Who Made This Muddy Mess?

Sediment Reduction for MS4 Permitting: “Who Made This Muddy Mess?”
Gregory R. Glitzer, P.E.

That quote was blared by my mother on too many occasions as she followed a trail of footprints through the back door to the kitchen (a common detour via the cookie jar in the cabinet) and to my bedroom, where I would invariably be caught, cookie in mouth, trying quite unsuccessfully to jettison my muddy sneakers and put on my best ‘not-me’ face.

At a time when many municipalities are grappling with PADEP’s review of their MS4 permits within sediment impaired waterways, maybe it’s time we ask “Who made this muddy mess?”

The Bucks County Planning Commission’s “Neshaminy Creek Watershed Sediment Reduction Plan for Municipal Implementation” states that over 75% of the total suspended solids (TSS) load in EPA’s Neshaminy Creek TMDL are the result of streambank erosion (Total Maximum Daily Load, is the calculated maximum amount of a pollutant that a waterbody can receive and still meet designated water quality standards). Can that be correct, considering the vast area of the watershed compared to the rather limited area of the riparian zone? Have our stormwater management systems failed to protect stream banks from accelerated runoff failures since the passage of the Stormwater Management Act 167 in 1978? Have erosion and sediment control plans failed to protect our streams? With our best management practices (BMPs) largely targeting upland areas, are we focusing 100% of our effort on less than 25% of the problem? Did Yogi Berra, who stated that 90% of baseball is half mental, calculate that statistic?*

Contrary to much of what we generally accept as fact or conventional wisdom, much of this sediment that contributes to the high (TSS) loading in our streams is already in place within the riparian zone. A growing body of research suggests we’ve inherited these incredible sediment loads, and it may go back much longer than most people could imagine. It’s time to add the term legacy sediment to our lexicon.

Legacy – (n) anything handed down from the past, as from an ancestor or predecessor

Research by Dr. Robert C. Walter and Dr. Dorothy J. Merritts at Franklin and Marshall College has challenged our long established concept of a meandering stream channel deeply eroding through its historic and naturally formed floodplain in response to increased urban and agricultural development. Walter and Merritts have documented the profound effect of water powered mill dams on stream morphology in the mid-Atlantic region.

Water powered mills once dominated the landscape of the mid-Atlantic region for well over 200 years, and every mill was accompanied by a dam to generate the requisite falling head of water. Those dams were often of timber crib and rock construction chinked with soil – some were little no more than felled tree trunks. All were vulnerable to failure.

The mill ponds behind these dams rapidly choked with sediment from the clear-cut watersheds. Soil conservation practices were unknown. In addition to the accumulation of sediment within the pool of the mill pond, the backwater effect of the mill dams decreased stream flow velocities, often for miles upstream of the dam, with the impact often spreading throughout tributaries. Sediment rapidly accumulated in the stream valley as solids settled out of the stream flow at higher elevations in the watershed compared to an undammed stream. Inevitable failure of the abandoned mill dams lead to stream channels cutting through these accumulated sediments. We see the results of those dam breaches today in stream channels deeply incised in legacy sediments.

Why should we care? Why bother trying to understand where this sediment came from and how streams evolved? Understanding the source of the sediment will help target successful planning and management. Quite simply, our planning efforts for sediment reduction must be informed by the best science available and executed by proven practices.

Restoration projects based on an ill-fitting template for understanding stream dynamics in our region may not achieve desired results or may actually fail. Take for example a simple stream buffer planting project, although I would argue there is nothing simple about installing and maintaining a stream buffer project. These are encouraged (and in some cases required) throughout our ordinances, and may be proposed as part of an MS-4 sediment reduction strategy. However, without an understanding of the impact of legacy sediments on stream channel form, we may be setting our stream buffer projects up for failure.

The photo below shows Dr. Walter surveying a reach of Big Spring Run** in Lancaster County as part of planning and design for a stream restoration project. Notice the tree plantings left over from an earlier riparian buffer project. These trees were installed on top of the legacy sediments – deer tubes have not even been removed, yet the stream channel has carved through the sediments and claimed some of the plantings. Conventional wisdom states that buffer plantings immediately adjacent to stream banks might be expected to stabilize eroded banks, but failure to understand stream dynamics and the impact of legacy sediments may prove that conventional wisdom is none too wise.

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Photo courtesy of Cheryl Schenk, Franklin and Marshall College

As opposed to an ill-fated riparian buffer project, even a limited-scale stream restoration project undertaken with an understanding of historical stream forms and the impact of legacy sediments can be a successful sediment reduction strategy. Stream channels incised within deep legacy sediments are essentially sediment quarries, but since their loads aren’t hauled away in large diesel powered trucks, they go unnoticed, operating without a mining permit or erosion control plan, often right under our noses.

Does this mean that township supervisors will need to don waders and canoes and take stream tours in addition to the yearly road tours to rout out this non-conforming land use?

I doubt it. Legacy sediments from mill dams are actually quite easy to locate, once you open your eyes and your mind. The photo below shows a rather typical stream bank along a tributary to the Neshaminy Creek. Over 4 feet of the stream bank is vertically exposed (that’s an old fashioned yard stick in the photo; if you read up more on this topic, you’ll encounter the metric system). Conventional wisdom suggests that a wooded riparian buffer should supplement the meadow grass cover, as the grass’s root penetration is insufficient to stabilize the bank. However, even the minor wave action caused by the photographer wading the stream is causing erosion.

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Photo courtesy of Joe Mihok, Bucks County Trout Unlimited

But pan that camera to the opposite bank and trudge slightly downstream and we find this:

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Photo courtesy of Joe Mihok, Bucks County Trout Unlimited

The core of a breached pre-industrial mill dam, complete with timber cribbing and rock fill. No surprise really, as the 1859 map below shows 5 mill ponds within a single mile reach of stream. This particular mill pond is site #3 below:

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Image via Joe Mihok, Bucks County Trout Unlimited

A wooded riparian buffer perched on top of these legacy sediments will do little to stabilize the bank, and may actually fail without a broader stream restoration plan. But armed with at least a basic understanding of legacy sediments, we now have the tools to plan an effective sediment reduction strategy that targets a major source of sediment pollution. And that understanding should prove a critical step for both municipalities and the Pennsylvania Department of Environmental Resources in dialogue involving planning for effective sediment reduction.

*I don’t think that’s exactly what Yogi Berra quoted, but then again, he would merely respond that “I didn’t really say everything I said.”

**For further reading on this subject, please visit www.bsr-project.org. This site contains helpful visualizations that document the restoration of Big Spring Run and assist in an understanding of the impact of legacy sediments. It also provides links to eye-opening research on the topic, including Dr. Walter’s and Dr. Merritt’s article “Natural Streams and the Legacy of Water-Powered Mills.”

Bucks County Association of Township Officials (BCATO) featured this article in the Township Tips section of the BCATO Monthly eNewsletter for September 2015.