Clay Slip

I noticed that the shale in this section of stream bank was decomposing much more rapidly than any of the other exposed sections. Cracks have actually formed near the top of the exposed face. This indicates a major shift of material downward toward the creek. I haven’t seen any of the other shale outcrops behave in this manner.

Instead of standard stream bank erosion, there is a slip in progress here. A slip is a condition where a mass of soil moves downhill as a single unit, as opposed to erosion where soil is stripped away only from the exposed surface. No amount of vegetative cover or root mass is going to stop a slip. As seen here, the soil just slides out from under the plants.

Plants that began life high up the bank, end up at the bottom. This Ironwood will eventually be washed on downstream. There’s a chance it could lodge somewhere and reestablish in a new location. This is another example of how plants colonize new areas.

As the toe of the slip moves into the creek channel, normal stream bank erosion carries it away. This cycle will continue until the slipping hillside stabilizes.

I was still puzzled by the rapid formation of the slip. There are larger areas of exposed shale that stand almost vertically against the creek and show no signs of slipping. The appearance of this shale bed is no different than any other.

A more physical examination shed some light on the occurrence. My thumbnail could easily carve a depression in the sample. The entire bank is composed of compressed clay in layers identical to the shale formations. Given enough geologic time, this clay could have become shale. As it is now, the clay has the look of shale, but not the strength. There’s no telling how large this pocket of clay could be. The slip could eventually reach far up the hillside. I know it won’t go to the top of the hill, because limestone takes over somewhere between here and there. I’ll just have to wait and see what happens.

Creek Obstructions

If water flowed downhill without encountering resistance, it would travel in a straight line. Some forms of resistance are obvious. A fallen tree that can’t be moved by the water, must be bypassed. I discussed this fallen tree back in the winter and it is still redesigning this section of creek.

The wider the creek gets, the slower the water flows. Eventually, the channel bypassing the tree will be large enough to accommodate all of the water flow and the creek will stop getting wider. The tree will gradually decompose and will finally be carried down stream. When that happens, this wide area will tend to slow the water and the temporary hole that has been created will be filled with deposited gravel.

Obstacles like fallen trees are easy to spot, but they are not the only type of resistance acting on the flowing water. Water is attracted to itself and other objects. Water flowing down the creek tries to hold on to the rocks, roots and soil of the stream bed. This slows the speed of water going down the channel and the result is exactly what you see when you have a slow moving car in heavy traffic; first there’s a back up and then people start to pass. Water not in contact with the stream bed is not affected by the strong attraction and can move at a faster speed. The faster moving water begins to pass the slower water with some going over the top and some trying to go around. The result is a stream that tries to change its shape.

We’ll stop here for a little follow-up on the perched island of vegetation. There seem to be a fewer number of plants surviving in this hanging mass of flood debris. Those that are left seem to be doing well. I suspect the Hog-Peanut Vine of getting its roots into the actual creek bank.

The creek banks themselves become obstacles. Masses of roots hang free on this curve and provide a buffer between the soil and the rushing water.

Of course, a big chunk of rock posing as a hill is a pretty insurmountable obstacle. The result has been a long stretch of creek with a wide gravel bottom. The gravel in this area is constantly being picked up and carried downstream, but the loss is usually followed by an equal amount of new deposition, so the gravel stretch is fairly stable even though it’s not always the same gravel.

It’s fun to watch the creek change as it encounters new obstacles. The creek changes constantly, but it’s always the creek. The life that’s adapted to this type of environment thrives amidst the chaos.

Storm Flow

Storm runoff yesterday offered some good examples of how increased water flow changes the stream course. Water above the dam is temporarily slowed by the obstruction. Water velocity increases as it is forced through a narrower opening and increases again as it rapidly falls to a lower elevation. Turbulence combines with the increased speed and the water is able to change the shape of the downstream channel.

The turbulence from the fall travels as waves of energy down the stream bed. Water striking the streambed is deflected upward where is can again fall. These energy pulses effectively knock pieces of loose materiel up into the water column where they can be washed away.

The water’s energy is directed to the bank on the outside of the curve. The bank suffers a little bit of erosion each time this happens. This increase in flow was produced by 1.25 inches of rain in a 12 hour period. Streams are subjected to this type of flow several times each year and are subtly shaped by the action of the water. Out of bank flood events are a much rarer occurrence and have a different effect on the stream.

Obstructions, such as the fallen tree, add to the complexity of the flow pattern. This obstruction is causing the water to accelerate through the narrow opening to the right and is causing an outward bow to the stream bank.

Two tributaries join in the center of Blue Jay Barrens. Both of these tributaries begin on neighboring properties and the water quality is a reflection of the activities taking place on those properties. The stream on the left always exhibits a higher degree of turbidity.

The amount of sediment attributed to stream bank erosion is minimal. The sediment shown here is a result of poor upland soil condition and cover. There are a few small watersheds that are entirely contained within Blue Jay Barrens. The clear water entering from the right is from one of these watersheds. Proper land management could allow these streams to flow clear despite the amount of rain received.

Tree Bridge

This small tributary once flowed over the top of these tree roots. Now it flows beneath. Trees don’t begin life floating in the air, so at one time the stream bed had to be quite a bit higher. This scene has changed very little since I bought the property. The stream has eroded down to the layers of shale and erosion of the shale is a much slower process.

At one time these roots probably stretched along the stream bed. The stream must have been stable for quite a while for the roots to reach this size. Something happened to cause the stream to go through a period of rapid change. It most likely had something to do with the way the land was being used at the time, but I could have been that the tree finally reached a size large enough to change the pattern of water flow.

The roots look like they could become animated and walk the tree right up the bank. They seem to be well anchored in each bank. The tree doesn’t seem in any danger of falling. Water passing over that tangle of roots would have had a varied pattern of drops and swirls. One tiny change could have set in motion a series of events that resulted in drastic changes in the stream bed.

The positioning of the tree certainly makes an odd sight. At some point in the past, water came around the left side of the tree and created a small channel. That channel has since stabilized and is no longer eroding. Water following that channel could have been the catalyst that started the stream channel to erode.

The tree, a Box Elder, is the largest in the area. The trunk has a bit of a bow, but it still appears to be healthy and vigorous. Box Elders are known for growing in wet situations, so it’s not surprising to find that it once grew in the stream. I expect it to be around for quite a while.