Coal Bank Road site: Watkins Creek at its best

One of the first sites our Stream Team began monitoring is about where Coal Bank Road crosses over Watkins Creek. This is near where the creek empties into the Mississippi River. We chose this site to monitor for two reasons. The first is to learn about the state of the creek near its end, to get an overall picture of the levels of any contaminants. As I mentioned in the previous post, we are particularly interested in the levels of E. coli and chloride (salt), two pollutants found in Watkins Creek at levels exceeding Missouri state standards. By monitoring near the mouth of the creek, we can see the cumulative effect of these pollutants.

The second reason we monitor at Watkins Creek is because this is one of the easiest sites to walk to along the main stem of the creek (the longest path the creek takes from source to mouth). The photo below shows the west bank at this site, where we walk to the creek.

Notice the gentle slope (for Watkins Creek, at least) from the stream bank up into the riparian corridor. Creeks in watersheds with only small amounts of impervious (hard) surfaces like roads, rooftops, parking lots, and the like exhibit this sort of gentle slope up from the stream bank into the riparian corridor (the area just above the first rise from the stream bank). In such a watershed, rain falls onto absorbent soil and percolates through the soil down to the water table. The water table keeps the stream flowing during dry periods. The soil holds a lot of water during wet periods and what water it can’t hold flows slowly over the ground and eventually enters the creek. When it rains, the water level in the creek rises slowly and goes back down even more slowly. This keeps the velocity of the water rather low and the water level doesn’t vary greatly, thus the stream tends to not erode the surrounding land to a large extent. As long as the impervious surface in the watershed is less than about 10%, the stream will have this sort of gentle slope up from the bank and it will be easy to walk right up to the stream.

Watkins Creek has a much higher level of impervious surface in its watershed than the ideal of 10% or less. Overall the impervious surface is probably closer to 25% (I cannot find the exact number as I am writing but this is about right). Looking at a map of the watershed, the impervious surface is unequally distributed; a higher proportion of the hard surfaces are upstream of Lilac Road than downstream. Once the stream crosses Lilac Road, it can spread out a bit and the water velocity slow down somewhat. By no means does it do this enough to be called unimpaired by the time the creek crosses under Coal Bank, but it does slow down enough above Coal Bank that we can access it without using a rope.

Next time I’ll discuss some of the biological and chemical testing that we’ve done at this site and the results that we’ve obtained.