Saturday, February 25, 2012

The Mysterious Island

A view upstream along my creek.  The "mysterious" island is on the right bank

Picking up on a theme of a current family adventure film, but without bees the size of rhinoceroses...

Ever since I came to my preserve, I've been fascinated by a fairly sizable island (2-3 acres) in my creek.  The smaller streams in the Mid-Atlantic don't have islands; the valleys are too deeply incised into our metamorphic and sedimentary bedrock to form islands.  These streams are straightforward down cutters.  So, how to explain this rocky, wooded island? 

I've also been concerned that I am "losing" the forest on the island.  The trees at the edge are being undercut by the current...
...and flooding from severe storms washes over the center of the island and topples trees and removes topsoil.  Surely, this landscape isn't long for this world.

I've worried and puzzled over this situation for years.  I had some suspicion that it was related to the long-breached colonial mill dam located just downstream from the lower tip of the island but couldn't put the puzzle together fully.

Then, in 2008, a seminal paper appeared in the journal Science.  The two authors posited that all of the streambeds in the Mid-Atlantic had been significantly modified through the construction of colonial water powered mill dams.  Now that the dams had either breached naturally or had been removed purposefully, the streams were regaining their natural channels and were quickly washing away the "legacy sediments" that had accumulated behind the mill dams.  Furthermore, the urbanization of these same watersheds generates rapid and excessive runoff, so legacy sediment removal was progressing even faster than it would if the watersheds had returned to more natural conditions after the milling operations ceased.  Exactly the explanation I had sought for the disappearance of my island.

I'm still distressed that the forest, with some remarkably large American beech, oaks, and tuliptrees is washing away, but this really only represents a return to more natural conditions in the streambed.  Of course, try to tell that to the aquatic invertebrates that are smothered by the eroding sediments after each storm.
A view downstream at the head of the island where the creek divides

Ruins of the stone foundation of a water powered mill located just upstream of the island


John Gray said...

that stone wall looks like something from North Wales!
glad you enjoyed The Artist

John "By Stargoose And Hanglands" said...

Here in the UK it's the normal state of affairs to have such islands associated with mills. The way they come about is this: the streams in their natural meandering state are too slow to power the mills for much of the year when the water is low. In order to obtain a steeper gradient and faster flow a straight channel is cut across a large meander to the site of the mill, the old watercourse is retained so that the flow to the mill can be regulated via a system of sluices. Not sure if that's the same as what you're describing.

Scott said...

John: No, that's not what happened with our streams. In the Mid-Atlantic states, the dams were build on rapidly flowing streams. Few if any streams are of such low gradient that they actually form meanders. When the water velocity dropped as the streams entered the slack water of the ponds, they dropped their loads of sediment (made all the worse because the forests in the watersheds were being cleared for agriculture at the same time). In fact, one of the reasons that milling became unprofitable in many places (in addition to the introduction of the relatively portable and dependable power provided by steam engines) was that some millponds had silted up so badly they didn't hold enough volume to turn the water wheels any longer.

When the mills were abandoned and their associated dams no longer maintained, the dams gradually breached, lowering the water level, exposing the sediments, and allowing plants to colonize--like the forest that developed on my island. Then, when the dams failed completely, the plant-stabilized sediments were left high and dry above the newly down-cutting streams. I don't know of any other such islands in this area, but most of the streambanks in my region are experiencing this erosion and down-cutting.

Scott said...

John Gray: The walls are built of the native stone in our area, a granitic metamorphic gneiss--dense and dark gray. I wonder about the origin of your rocks? Just a few miles west, similar rocks bear considerable quantities of mica, which makes them glisten when the sunlight falls on them. They're prized for building stone because of the distinctive mica flecks.

I think that "The Descendents" deserved Best Picture a bit more than "The Artist," but I'm not quibbling--they're very different films so it's like comparing apples to oranges.