Whether floating or wading the Delaware River, swimming in or just gazing at its waters, the most compelling scenery, for me, has always been the view beneath the surface. The riverbed itself and all the engaging aquatic life there and within the flow have been what’s held my attention. And the picture is usually clear and captivating.
When the Delaware flows high and muddy, as it sometimes does temporarily after rains and runoff events, my interest in being on the river drops off dramatically, though the scenery above the surface is plenty pleasing in its own right. But even a turbid Delaware clears up faster than most rivers under these circumstances, thanks in large part to an extensive and well-developed natural system that efficiently strains suspended particulate matter out of the water column. This powerful filtration system oftentimes helps to produce water of clarity equal to distilled water, as measured on a numerical scale (in Nephelometric Turbidity Units) and documented by the National Park Service’s water quality monitoring program.
So how did we get so lucky to have a widespread, built-in filtration system in the Delaware River that enables this marvelous viewing, provides numerous additional ecosystem benefits and works for free with no carbon footprint?
Freshwater mussels are not the most charismatic animals found in the Delaware, nor the most celebrated, but in terms of ecological function and benefit, they’re among the most important. They’re simple creatures that do a few key things, and do them well.
Mussels filter water. They do this by ingesting suspended particulate matter through their incurrent siphon, and running it through internal gills that strain out food particles. They then expel the newly-cleansed, filtered water back to the river through their excurrent siphon. Mussels subsist on a mixed diet of algae, detritus (decaying plant and animal matter) and microbes that are digested and assimilated into their body tissue. Undigested materials are excreted as bio-deposits—organic-rich materials full of nutrients—that enter the river’s food chain to become incorporated into the bodies of aquatic insects, or swim back out into the ocean in the biomass of out-migrating fish. (These include immense numbers of young-of-year American shad, or the still-significant numbers of mature American eels that manage to avoid the hazards of predators and eel weirs on their end-of-life journey back to the Sargasso Sea in the North Atlantic, where they will breed and begin a new generation.) Biogeochemical cycles that have occurred for thousands of years are still relatively intact in the free-flowing Delaware, maintaining an age-old balance through biomass interchange with the ocean that benefits both systems. Freshwater mussels are central to sustaining this balance.
Freshwater mussels, like many bivalves, are regarded as “ecosystem engineers” for their ability to modify habitat complexity and improve water quality. Their bio-deposits enrich sediments with organic material and biochemical compounds, enabling enhanced river-bottom algal growth and greater food resources for aquatic insects, fish, and other fauna.
Mussels also help stabilize substrates and stream channels and reduce streambed transport of sediments during high-flow events. They are important links in aquatic food webs as well, feeding on microscopic matter at the base of the food chain, then themselves being eaten by secondary consumers.
The sheer numbers of freshwater mussels in the Delaware River magnify their positive influence here. Though little noticed, freshwater mussels make up the greatest animal biomass in the river. Quantitative sampling done here by scientists from the U.S. Geological Survey’s Northern Appalachian Research Lab in 2002, and based on nearly 16,000 random sample plots, documented an average of 76 mussels per square meter of riverbed. Some sections of the river had over 634 mussels per square meter of substrate.
To the casual observer, these numbers may seem inconceivable, as most people only notice the shiny inner shells of dead mussels sparsely strewn across the bottom of the Delaware. But having worked alongside the dedicated USGS crew here during much of the sultry summer of 2002, I can attest to the astounding numbers of mussels found in the riverbed. The live mussels are much less visible, mostly buried in the substrate, oriented vertically, often with only the posterior end of their shells and their siphons exposed, and require an attuned eye to detect.
There are literally hundreds of millions of mussels in the Delaware River, each with the ability to filter multiple gallons of water a day. Collectively, they filter billions of gallons of water on a daily basis, greatly influencing and contributing to the superb water quality found here and on down through the entire non-tidal Delaware. The flow volume of the Delaware River is filtered many times over through the bodies of freshwater mussels on its way to the ocean.
Connectivity with the ocean is key in maintaining the Delaware’s robust mussel fauna, as well as the rest of its biodiversity and resilience. During most of the period of escalating dam-building in the last century, we weren’t aware of important tradeoffs being made. Freshwater mussels, in order to reproduce, require a fish host for their larvae to live on briefly during the early stages of their development, without harm to the fish. This fish host is often specific for each species of mussel (the Delaware has 12 of the 14 species found on the Atlantic Slope), and is usually a fish species with which they’ve co-evolved over thousands of years. When they’re de-coupled from their fish hosts, mussels can’t reproduce.
