Problems Facing Urban Streams

Water Quantity: too little or too much water.

Increased/Decreased Flow:   When rain hits grassy, forested, or other unpaved surfaces, the water soaks into the ground. Much of this water is absorbed by plant roots and is stored in soils. The water that is not absorbed or stored, gradually enters streams and creeks. Stream flow slowly increases to a peak flow, and later slowly decreases to a stable flow maintained by water stored in soils.

When rain hits paved, concreted or other impervious surfaces (rooftops, metal, etc.), the water does not get a chance to filter through the soil. The water immediately becomes runoff. This runoff rapidly moves to the streams and rivers, picking up sediment and pollutants as it travels. Once it reaches the streams and rivers, it causes the water to rise very rapidly and increases the size of the peak flow. This rapid increase flushes fish, insects and other creatures away before they have time to retreat to safe areas.

Particles transported by the rapidly moving water can act as a sandblaster. This process is called scouring. Scouring can undercut root systems and destroy aquatic vegetation and aquatic animals. In addition to the negative effects on larger animals and plants, it can also destroy microscopic animals in the streams. The larger animals, such as fish, eat the microscopic animals. Once the microscopic animals are destroyed, fish no longer have food to survive and prosper.

Large quantities of water flowing quickly through stream channels can cause the banks of the stream to erode, adding sediment to the stream and causing habitat loss. Stream banks may become unstable and fail, resulting in sloughing of adjoining property (such as backyards and roadways). This same increased flow drastically changes the structure of the stream bottom by washing out rocks, logs, and vegetation. These structures provide shelter and food for animals living in the stream.

With more water moving across the land surface and less water making its way into the ground, there is little water being stored in the soils. Consequently, streams are subjected to decreased flow. Water once gradually released from soils is no longer available to maintain stream flows during drier periods. This decrease in flow is also contributing to habitat destruction and causing stream temperatures to increase. Increased stream water temperature results in lowering the ability of the water to carry oxygen. Lower oxygen levels may kill fish.

Habitat Loss - Removal of Riparian Vegetation:  A riparian area is defined as an area of trees, shrubs and other plants located next to, and upslope from, a body of water. Riparian buffer zones are vital to healthy streams, water quality, and healthy ecosystems. Riparian zones provide for the uptake and long term storage of nutrients, water. They also regulate water temperatures by shading the stream, which increases the stream’s ability to hold oxygen. Riparian vegetation also helps filter pollutants and debris, stabilizes stream banks (reducing erosion and sediment transport), and provides for wildlife habitat.

Wetlands:  Wetlands are areas where water is near, at, or above the level of the ground. Biologically, wetlands are rich and diverse ecosystems. Yet, they are often under-appreciated and generally the first to be destroyed by either filling or draining. Wetlands contain plants and microorganisms that help filter excess nutrients and toxins in the water. Wetland areas also retain water, which assists in the prevention of flooding in low-lying developed areas. Wetlands support a large number of threatened and endangered species, including migratory birds, fish, amphibians and wildflowers. Protection of natural wetlands and the construction of new wetlands to replace what was destroyed is key to the development of an effective water quality program and a healthy environment.

Channelization:  Channelization is the practice of dredging and straightening stream channels to increase flow rates and carrying capacities. Channelization acts to force as much water as possible away from an area in a short period of time. Traditionally this was done to prevent flooding. Beargrass Creek concrete channels are a good example. However, there are problems resulting from channelization. Channelization is detrimental for the "well being" of streams and rivers through the elimination of suitable habitat and the creation of excessive flows. Stream bottoms are made more uniform. Habitats of natural streams contain numerous bends, riffles, runs, pools and varied flows, and tend to support healthier and more diversified plant and animal communities than those in channelized streams. A riffle is the shallow area within a stream where the flow of water is constricted and forced over rocks and ledges. Deeper, slow moving water and finer sediments characterize a pool. A run is an area between a pool and a riffle where the water flows a little quicker but is not broken up by rocks and ledges. Historically beaver dams were a very important part of a natural stream habitat. Most channelization and maintenance practices restrict the growth of riparian (streamside) vegetation. This vegetation filters sediments, shades the stream, and provides needed plant materials that seasonally fall into the stream. These plant materials become food and habitat for organisms. The elimination of shading, coupled with the lower water levels, result in temperature elevations of 5-20 degrees Fahrenheit above those found in streams with trees in riparian areas. These temperature elevations create increased stress on fish during the warm weather months due to the warmer water holding less oxygen. Dissolved oxygen stress is common in channelized streams.