Stormwater: The Patapsco River Non-tidal Watershed

Acting as a buffer between the Baltimore suburbs and the Patapsco River is Patapsco Valley State Park. The park is one of my favorite places to visit because it’s close to home and an easy escape into nature. Every summer I come to the park and swim in the Patapsco River, which flows into the Chesapeake Bay. I recently took a hike along a path that follows the river and found myself eagerly anticipating warm weather when I could finally jump back in (it’s February and in the 40s now). Along the path you’ll see pipes and canals that flow water into the river. Considering that I wade through the river each year, it got me wondering where that water comes from and how, if at all, it’s treated before reaching the river. The answer? Well…it’s stormwater. Some of it is treated and filtered naturally, and some of it is not.

Stormwater is a leading contributor of nutrients entering our waterways from Anne Arundel County . Rain washes nutrients like nitrogen and phosphorus into our storm drains and then into streams and rivers. Impervious surfaces like roofs and streets don’t give the water any chance to be filtered before entering these streams. Runoff from impervious surfaces is much more polluted than runoff from pervious surfaces. Once an area has over 10% impervious surface coverage, stream quality declines. The Patapsco drainage area has about 27% imperviousness. Furthermore, the stormwater running off these surfaces is more easily heated. Warm water holds less oxygen. So, when you have water in the Chesapeake that is less oxygenated and contains more nutrients, you’ll increase the chances of algae blooms and dead zones in the Bay. Treating rainwater before it enters the Bay is essential to the Bay’s health.

The part of the park that I visit and I live in is located in the Patapsco non-tidal watershed area. This watershed is one of 12 in Anne Arundel County, the county directly south of Baltimore. I wanted to understand how my area is contributing to keeping the Chesapeake Bay healthy through their stormwater management. The county stormwater management strategy includes dry and wet ponds, stream restoration, and street cleaning. Many of these strategies were within walking distance of my own backyard. You’ll recognize these systems if you live in any area that is even remotely populated.

The dry and wet ponds are part of the municipal separate storm sewer systems. MS4s for short. An MS4 is a conveyance system (a drain or sewer system) that is owned by a city, state, or public entity that is used to collect and convey stormwater, and is separate from a wastewater system. You’ll spot them along highways and in neighborhoods. They’re essentially artificial ponds that collect and even treat our rainwater. The dry pond (detention pond), is usually not filled with water and only fills during heavy rain. At the bottom of this dry pond is a drain that will lead to a nearby stream or river. This type of stormwater best management practice (BMP), a way of treating and/or storing stormwater before it reaches our waterways, is designed to hold excess water and slowly release it. This allows nutrients like nitrogen and phosphorus to filter out of the water before entering the Bay.

The other version of this BMP that the county uses is the wet pond. A wet pond (retention pond) is different as it permanently holds water. The system has a point where stormwater is drained in, and an elevated point where water can drain out. The sources into the pond come from the gutters and sewers you see along the street. The pipes leading into the pond allow large amounts of water to enter, while the pipe that drains the pond is usually smaller which forces water to exit more slowly. This gives the system more time to process and filter out harmful nutrients. The water leaving the system can only drain when the water level is high enough to reach the elevated exit point. Without rain and stormwater entering the pond, the water has a chance to be filtered by natural vegetation. An added benefit is that these ponds, if properly maintained, provide a habitat for other species, like ducks, geese, and other waterfowl. Both dry and wet ponds drain water to nearby streams and rivers. Where I live, they very well may lead to Holly Creek which flows into the Patapsco.

Once the water leaves these BMP’s, they still have a chance to be treated even further. As I mentioned, Holly Creek is the next step where water in my neck of the woods can be filtered…at least for a portion of the stream. Much of Holly Creek has been converted into an urban, concrete stream. Stormwater that flows through this section of the stream moves rapidly and isn’t filtered by any sort of vegetation. When the water finally leaves the culvert at the artificial end of the section, it enters the natural half of its journey. Here, water meanders through the stream and wetlands until it reaches the Patapsco. Some issues the area around the natural part of the stream has are erosion, invasive vines, and flooding. Often when a creek is converted to an urban stream, the rapid moving water can cause erosion and flooding further downstream. And the roads and neighborhoods near the natural part of Holly Creek are often flooded during steady or heavy rains. When erosion begins deteriorating the floodplains around the creek it invites invasive species in, like vines, which can climb and kill trees. Trees are important in filtering water and stabilizing the ground to prevent sediment runoff and further erosion. The more erosion that happens, the faster the water can travel. You want to slow down the velocity of water to help stabilize the stream. In soil, there are trace amounts of phosphorus, nitrogen, and some heavy metals, which the Bay can handle in small amounts. But when you have heavy rain and high levels of water that degrade and erode streams, it washes away those legacy sediments from the ground and their levels entering the Bay become too high for it to withstand. The county has continued to work on stream restoration to help slow down water and limit the effects of erosion. The restoration also brings in native species that help filter the stormwater and limit sediment runoff and erosion.

One area of the non-tidal watershed that has received restoration is right on the Patapsco River at the site of the former Bloede Dam. The dam ran from 1907 until 1924 when it ceased operation due to sediment build up causing complications (thinking about the Conowingo Dam here). In 2018, the dam was finally demolished by a planned explosion. Where there was once a resevoir is now just a river. On the old banks, the State planted native trees to help stabilize the ground and be a buffer for stormwater. Just a couple hundred yards up from the old dam site is a set of culvert which have recently gone under construction. The culverts now drain into a stepped and graded decline into the river, which helps slow the water down and prevent sediment runoff.

Finally, one more strategy that Anne Arundel County uses in their stormwater management, and the most cost effective according to them, is street sweeping. Street sweeping is effective at removing nitrogen from the streets, and especially cost effective when removing phosphorus. At first, I was unsure how a street sweeping vehicle would remove nutrients. I did not know that the county used vactor (vacuum-assisted) trucks to clean both the streets and the inlets, or storm drains. According to the county, sweeping the streets near the watershed twice a month reduces the load into the waterways by 25%.

While the county has their own system in place to manage stormwater, it’s important that residents do their part to limit the harmful nutrients and chemicals from entering the Chesapeake. You can use a barrel to collect rainwater (which can be used later in your garden), wash your car on your lawn so that the grass can absorb the runoff, pick up after your pets, or create a rain garden in your yard. Are there any habits or strategies you use to combat harmful storm runoff?