Dam removal brings species back to long-hidden habitats
Part 2 of our series exploring how dam removals can reconnect species to habitats that have long been inaccessible.
Dams have long been used to benefit people and communities as impounded rivers are used to power mills, harness hydropower, store water for drinking and agricultural irrigation, and create navigation channels. However, these societal benefits often come at a cost to natural resources. The central purpose of dams, to hold back water and regulate flow, is counterproductive to the natural flow regimes (movement of water) that support biological diversity and healthy ecosystems.
The Muskegon River in Big Prairie Township near Newaygo, County, MI. This section of the river is confined between the Croton and Hardy dams.
Water confined behind a dam can dramatically impact habitat up and downstream, reduce water quality, and block species from moving throughout a river. In free-flowing river systems without dams, sediment – or the small sand and gravel pieces in the substrate – move throughout the system – creating stream beds, dispersing nutrients and food, and depositing material on channel bars and in floodplains, creating habitat patches throughout a river. When a dam is present, the structure disrupts sediment transport downstream, causing it to accumulate above the dam while very little reaches areas below. Sediment builds up behind the dam covering rocky areas that are important for fish to lay their eggs. This loss of spawning areas and habitat structure can have a ripple effect on biodiversity through decreased reproductive success, changes in the aquatic community make-up, and the loss of species from disjunct sections of the river.
While sediment builds up behind a dam, the downstream areas experience disruptions in sediment transport, which is knows as sediment starvation. Preventing sediment from moving downstream can result in scoured or eroded stream beds with no sediment to replace what is washed away. Without fresh sediment supplies to replace the lost material, habitats are degraded, and species are forced to leave the impacted area to find new homes. Sediment particles also carry nutrients necessary to sustain the base of a river’s food chain, and if a river is sediment starved, this can quite literally mean the aquatic organisms are starved and forced to relocate as aquatic plants and macro-invertebrates that form the base of the food chain can no longer be supported.
In addition to these habitat impacts, dams can also cause changes to water quality through increased waters temperatures and decreased oxygen in the upstream impoundments, making sections of rivers inhospitable to species that used to be common in those areas. Dams also impede aquatic organism passage, blocking important migration routes that provide access to spawning, feeding, and thermal refuge habitats required for species to thrive. Disruptions to aquatic organism passage is one of the most common causes of biodiversity declines in rivers.
With all the detrimental impacts of dams in mind, it may seem like a herculean effort to restore habitat and natural flow regimes to a river, but rivers are inherently resilient. When dams are removed, rivers often carve out their own path and return to the rhythmic flows that support abundant wildlife. The impounded sediment flushes downstream and stabilizes into diverse habitat patches again while the restored seasonal water flows and stabile water temperatures cue species to migrate, reproduce, and find food. Species are often resilient too and much like the oft quoted line from the 1989 movie Field of Dreams, ‘if you build it, he will come’, a similar notion can be said of species and dam removal – if you unbuild it, they will come.
Ballville Dam on the Sandusky River near Fremont, Ohio. The structure, removed in 2018, was one of the largest dams in the Great Lakes region measuring 407 feet wide and 34 feet tall. Photo Credit: MWH Constructors (www.mwhconstructors.com)
There are over 7,000 dams on rivers, creeks, and streams that flow into the Great Lakes, degrading habitat and blocking access for aquatic organism passage. Removing these impediments reconnects species to habitats that have long been inaccessible and restores populations long-lost, and sometimes forgotten, from our rivers. A notable example of this is the Ballville Dam, a 407-foot-wide, 34-foot-tall structure that spanned the Sandusky River near Fremont, Ohio. The Ballville Dam was in place for 105 years, blocking access to 22 miles of critical habitat for lake sturgeon, walleye, and white bass. The dam was built for hydroelectric power generation in 1913 and the structure created an impounded reservoir that extended 2.1 miles upstream and covered an area of 89 acres. The hydropower facility only operated for 46 years and the structure slowly deteriorated over several decades. After years of planning and coordination, the Ballville Dam was removed in August 2018, reconnecting 22 miles of the Sandusky River to Lake Erie and restoring 21 acres of wetlands along the river. In May 2019, a few short months after dam removal, a large lake sturgeon was hooked by a recreational angler four miles upstream of the former dam site. Lake sturgeon historically spawned in the Sandusky River, one of 19 known spawning locations for the Lake Erie population, but after the massive Ballville dam was constructed, the Sandusky River lake sturgeon population disappeared. Now, after more than a hundred years, the dam removal and sighting of a travelling sturgeon has renewed hope for the species’ return. Of the 19 original lake sturgeon spawning stocks in Lake Erie, only four remained as the others were lost in the wave of dam construction and habitat degradation that swept the region in the early 20th century. The removal of dams like the Ballville is the first step in reconnecting long-separated habitats and restoring species that once called these rivers home.
The example of the Sandusky River lake sturgeon is exceptional and exciting, and while there are great examples of species returning immediately after a dam is removed, there are instances where barrier removal is just the first of many steps. While rivers will carve their natural path again when the restraints of a dam are shed, and species often return to the newly opened river systems, there are cases when effort and investment are needed to give species and habitats a helping hand.
Long-lived and slow maturing species, like the lake sturgeon that can live 80 – 100 years, could take decades to return and reestablish on their own. While a singular fish coming back to a newly reconnected river provides optimism for the future of restoration, a healthy population needs hundreds of individuals of varying ages. When a species only exists at 1% of its historic population, like lake sturgeon, we often need to take actions to assist these species down the long road to recovery. Reintroducing a species or doing population augmentation (adding more to increase their numbers) in a reconnected river can help give the species a boost and decrease the recovery time.
A small lake sturgeon awaiting release into the Maumee River as part of the reintroduction efforts to reestablish this species. Credit: Jessica Collier/USFWS
We are reaching a point in time when many dams built in the late 18th and early 19th century are reaching the end of their life expectancy and opportunities to reconnect and restore our rivers are arriving. Through dam removal, we can open the door for species to come back and thrive in the habitats that lay hidden beneath and behind these dams. The Muskegon River and its tributaries are a prime example – with over a hundred dams throughout the watershed, opportunities await to reconnect and restore the habitat and biodiversity in this beloved watershed.
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 3 of If You Unbuild It, They Will Come: Reconnecting the Muskegon River watershed. Part 1 of this series was featured in the January newsletter and can be found online at: https://mrwa.org/if-you-unbuild-it-they-will-come/
Use this web address to learn more about Lake Sturgeon restoration in Lake Erie, including the sturgeon caught upstream from the former Ballville Dam site: https://www.greatlakesnow.org/2019/10/lake-erie-sturgeon-release-population/
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.