The most important fish host for eastern elliptio mussels (Elliptio complanata), which make up 98.7% of the Upper Delaware River’s mussels, is the American eel, a sea-run migratory fish that is still able to access its important historic habitat here (all the more unique to this system, when 84% of historic stream habitat on the Atlantic Coast is blocked by dams). Consequently, we have hundreds of millions of eastern elliptio mussels each filtering multiple gallons of water per day and providing other important and economically valuable ecosystem services for free, just because we’ve kept the system healthy enough to sustain the native biota of the Delaware River.
In contrast, the numerous dams that exclude American eels from the upper reaches of the Susquehanna River have kept much of its aging population (some live over 100 years) of freshwater mussels from reproducing (and in decline), water quality is not all that it could be, excess nutrients and sediments are being flushed to Chesapeake Bay, and hundreds of millions of state and federal dollars are spent annually in an effort to clean up significant water quality problems there. Today’s Chesapeake Bay has about 1% of the oysters it had historically. Those historic oyster populations could filter the entire volume of water in that vast bay in four days. If we’d done enough to sustain them, this natural capital would still be able to provide their numerous valuable ecosystem services, for free.
The Delaware River is unique in that originates from a drainage that is relatively undeveloped, an upper basin that is still over 80% forested, and a landscape that retains its ability to produce clean water. It is the Delaware’s high-quality water, complemented by diverse habitats and connectivity with its floodplain, a complex food web, and an absence of dams on the main stem, that enables the superb aquatic resources we have here to flourish. These resources, such as freshwater mussels, in turn provide a positive feedback mechanism that further improves habitat and water quality and extends this water quality downriver for ecosystem, drinking water, industrial and other human needs.
In the face of now certain climate change, this Delaware River, because of all these features, will fare better than most rivers in their ability to continue to produce high-quality water, which will become an even more precious, and absolutely essential, resource for human and ecosystem needs into the future
North America was once home to 297 species of freshwater mussels, by far the highest diversity in the world. Today, they’re the most rapidly declining animal group in the U.S., over 70% of which are either extinct, endangered, threatened, or potentially justifying federal protection. The majority of the Delaware River’s species (9 of 12 species) fall into the categories of critically imperiled/endangered (dwarf wedgemussel brook floater mussel); imperiled/threatened (eastern pearlshell, alewife floater, triangle floater, yellow lampmussel, and eastern lampmussel); or vulnerable/species of concern (eastern floater, squawfoot or creeper mussel) on either the state or federal levels, due to population declines range wide. Some species once found throughout the Delaware now only survive in certain sections. Only one species in the system (eastern elliptio) has consensus as being secure, though their numbers decline dramatically below some problematic tributaries in which water quality is known to be impaired.
There aren’t many rivers left like the Delaware. As the last major river on the Atlantic Coast undammed the entire length of its main stem (330 miles), it provides unparalleled access to the full range of habitats for nearly all migratory fish species of this seaboard, and retains ecological integrity that is exceptional among the large river systems of the mid-Atlantic and Northeastern United States. It functions still as other rivers did prior to dams, with a pulse of life (in the form of sea-run migratory fish) that ascends its waters and nourishes its inhabitants, timed perfectly to provide for wildlife such as hungry bald eagles feeding their growing young. An accompanying downstream pulse later in the season benefits an array of marine species, balancing an age-old cycle of biomass interchange and completing an important ecological link.
The Delaware River possesses a super-efficient “operating system,” which has an extraordinary ability to produce clean water and benefit numerous life forms, including ourselves. Such a system is not something we could easily or inexpensively re-engineer. Its wondrous function is largely influenced by simple mollusks that have been at work here in the Delaware for perhaps the past 15,000 years, since the river settled into its present form following the retreat of glaciers after the last ice age. They don’t require a lot to survive. A landscape and stream network that sends them relatively clean, cool, well-oxygenated water that is free of pollutants and an overabundance of sediment, and that provides adequate food material and connectivity with their fish hosts will do just fine. We would do well to maintain conditions that will keep their populations healthy here, ensuring that the yeoman’s work they do, gratis, for the river and all its stakeholders, continues into the future.
[Don Hamilton is the Chief of Resource Management for the National Park Service’s Upper Delaware Scenic and Recreational River.]