By Faryn Steckiel
Part 1: Legacy Sediment and a New Approach to Stream Restoration
In the early 2000s, something compelling was found by a group of four college students from Franklin and Marshall College in Lancaster, Pennsylvania: nearly 20 feet of stratified sediment in the banks of the Little Conestoga River. Upon upstream investigation by the students’ professor, Dorothy Merritts, and another surveyor, Robert Walter, they found the culprit of this massive quantity of sediment in the remnants of an early American milldam.
Merritts, a geomorphologist, and Walter, a geochemist, share an upbringing similar to many other Pennsylvanians from rural areas. They spent much of their childhoods in nature, playing and fishing along streams and inadvertently becoming naturalists as they did so. Their reconvening in Lancaster as seasoned field technicians and scholars was largely serendipitous, but their first milldam observation has radically affected the way that we approach mid-Atlantic stream bank and floodplain restoration. In some ways, Merritts and Walter have altered our perception of stream morphology in some parts of the country altogether– but getting to this point wasn’t necessarily simple.
So how did all of that sediment end up along the Little Conestoga River, and why can similar conditions be found by so many other mid-Atlantic streams? For a long time, geomorphologists believed that a buildup of sediment was just a characteristic of natural stream evolution. We’ve long been aware that waterways are highly dynamic systems, and even disturbances that seem small can have drastic effects, especially downstream of wherever the disturbance took place. The disturbances that happen in our part of the continent are rarely tectonic. Instead, they’re usually caused by weather events or mammalian influence, such as beaver dams, agriculture, deforestation, grade changes, or the building of structures which alter the course of a waterway.
Walter’s background in terrain dating and mineralogy, coupled with having grown up in Lancaster, helped him come to the hypothesis that the meters-high sediment deposits they were seeing were the byproduct of pre-existing dams rather than explanations proposed by the popular theory of river evolution. As the two F&M professors conducted further research, evidence of hundreds of dams were discovered in Lancaster County’s waterways, and they began to use a term "legacy sediment" to describe these anomalously large anthropogenic deposits.
When Europeans settled on the east coast of what is now the United States, many anthropogenic disturbances happened all at once. The deforestation that took place, as well as subsequent/simultaneous agricultural operations, were a couple of reasons why so much sediment was released into waterways in the first place. All the while, colonizers were constructing those hundreds of dams in the bottoms of valleys. Thousands of pounds of sediment were held in the reservoirs of these dams from the 18th to the 20th century. As dams were breached or deconstructed, all that sediment, combined with a sudden high velocity of water, created the incised streams we see today.
The ramifications of Walter and Merritts’ hypothesis and subsequent findings were far from insignificant. What they found beneath the sediment suggested that pre-colonization, the bottoms of these valleys did not resemble their modern counterparts at all. Instead of meandering, single-channel streams in fine-grained floodplains, the researchers had found that, without such drastic anthropogenic influence, “valley bottoms along eastern streams were characterized by laterally extensive, wetland-dominated systems of forested meadows with stable vegetated islands and multiple small channels during the Holocene” (Walter & Merritts 2008).
After collecting years of data, Merritts, Walter, and their extensive team put their hypothesis to the test, and in 2011, they broke ground at Big Spring Run in Lancaster County. In the end, 22,000 tons of legacy sediment were removed, exposing rich pre-colonial soil and reuniting the ground surface with the natural groundwater table. Native plants were installed, but many plants came up on their own as the dormant seed bank hiding beneath swaths of sediment could finally receive the nutrients it needed to thrive. Big Spring Run was transformed from an incised, single-channel stream into a sprawling, vegetated, multithreaded wetland.
When actions are taken to slow down the flow of waterways, it gives the habitat an opportunity to process sediments that are washed downstream and also decreases the chance of flooding events washing away trees and soil. The longer a vegetated habitat can retain that water, the better it can remove polluting nutrients like nitrogen and phosphorus, which legacy sediment is rife with. It also facilitates the mixing of ground and surface water, a phenomenon called hyporheic exchange, which helps to cool waters and better support aquatic invertebrates.
Follow-up monitoring of Big Spring Run confirmed that all of these benefits and more were taking place. They found that the native plants were taking up excess nitrate, that 85% less sediment was being washed downstream, and that phosphorus levels had dropped nearly 80%. The success of the project has prompted many others like it, particularly in southeastern Pennsylvania and neighboring Maryland, despite initial skepticism from many.
