If You Unbuild It, They Will Come

Jessica Collier
Fish and Wildlife Biologist
United States Fish and Wildlife Service

The double-edge sword of dams: revolutionary and restrictive

Part 1 of our series exploring how dam removals can reconnect species to habitats that have long been inaccessible.

Dams have a long and storied history in the United States. The Anasazi constructed small dams to capture water for their crops over 800 years ago on the Mesa Verde in Colorado. As Europeans colonized the east coast in the early 1600s, one of the first structures created in each settlement was a dam to store drinking water and power gears to mill grains and saw timber. Later, the impoundments created behind dams were crucial to pre-refrigerator food storage: the slack water was frozen and sawed out in large chunks, a more reliable source of ice than free-flowing rivers. Dams have been an integral part of American development and an essential part of commerce in the 19th century. The 1920’s brought a boom in hydropower with dams constructed across the country to produce cheap electricity, but it was the 1930’s that ushered in the golden age of dams.  The CCC (Civilian Conservation Corps) and the WPA (Works Progress Administration) constructed thousands of dams between 1933 – 1942 with even more to follow between 1950 -1980.

The Reedsburg Dam on the upper Muskegon River in eastern Missaukee County near Houghton Lake, MI. Photo Credit: Wikimedia Commons

Dams revolutionized our society. They were crucial to power mills for food, lumber, and textiles, and as our society advanced, so did the utilization of dams and their use expanded to include orchard and cropland irrigation, drinking water and irrigation storage, facilitating shipping and navigation, electricity production, reducing flood capacity to safeguard infrastructure, and creating areas for recreation. Today, there are over 2 million dams in the U.S. and while most are small and privately owned, approximately 75,000 are large structures more than 6 feet tall and about 6,000 of those are over 50 feet tall. If you accounted for all the water backed up in reservoirs by dams, it would cover about 3% of the land surface in the United States, roughly equivalent to the state of Arizona. In Michigan, there are more than 2,500 dams. The Muskegon River watershed, one of the largest watersheds in the state, has 32 registered dams and many unaccounted structures that are smaller privately-owned.

Trowbridge Dam on the Kalamazoo River near Otsego, MI. Photo Credit: Jerry Campbell via MLive.com

The damming of American rivers has been a double-edged sword – for as many benefits as society has gained by utilizing dams, the rivers have lost even more due to the consequences of building dams. The lovely, winding, intricate nature of meandering rivers becomes truncated, constrained, and disrupted when a dam is built. Dams and their impoundments fragment rivers and watersheds, breaking them up into parts that are no longer inter-connected and functioning as a whole system. Without the crucial connectedness of rivers, fisheries become depleted as migration and passage is blocked, water quality degrades, habitat is lost, paddling and other recreational activities are inhibited, and the altered river flow changes the very dynamics and rhythms these ecosystems rely upon. When we look at a dam, there is a transfixing and serene nature to the cascading waterfall created but hidden from view are the habitat alterations that result from these structures. Dams can hold back large wood and trap gravel and sand behind them, a term often referred to as sediment starvation because it causes the loss of sandbars and gravel deposits downstream leading to the loss of habitat for aquatic organisms. Sand bars and barrier beaches play a pivotal role to stop erosion and mitigate floods and the loss of these natural structures through sediment starvation can lead to severe impacts on downstream infrastructure. The water restrained behind dams is often slow moving and sluggish becoming a reservoir for toxic algal blooms, increased water temperatures, and low levels of dissolved oxygen. Hydropower plants alter river flow as they withhold water at times and then release significant amounts at other times, disrupting the natural and seasonal flows that often signal reproduction and growth cycles for many aquatic species. The physical structure itself poses a significant barrier to aquatic organisms as even small dams only a few feet tall can completely block access throughout the river.

Many species rely upon interconnected river systems in Michigan including lake sturgeon, brook trout, walleye, lake whitefish, salmon, white sucker, American eel, herring, shad, freshwater mussels, aquatic insects like mayflies and caddisflies, and even semi-aquatic species like turtles, mink, otters, muskrats and more. Freshwater mussels have a unique life history that requires a host, often fish and sometimes amphibians, to carry their larval stage for a short period of time.  The mussel larvae, called glochidia, develop on the host without harm and then drop off to continue their life cycle. As fish passage is blocked in river systems, so is the population expansion of freshwater mussels.  When dams fragment populations that need access throughout the river system to complete different life stages or seek refuge from changing conditions, there is a loss of genetic diversity and resiliency in these populations which can lead to small-scale, local extinctions. For turtles, and other reptiles, amphibians, and mammals, when access throughout a river is blocked, the species have to venture up the river banks and cross roads or highways to get to the next section of river – a perilous journey with unnecessary exposure to road vehicle traffic and predators. In the case of lake sturgeon, salmon, and other species that imprint, or remember, the rivers where they were born, dams and other barriers mean they cannot return to their natal streams to reproduce.

Croton Dam on the Muskegon River west of Newayo. Photo credit: Larry Pieniazek, wikimedia commons.

For all these species, and many more, the loss of habitat, migration routes, and access to refuges in rivers due to dams fragmenting and blocking movement means the loss of biodiversity, resilient populations, and well-functioning ecosystems. It is a difficult process to balance ecological integrity and human needs to ensure that species and habitats not only exist but flourish while also helping society prosper alongside. For many of the dams across Michigan and the U.S. their utility has long expired as new technologies advanced, but these structures often persist, silently opposing the natural stream order and connectedness of rivers and their species. But in light of the harm dams have caused, rivers themselves are inherently resilient and once the constraints are released, they settle back to the natural rhythm they have created for millennia. When a dam is removed, rivers can reform their old channels, flow patterns, and rebuild habitat complexity.  In doing so, species return to rivers and habitats that were long inaccessible when dams blocked the way.

Join us in this multi-part series as we explore how dam removals can reconnect species to habitats that have long been inaccessible. Stay tuned for Part 2 of If You Unbuild It, They Will Come: Dam removal brings species back to long-hidden habitats and Part 3 of If You Unbuild It, They Will Come: Reconnecting the Muskegon River watershed.


Jessica Collier is a Fish and Wildlife Biologist with the U.S. Fish & Wildlife Service. She works out of the Green Bay, WI field office with the branch of Partnerships and Habitat to assist with restoration and conservation across the Lake Michigan basin. Jessica is passionate about Lake Sturgeon and freshwater mussel conservation and she is an avid gardener, runner, and stained glass artist.