir continues to fill, the normal surface slope in the reservoir will increase resulting in increased velocity of the water on a day to day basis. Eventually the “scour threshold” will be at a much lower discharge rate than 390,000 cubic feet per second. So people who live along the Chesapeake Bay will be in for more muddy messes flowing through their lovely estuary. Also coming with all those suspended solids will be all the other contaminants that the dam has been holding back. Of greatest concern will be the phosphorous and nitrogen. The Geological Survey pointed to an earlier report from the mid-1990’s which calculated reservoirs were trapping 2 percent of the nitrogen and 40 percent of the phosphorous the Susquehanna River would have otherwise been contributing to the Chesapeake Bay. Bad news for the denizens of the Bay, like Rockfish and blue crab as algal blooms will increase, consume oxygen and result in zones where fish and crustacean cannot survive. Here’s a link to the Geological Survey’s great report: http://pubs.usgs.gov/sir/2012/5185/pdf/sir2012-5185-508.pdf