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.
In this installment, I’ll take a closer look at how the channel functions today, and what that means for its water quality. Other posts can be found using the tag watershed.
Last week, I examined how the silty Potomac started to clog the harbors around Washington, and mentioned how that resulted in the 1896 construction of the Tidal Basin and the Washington Channel. Today, we’ll investigate in some more detail just how the basin and channel work.
The satellite photo currently shown on Bing Maps appears to have been taken at low tide, when the Potomac River flows more or less as it always has, carrying fresh water east to the Chesapeake Bay and Atlantic Ocean:
Note the high sediment levels in the Potomac, as indicated by its brownish color — it appears to carry more sediment than the Anacostia. I’ve highlighted the rivers’ silty water flows with greenish arrows.
At high tide, the picture looks very different, as the silty rivers collide with waves of clearer, darker ocean water racing upriver:
The Tidal Basin-Washington Channel system uses the tidal surge to activate a pair of one-way gates. Imagine the Tidal Basin as being bound by two sets of doors that both only open inwards. The first set, the Potomac inlet gate, is at the Basin’s southern end: facing the flow of tidal water but perpendicular to the river’s flow. When the tide rises, the pressure of water flowing upstream pushes open the inlet gate, filling the relatively low basin with 250 million gallons of water:
When low tide occurs, water recedes away from the Tidal Basin at the Potomac gate. The now-higher water in the basin tries to escape back out to the Potomac, but because those gates only open inwards, the water instead pushes the gates shut. (Water only pushes, not pulls.)
Meanwhile, at the Tidal Basin’s eastern end, an outlet gate leads to the Washington Channel. There, the gates open from the Basin to the Channel: they’re pushed open when the tide falls away from the Basin, spilling those 250 million gallons into the Channel, and they’re pushed shut when the tide rises.
Since the Channel is scoured during both the high and low tides, and is fed by the clearer tides and not by the muddy Potomac, sedimentation is no longer a problem and the channel retains navigable depth without the need for dredging. And since the Tidal Basin is fed by the tides — it’s always “high tide” in the Channel — the water within Washington Channel more closely resembles that of the Chesapeake Bay than the Potomac River next door. Thus its water is relatively clean, which is curious for a water body within the District of Columbia.
As a habitat, the Channel more closely resembles the brackish downstream Potomac than any of the neighboring freshwater rivers and streams. Fishermen know this, and set up along its banks to catch fish that have been swept upstream by the tide:
In further installments, I’ll take a closer look at the shorelines, adjacent land use context, drainage, and water quality measures within the Channel.