A natural resource development like this one can be disconcerting. One of the main reasons it was met with such severe criticism upon its release is because it potentially threatens decades of other scientists’ restoration efforts. The line of thinking was that, if Merritts and Walter were correct, it’s possible that very expensive and labor-intensive past projects that were planted atop legacy sediment could be washed away at any moment, essentially causing more harm than good. They also called into question much of what Leopold and Wolman’s well-known and largely accepted framework of channel morphology was built on.
For some, the most jarring aspect of this habitat restoration method is the way it changes the appearance of places they love. The places we find beauty, meaning, and solace in are not inherently well-engineered or the most ecologically accurate, and even if we know habitat restoration is the right thing to do, it can still hurt to watch things change. However, the benefits of proper floodplain restoration cannot be understated, and although the upfront costs are high, Walter and Merritts’ approach was found to be as much as 16 times more cost-effective than other strategies. After all, who couldn’t love a wetland sprawling with native vegetation and wildlife that protects people from flooding events and nutrient pollution?
Part 2: Pre-Columbian Seed Banks and Volunteer Plants
When habitats are restored to pre-colonization renditions of themselves, the land begins to tell the story of its past. When suffocating legacy sediment is removed, the dormant seed banks of historic floodplains can finally wake up. Sediment removal and regrading also provides the perfect opportunity to remove invasive species, giving archived native seeds a low-competition environment to get started in.
Volunteer plants are those that are not purposefully installed during a habitat restoration project. Their seeds are carried by the wind and water or spread by birds or mammals. Alternatively, they could have been lying dormant for varying lengths of time before their ideal conditions returned. Native volunteer plants that come from seed banks under legacy sediment are typically understood to be quite hardy and resistant to droughts and flooding. They also tend to form highly complex, multi-species cooperative plant communities both with other volunteers and with deliberately planted specimens.
It is vital that landscape technicians use cultivated native plants and cover crops to prevent a project from immediately eroding away or being overtaken by undesirable species. Although there are many native plant nurseries that sell high quality, regionally relevant species precisely for habitat restoration, as well as many well-trained people who meticulously plan the way they’re arranged in restoration projects, these specimens occasionally have a more difficult time adjusting to the conditions of a particular environment compared to volunteer plants that have been adapting to it for countless generations. It could also be argued that there is less risk of a plant failing due to being planted at the wrong time or in the wrong place, despite best planning efforts.
Unfortunately, there don’t appear to be comprehensive reports on exactly which species and species communities have cropped up in places where the Walter and Merritts approach to floodplain restoration was utilized. For Big Spring Run, however, the primary species transitioned from Kentucky Bluegrass, common fescue, and orchard grass to watercress, bulrush, and reed canary grass– from facultative upland,
non-native species to facultative wetland, primarily native species. As more of these projects are initiated and data is collected on their progress, our understanding of Holocene-era riparian forest and floodplain communities will become more extensive and will hopefully inspire planting schedules for places with less robust preserved seed banks.
Sources:
Walter, R.C. & Merritts, D. (2008). Natural Streams and the Legacy of Water-Powered Mills. Science, 319, 299-304. 10.1126/science.1151716
Voosen, P. (2020) A Muddy Legacy. Science, 369(6506), 898-901.
LandStudies, Inc. (2010). Floodplain Restoration. LandStudies, Inc.
Feibel, S. (2022, January 24). Removing Sediment to Restore Wetlands Clears Revenue Paths. Conservation Finance Network. https://www.conservationfinancenetwork.org/2022/01/24/removing-sediment-to-restore-wetlands-clears-revenue-paths
Fleming, P., Merritts, D., & Walter, R. C. (2019). Legacy sediment erosion hot spots: A cost-effective approach for targeting water quality improvements. Journal of Soil and Water Conservation, 74(4), 67-73. 10.2489/jswc.74.4.67A
Big Spring Run Project. (2014). Results. http://www.bsr-project.org/results.html
Lawson, N. (2024, August 30). What Lies Beneath: Treasures in the Seed Bank. The Humane Gardener. https://www.humanegardener.com/what-lies-beneath-treasures-in-the-seed-bank/