Few human actions have more significant impacts on a river system than the presence of a dam. As a result, dams occupy a central role in the debate about protecting and restoring our river resources. Many of the major environmental campaigns in the United States, and around the world, have revolved around efforts to fight construction of large dams.
A less known page in the history of rivers is the large number of dams that have been removed. Relatively little attention has been paid to the hundreds of smaller dams that have been torn down and the thousands of miles of free flowing rivers that have been restored. For decades dam removal has been an accepted approach for dam owners to deals with unsafe, unwanted, or obsolete dams. The decision to remove a dam is not as radical an idea as some today may suggest; dams are removed all the time, by a variety of entities, for a variety of reasons. Just as for any building or other human construction, dams have finite lifetimes and are often removed when they become obsolete or dangerous.
Although dams can provide important societal benefits, dams also cause negative impacts to rivers, wildlife and sometimes local communities. Some dams no longer provide any benefits, while continuing to harm the river. Others have significant negative impacts that outweigh the dam’s benefits. Still others simply are so old and/or unsafe that they cost too much money to maintain. In these situations, dam removal has been demonstrated to ve a reasonable option to eliminate negative impacts and safety concerns. Excerpts from Dam Removal Success Stories, American Rivers, 1999
How do dams impact rivers?
The dams themselves do not pollute the rivers, but the resulting impoundment they create behind them have significantly negative effects. Because water slows down behind the dams, suspended solids in the water column drop out and settle, forming huge sediment build-up. The increasing depth of sediment decreases the depth of the water and covers the diverse habitat the bottom of the river provides to fish and other aquatic life. The slowing of the water also allows nutrients to build-up behind the dams. The increased concentration of nutrients feeds algae, allowing it to grow at an accelerated rate. As algae photosynthesizes during the day it produces large amounts of dissolved oxygen, sometimes to a point of super-saturation. But, in the evening the algae respires and uses oxygen, so much oxygen in some places, that the level of dissolved oxygen drops well below the state standards of 5 mg/L. The swings in oxygen levels put great stresses on the aquatic life that lives in our rivers. As a result we tend to see pollution tolerant fish species, like carp and green sunfish, dominate communities behind dams.
Dams also pose significant physical barriers in our streams and rivers, blocking passage of fish and freshwater mussels. Most of the native fish found in the Midwest are not strong swimmers and are not able to make it past even some of the smaller low-head dams on our river systems. Many fish species like small-mouth bass and golden redhorse suckers need to move into smaller streams to spawn and their young need access to the varied habitat provide by these smaller streams.
Dams on the DuPage River block the passage of many species of fish and mussels, which require the movement of fish to carry their young upstream. Even though the DuPage has greatly improved over the last twenty years we are not seeing all of the species that once swam here because we have no place to recruit them from because the dams block their passage. From the first dam on the DuPage River in Channahon, less than one mile from the confluence with the DesPlaines River, we lose four species of fish, 10% of species found in the watershed. Several others drop out as you move upstream. For every dam that is removed or modified to facilitate fish passage we open up the main stem to many tributary streams and increase available habitat to new species of fish.
As we think about the issues of dam removal we need to remember to look at the big picture, the health of the whole stream ecosystem and the economic and social benefits or detriments to all of the communities in the watershed.
Dams on the East & West Branches of the DuPage River
The East and West Branch DuPage River contain five relatively major hydraulic structures of interest. Three are small head dams with considerable impoundments McDowell Grove Dam, Warrenville Grove Dam, on the West Branch and Churchill Woods on the East Branch. The other two structures, the Fawell Dam on the West Branch in Naperville and the Prentiss Dam in Woodridge on the East Branch are flood control structures and are not under review at this time.
Impacts of the dams on water quality and stream ecology
The impoundment areas for all three dams under review in this watershed plan are listed for dissolved oxygen and/or phosphorous impairments. The nature of the DO problems at all three sites is similar in nature if not magnitude. As water slows down behind the dam the rate of sediment deposition increases. This sediment not only covers aquatic habitat, microbes within it exert a strong oxygen demand during the breakdown of decaying materials, lowering DO levels in the impoundment in question. These accumulated sediments also tend to be high in phosphorus and nitrogen. The DO is also affected by photosynthetic cycle of aquatic plants - free-floating algae, fixed algae and macrophytes. Algae production is fed by the excess nutrients in the accumulated sediments and tends to "bloom" in the peak of the summer in August when water levels are at their lowest. All of these processes put together creates a complex problem.
As the water passes over the weir and plunges into the downstream river section a large amount of aeration occurs but this does not affect the depleted upstream area, it is generally recognized that dams are net negative in regards to DO.
Modeling of the impacts of Churchill Woods dam, the largest of the three structures, by the DRSCW in 2006 showed that under low flow conditions (approximately the lowest flows to be expected in a ten year period) the dam was associated with the lowest DO area on the East Branch. The results of this model are shown in the figure below. Low flow conditions were selected for this study as they are often conducive to high temperature and low DO conditions. These conditions tend to peak in July and August.
*A DO profile for the East Branch of the DuPage River. Source DRSCW QUAL 2K model for East Branch 2006. The dams are marked as vertical grey lines. Churchill Woods shows a large fall in DO north of the dam while Prentiss Creek dam, a through flow dam, does not. The model shows that Green Triangles represent POTWs discharging into the basin, the triangles relative size is representative of the plants design average flows. Vertical black lines represents major roads
QUAL 2K modeling has not been carried out for the West Branch.
Empirical DO data collection in 2003 (for McDowell and Warrenville Dams) and in 2006 (for McDowell and Churchill Woods Dams) show DO falling to below the Illinois DO standard of 5 mg/l at all three sites.
Dam impoundments generally make for poor habitat for fish and macroinvertebrates. Sediment covers critical feeding and breeding habitat with a depth ranging from a few inches to several feet. This sediment, when re-suspended, clouds the water making it difficult for fish like small mouth bass to see their prey. It also invites infestation by invasive plant species like cattails, reed canary grass and giant reed. The graph shows a 2006 habitat study for the West Branch. The three lowest scores recorded by the survey were found in streams head waters and the impoundments of the McDowell Grove dam and Warrenville dam.
*QHEI scores for the West Branch of the DuPage River. Study produced by DRSCW, data collected in 2006. Note QHEI scores shown in the 2003 reports on the Warrenville and McDowell Grove dams are from a separate study
Dams of course, most famously, fragment the river ecology by limiting the movement of aquatic organisms (reference). Data collected by the Conservation Foundation during the 2003 study of the West Branch suggest that species diversity is lower upstream of the dams. This was confirmed by the Index of Biodiversity scores (an aggregate value for the diversity and quantity of aquatic species) generated during the 2006 Bioassessment study for the West Branch.
The Bioassessment for the East Branch was executed in 2007 and the results for both watersheds will be available in early 2008.