The rain falls upon this plain, but then what?

Washington Monument in a different reflecting pool

As I mentioned in previous posts, the Washington Channel is quite unique in that the water contained within it has little to do with its drainage basin: instead, its water is essentially imported from downstream via the tidal cycle. As such, its water quality (unlike almost all other waterways) largely does not reflect the land and water context adjacent to it. In addition, the Channel benefits from being entirely within the District of Columbia: the federal government has long held title over the waters (Morris vs. United States), and used parkland to create and frame the Basin and Channel. As a result of that unique context, not only does parkland surround all of the Tidal Basin and most of the Washington Channel, but the surface waters are also under federal protection.

Tidal Basin & Washington Channel hydrological maps

The Tidal Basin and Washington Channel do receive surface and groundwater runoff from their immediate areas, which together add up to 1.412 square miles of the District. The Tidal Basin drains 0.423 sq. mi., of which 0.169 sq. mi. (almost 40%) is surface water. 43% of the watershed is parklands and grass areas, including parts of the National Mall, the monuments ringing the Basin, and even the small hill underneath the Washington Monument.

Tidal Basin & Washington Channel hydrological maps

The Washington Channel drains 0.989 sq. mi., of which 0.3 sq. mi. (25%) is surface water. As its north bank is heavily developed, 53% of its watershed includes urban development, and the remaining 22% is parkland.

DC’s largest water pollution problem is its combined sewer/stormwater system, or “CSO.” (I’ll write more on these systems in later posts; they’re super-important for understanding urban water quality but not entirely relevant to this post.) This system, which is responsible for dumping a toxic brew of sewage and rainwater directly into many local waterways, drains one-third of the city, including most areas built before World War 2. However, since the immediate environs of the Tidal Basin and Washington Channel were redeveloped in a somewhat recent era, they have separate sewer and stormwater systems. This map shows the large parts of the city which have combined sewers — many of which, incidentally, are named after the creeks that they replaced:

Instead, smaller, separated storm drain systems — nine along the Channel and three along the Basin, delineated by the faint lines running roughly perpendicular to the water on the map below — intercept rainwater that falls on Southwest Washington’s roofs and streets, and dumps that untreated water into either the Tidal Basin (light blue on this map) or the Washington Channel (tan on this map):

Tidal Basin & Washington Channel hydrological maps

As you can see by comparing the first and last maps, the inland boundaries of the two watersheds are defined by these artificial drainages rather than the natural contour lines seen in the first map. If you look in the vicinity of N and O Streets SW, for instance, you’ll see that there’s a valley roughly between Third Street and Half Street. Historic maps show this as what was James Creek, which drained pretty much due south to the Anacostia River, but instead these blocks now drain “uphill” to the Potomac to the west.

The storm drains dump unfiltered water, often contaminated with urban pollutants, directly into the Basin and Channel. This contributes substantially to the substantial water quality problems within these two water bodies, but plans are underway to substantially reduce the quantity of these flows in the near future.

Sources for this post, notably the maps and geographic analysis of watersheds, include the DC Department of Environment’s water quality standards documents, which I’ll report on in greater detail in an upcoming post.


This semester, I’m taking a Natural Resources class through Virginia Tech about understanding local watersheds, wherein I’ll be researching and posting knowledge about the Washington Channel. You can explore the other watersheds that my classmates are investigating over at the class blog’s page. Other posts in this series can be found using the tag watershed.

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  1. Pingback: Fresh policies will freshen the flow into the Washington Channel | west north

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