Geology of Shades Creek Greenway

Frog stranglers, gully washers, turtle floaters. Whatever you call an intense downpour, Birmingham gets a lot of them. When it rains that heavily, most of that stormwater flows downhill to our creeks. As the creeks rise, they spill over into adjacent low areas called floodplains.

Shades Creek Greenway is built on a floodplain. The soils of the floodplain are known as alluvium. Alluvium is a geological term for sediment deposited by a river or stream. In Shades Creek’s floodplain, the sediments are derived from the weathering and erosion of rocks in the mountains surrounding Shades Valley, especially chert, sandstone, and shale. Over long periods of time, these sediments have washed down and accumulated in Shades Valley to create the floodplain.

You can find a sample of these rocks by wading into the creek when the water is low and looking for rocks of different colors and textures. With every flood, these rocks tumble farther along the stream bottom and – with time – are ground down into fine sediments. During floods, these fine sediments are lifted and carried by swift currents and then deposited wherever the stream rises above the bank. The sediments alongside the Greenway were all deposited within the past 2.6 million years, during the current geologic period, known as the Quaternary.

Floodplains are constantly changing. Visit the Greenway after a big flood and look for places where swift currents cause the bank to collapse. Over time, streams and rivers carve their way through the floodplain as water follows the path of least resistance. The result is often a meandering path through the landscape.

Within the Greenway, however, Shades Creek is unnaturally straight. Why?  Curves in a stream slow down the water. This causes water to back up during a heavy rainstorm and flood adjacent areas. Flooding, plus the fact that meandering streams conflict with conventional construction methods that straighten stream channels as a common practice. Innovative methods such as Natural Channel Design can be used to benefit both construction needs and urban stream health.   Unfortunately, channelization – as river scientists call this process – destroys stream habitats for wildlife and increases flooding and erosion downstream. Farther downstream, however, Shades Creek still meanders freely. Efforts have been made to restore some of the stream’s natural habitat features with the use of cross vanes (learn more).