Streams Need Shade
As summer fades into fall in central North Carolina the weather can still be hot. On a hot day there is nothing better than a cool spot in the deep shade under a tree. Humans love shady spots. How about fish and frogs? It turns out they have to have it. Under natural conditions streams and rivers in North Carolina will have heavily wooded banks. Small streams will be completely shaded, and river banks will have pockets of deep shade even on the widest rivers. The reason frogs and fish and other aquatic animals have to have shade is they have to have cool water. Hot water will kill them. Under natural conditions overhanging trees and shrubs keep flowing water cool naturally. Sunlight and stormwater runoff from asphalt and concrete covered surfaces warms the water and the result is what is known as thermal pollution. To visualize the impact of hot paved surfaces on receiving streams, imagine walking bare-footed across an asphalt parking lot on a sunny August day. Bare feet would get first and second degree burns from the hot asphalt. When rain falls on the hot pavement, the heat from the pavement is transferred to the stormwater runoff and hot water runs directly into the nearest stream.
Elevated temperature typically decreases the level of dissolved oxygen of water. This can harm aquatic animals such as fish, amphibians and other aquatic organisms. High temperature limits oxygen dispersion into deeper waters, contributing to anaerobic conditions. This can lead to increased bacteria levels when there is ample food supply. Many aquatic species will fail to reproduce at elevated temperatures. A prime example of the harm hot water can do is the trout fishery in western North Carolina. The North Carolina mountains are the southern limit of the range of trout. Trout need cool water to grow and reproduce. In fact, water temperatures over 70 degrees F will kill trout. Water temperatures flowing off hot asphalt parking lots in August can reach temperatures as high as 120 degrees F. In terms of economic impact, a 2008 study in Western North Carolina valued the trout fishery at 174 million dollars per year.
While the trout fishery is high profile, all healthy streams should have fish and frogs and a complex aquatic food chain that depends on clean, cool water. The best way to protect streams is to allow stream-side vegetation to grow and thrive. Trees and shrubs not only provide shade, but they also stabilize stream banks and prevent them from washing away in times of high water. Trees, shrubs, and other vegetation filter surface waters before they flow into streams, and also purify subsurface water that feeds streams during dry weather. If you have streams on your property let them grow. Avoid spraying herbicides on stream banks as even grass and weeds can help stabilize stream banks during times of high flow. These suggestions apply to even the smallest steams, as even small streams eventually run to rivers. The forest industry in North Carolina adopted best management practices over 25 years ago that establish streamside management zones – SMZ’s – left alone along streams to protect water quality. As you ride around eastern North Carolina and see forests being clear cut, notice how the loggers leave the trees and shrubs along the streams alone. They are practicing good forestry, harvesting and replanting trees as a renewable resource, and protecting water quality at the same time.
A unprotected urban stream in Franklin County washing away in a heavy storm. Note the lack of any trees or shurbs on the stream bank to protect the soil from erosion.
A well-vegetated stream with healthy stream banks.
N.C. Cooperative Extension Service