Multiple, interrelated sources contribute to waterway degradation in Philadelphia's streams.
A degraded waterway in dry conditions.
A degraded waterway in wet conditions.
Due to the increased volume of runoff in urban areas, the aggregate flow of stormwater becomes a large volume that de-stabilizes streambanks. In turn, natural stream banks become eroded, making it very difficult for natural vegetation to become sufficiently rooted. Erosion is a self-reinforcing process; as stream channels become wider and deeper, they confine more and more of the stormwater stress within the channel itself, rather than distributing the force of stormwater out over the floodplain.
Erosion and sedimentation go hand in hand. Sediment is carried from land surfaces via stormwater runoff and washed off streambanks. It accumulates within pool habitats, fills in spaces between rocks, and settles to the stream bottom. It can be one of the primary causes of habitat loss for fish and aquatic invertebrates.
Lack of Baseflow
Baseflow in streams is primarily made up of water that soaks into the ground, making its way slowly to the stream channel. In perennial streams, baseflow is the portion of streamflow caused by groundwater inflow and is present in dry and wet weather. In a natural setting, about one-third of the total flow in a stream is runoff and the rest is baseflow. This relationship is reversed in Philadelphia's streams due to expansive impervious surfaces and excessive runoff; only one-third of the total flow is baseflow. In fact, a number of waterways in Philadelphia can be described as flashy, meaning that there is a dramatic fluctuation in flow between dry and wet weather. These waterways have very little or even no flow during dry weather and very heavy flow during wet weather.
Just as Philadelphia's streets and neighborhoods are often warmer than the surrounding areas, streams can be artificially warmer in urban areas than in more rural settings. Impervious surfaces that absorb heat during the day and emit stored heat at night are a primary cause of higher air temperatures in urban areas. Stream temperature is related most directly to air temperature, resulting in varied patterns throughout the seasons. Stormwater runoff is usually generated when there is cloud cover and cooler temperatures overall. But in some cases, stormwater runoff can increase stream temperatures. Warmer stream temperatures are not thought to be particularly harmful. However, the lower dissolved oxygen capacity of warmer water may indirectly contribute to oxygen stress for aquatic life.
All plants require nutrients for healthy growth, but when nutrients are present in water in excessive quantities, aquatic ecosystems become unbalanced and harmful algal blooms can result. Though producing plenty of oxygen during the day by the process of photosynthesis, algae and their associated microbial communities also consume dissolved oxygen throughout the night. When algal densities are very high due to excess nutrient concentrations, dissolved oxygen may fluctuate, a major stress for fish and aquatic invertebrates. In Philadelphia's streams, the primary nutrients of concern are phosphorus and nitrogen; these nutrients are present in stormwater runoff, fertilizers, and treated wastewater discharges. Pennsylvania does not have water quality standards for nutrients, and because nutrients may not always affect streams in the same way, nutrient water quality standards are more difficult to develop than standards for compounds that cause predictable effects.
The health of aquatic communities within and wildlife adjacent to a waterbody hinges on many factors. Waterways that lack appropriate depth, flow, and stream bottom characteristics cannot support the needs of fish and other aquatic animals. Erosion increases turbidity, bacteria growth compromises oxygen levels, and pollutants impair the ability for aquatic life to survive.
Human-Impacted Stream Channels
Bridges and culverts provide a path to convey natural water bodies beneath walkway, highway and rail embankments. Any man-made structure placed within the natural floodplain alters the shape of the stream/river and can result in undesirable social, environmental or structural conditions. Structures encapsulating a waterbody additionally impacts local environments as the covered section is no longer available to the surrounding ecosystem.
If the bridge or culvert is restricting flow, water velocity increases and causes local erosion and additional sediment transport downstream. During wet weather flow conditions, a constricting bridge can result in local flooding of roadways and houses as water depth increases behind the structure to move flow downstream.
Man-made dams act as a barrier that impounds water. A dam essentially limits the flow of water downstream, causing water to pond behind the structure. This ponded area generally contains stagnant flow which in turn results in the deposition of sediment and pollutants. The area directly upstream of a dam will generally suffer from severe environmental degradation.
Dams can prevent the passage of aquatic species, essentially dividing an ecosystem. Fish ladders constructed parallel to a dam enable aquatic species to travel upstream of dams. A fish ladder can provide a cost-effective alternative to dam removal.
Water flowing through a dam or overtopping it can result in scouring of downstream riverbeds and loss of riverbanks.