by Steve Saari
In nature, streams change over time. They meander and carve new channels. They abandon courses and leave behind ponds that then, over time, become wetlands. They don’t tend to disappear except over millennia. In urban areas, however, streams do disappear – and they can do so quickly.
An example of the loss of streams in urbanizing areas can be seen in current and historic maps of Broad Branch. Broad Branch and its tributaries once flowed as an above-ground stream from Chevy Chase, Md. down to where it meets the main stem of Rock Creek. This can be seen in this 1917 United States Geological Survey map of the District. The extent of Broad Branch is highlighted in blue so that it is easier to see.
Now compare the 1917 map with that of the current state of Broad Branch from Google Maps (again, the current above-ground portion of Broad Branch and its tributaries is highlighted in blue).
So where did these creeks and streams go? There are really two reasons for their disappearance:
1) Some were piped underground for development, and
2) Some were lost because their source of water has disappeared.
The first reason is easy to see by comparing the two maps. In the map from 1917, there is very little development and that development is concentrated outside of the stream valleys. In the second map, many of the streams that once bordered the roads in this area have been piped and homes have been built in these stream valleys.
The second reason is more difficult to envision. The general topography of the Broad Branch stream valley has not changed between 1917 and 2012, and essentially the same amount of water falls on the lands that drain towards Broad Branch today, so how has its source of water disappeared?
The answer is in how that water was conveyed to Broad Branch in 1917 and how it is conveyed there now. In 1917, there was much less impervious surface than there is now. There were few roads and even fewer were paved. There were fewer houses, sidewalks and alleys, and the houses and roads that were present were generally smaller.
A raindrop that fell in this area in 1917 had a good chance to land on a surface where it could infiltrate the ground and slowly make its way to the water table – the level of soil and subsurface materials that are saturated with groundwater in a given area. This groundwater flows slowly down gradient to the point where the soil is saturated at ground level and at that point a stream forms at the surface.In 2012 much more of the city and the area around Broad Branch are covered with impervious surfaces. So instead of soaking into the ground and feeding a stream through groundwater it quickly runs off our rooftops, sidewalks, and roadways and ends up in a pipe where it is shot quickly into the remaining above ground portions of Broad Branch.
The type of development that occurred between the 1917 map and the map from 2012 is what has led to stormwater pollution. Polluted stormwater has three components: It is hot, it is fast, and it is dirty.
Stormwater is almost always hotter than the water it flows into. This is true because our impervious roadways and rooftops are good at collecting and storing solar heat which they give off to the rainwater that falls on them. Also, rainwater that arrives at a stream by flowing over impervious surfaces and in pipes gets there much faster than water that infiltrates slowly into the ground. Finally, stormwater is dirty because it has picked up pollutants from our cars, from the fossil fuels we burn, from fertilizers, pet waste and more. Water that infiltrates slowly into the ground is cleaned of these pollutants by bacteria and natural chemical processes.
So how do we undo the damage from the 100 years from the first map to the present? Obviously we cannot raze all the houses and tear up all the roads and alleys. Instead we need to develop streams of “green” infrastructure to replace the real creeks and streams we’ve lost. Green infrastructure like rain gardens, permeable pavements, trees, green roofs, and cisterns can help us slow down, cool off and clean up and infiltrate stormwater close to where it falls.The District has started to move in that direction through incentive programs such as RiverSmart Homes that offers homeowners up to $1,200 to adopt practices that infiltrate stormwater on their properties. The District currently has new draft regulations that will require large development projects to capture and infiltrate 95% of storm events on their property. Finally, the District has started to retrofit its lands through the installation of green alleys and through award-winning projects such as RiverSmart Washington – a project that is working to retrofit almost 30 acres of District neighborhoods with this type of green infrastructure.
We can’t go back to the past, but we can work to undo the damage that we unknowingly have done. Let’s just hope it doesn’t take 100 years for us to do so!