The previous post talked about Crabtree Creek’s tendency to flood – last week again brought heavy rains over the Crabtree watershed that brought the creek up to the edge of  its large channels.  This also sends an impressive load of water over Lassiter Mill Dam, as seen above.  I shot a video clip of the rushing water from below the tailrace, as linked below.

video- Lassiter Mill dam at high water

What happens at Lassiter Mill vividily and intensely demonstrates what happens lots of places more gradually – the deposition of new soil by spreading flood waters.  This is an essential part of the natural systems of the Piedmont, and our flood control measures prevent the process from periodically enriching the soil with a layer of mud and silt – though the process continues to work just fine in the “waste” lowlands that remain in Raleigh.  An astounding number of these lowlands have become major thoroughfares – roads built relatively later in Raleigh’s long history, on land left undeveloped due to the floodplain.  The Beltline follows the low contours of Walnut Creek, House Creek, Crabtree Creek, Big Branch, Marsh Creek and then Walnut Creek quite precisely as it curves from Cary’s Buck Jones to Glenwood, over the crest of North Hills, and around southeast to Poole Rd and then Lake Wheeler Road.  The water is piped and rushed away from underneath these elevated roadways, carrying its minerals with it.

The suburbs and businesses near these roads certainly don’t need the sediments!  But the stuff has to go somewhere, and these days there is a lot of stuff.  When streams are buffered by a healthy band of water-loving trees and shrubs, erosion material is reduced greatly. In central Raleigh, Crabtree is clogged with lots of dislodged soil, construction materials and unnaturally exposed red clay.   But the deposition process is a vital one, and it gets exaggerated at Lassiter Mill, where the water brings its load of suspended minerals hurtling over the dam and then slows and spreads its course below.  As it slows, it drops much of its sediment load.  The area below Lassiter Mill changes yearly as the creek alternately erodes and builds up materials.  Check out the new load of sand deposited by the recent high waters.

There are several caveats and complications to consider.  This is a large load of sand!  Eight or nine inches at a dose, and not the silt and mud that the plants would prefer. But nature adapts, and the Lassiter “beach” is fun to browse, with a wide variety of weeds incubated from the loads of  soil and debris.  The silt and sand that currently washes down Crabtree is terribly unhealthy for the filtering mussels and other delicate aquatic life.  The red clay that paints Crabtree brown is such a strong pigment that Crabtree often changes the color of the Neuse where it conjoins.

Crabtree builds itself “shoulders”  as it repeatedly overflows, dropping the heaviest particles first as the water disperse into the floodplain.  This is why Crabtree presents such a tall ditched appearance as above at Hodge Road.  The plants arrange themselves in an orderly sequence beside or on top of these embankments according to their tolerance for flooding.

The next time Crabtree rises over it’s banks, put on your rubber boots and check out the glistening mica-rich silt that covers the greenways before the city sends its scrapers to clear it off to the side, where it enriches the plants as well as any landscaper’s mulch.  You are walking in the stuff that makes our floodplain soils, a rich muck delivered by the yearly floods.

Nature.org floodplain info

BBC floodplain story