Saturday, December 5, 2009

Rock at Creek Curve - Part 1

Blue Jay Barrens is located in a very hilly area and the creeks have a rather steep gradient. This causes runoff water to move quickly down the creek channel. As water accelerates, it has more energy to do such things as move stones along the creek bed or remove and carry away soil from the creek bank. The photo above shows a fairly long stretch of creek channel.

The water doesn’t continue for long in this direction. From a point on the bank above the creek, you can see the straight section coming in on the left and a curved section almost doubling back and taking off to the right.

Here’s the water flowing away. Notice the abundance of large rock suddenly appearing in the stream channel. That rock had to come from somewhere.

Getting nearer to the edge of this high bank allows us to see part of the rock supply. A scree beach has developed on the outside of this curve. The flowing water must be moving rock pieces slowly down the channel.

Things become clearer as we move from the top of the bank. The rushing water from a storm, coming from the left in the photo, enters the curve and smashes into the high bank. The water expends some of its energy against the bank causing the water to slow slightly. The current is redirected into a narrower channel that has trouble handling all the water coming its way. The fast moving water from above continues to supply more energy to the situation at the curve. As the water rises, some of the water turns back on itself and creates a rotating current. The result is the pool shown in the foreground.

Water near the surface of the flow is not influenced by obstacles on the banks or streambed and moves faster than that below. As the water rises, this power zone slams against the bank at the curve and dislodges increasing amounts of rock and debris. It’s easiest for moving water to carry rocks that are falling through the water column, so rocks detached from the bank can move a considerable distance before they finally come to rest on the bottom. As rocks are removed from the bank, the grass and trees on top begin to come down.

Jumping to the other side of the creek we find one of my favorite trees, Ironwood, Carpinus caroliniana, growing on the point across from the rocky bank. Water moves more slowly around the inside of a curve, so while the outside bank is being torn to bits by the water, this side is actually accumulating material. The Ironwood seems to be thriving here.

But there’s a problem. A cedar from the opposite bank is slowly leaning and is just beginning to touch the top of the Ironwood. It won’t be good for the Ironwood when the cedar completes its fall.

The cedar won’t be standing much longer. I’ll cut it this winter to avoid damage to the Ironwood and to take some of the stress off the bank. It’s inevitable that this bank is going to continue collapsing, but I’d rather it happen in little stages and not have a massive hole left from the cedar root mass ripping out of the ground. Tomorrow, we’ll have a closer look at those rocks.

No comments:

Post a Comment