3.1 Explanation
Humans affect stream ecosystems in a number of ways, both directly and indirectly. Streams in urban areas are exposed to particular types of human impacts that cause characteristic physical, chemical, and biological attributes. These attributes of urban streams are sometimes referred to as the “urban stream syndrome.”
Humans in towns and cities impact urban streams by creating large amounts of impervious surfaces. Impervious surfaces include roads, parking lots, rooftops, sidewalks, and driveways — constructed surfaces that prevent rainfall from penetrating the soil. Since a large portion of the rainfall in urban areas no longer penetrates into the ground, it is intentionally routed to storm drains and enters directly into urban streams as run off.
Relationship between impervious cover and surface runoff. Impervious cover in a watershed results in increased surface runoff. As little as 10 percent impervious cover in a watershed can result in stream degradation. From Stream Corridor Restoration: Principles, Processes, and Practices. Federal Interagency Stream Restoration Working Group, 1998. .
Runoff entering urban streams can carry all kinds of chemicals and pollutants with it (e.g., motor oil, pet waste, lawn fertilizers, and even sewer leakage). Perhaps the biggest impact of runoff entering urban streams, though, is the increased rate of storm flow. When an urban (post-development) stream experiences a rain event, the flow rate is much higher than in a natural stream, and this high flow rate brings with it energy that can erode stream banks and push sediment into the water, increasing turbidity. Urban streams may also have lower base flow rates (rates of water flow in the absence of rain events) than natural streams. This means that organisms in urban streams must be able to cope with both fast flows just after storm events and slow flows between storm events.
Stream flow after a storm surge, pre- and post-development.
Over time, the faster storm flow rates and resulting erosion in urban streams can lead stream channels to become deeper and more incised. The water table then typically becomes lower because of the deeper, narrower streambed. The result is a “floodplain” that rarely floods, a riparian zone where tree roots cannot reach the water table, and riparian trees that die of drought stress. When riparian trees die, urban streams receive more direct sunlight and experience increased temperatures (and thus have lower saturated dissolved oxygen levels) and greater diurnal variation in temperature.
Changes in stream channel shape, floodplain, and water table after development.
The high energy storm flow in urban streams also affects the diversity of habitats present. The stream channel can become straighter and more spatially homogenous in terms of width, depth, flow rate and substrate type. Thus, urban streams tend to have decreased habitat diversity, which affects the type and diversity of organisms they can support. The diagram below shows a natural stream that meanders, varies spatially in flow rates, and has both riffle and pool habit. It shows an urban stream with a straight channel and uniformity of flow rates, substrate sizes, and habitat types.
Comparison of flow rates, substrate sizes, and habitat types provided by natural and urban streams.
Stream health is a measure of the ability of a stream to support life. Stream health can be assessed in a number of ways, including physical measures (e.g., habitat diversity, channel shape, variation in substrate size), chemical measures (e.g., pH, dissolved oxygen, total dissolved solids, turbidity), and biological measures (e.g., the types and diversity of organisms found).
The “urban stream syndrome” described above results in deceased habitat diversity, increased erosion, and increased turbidity, among other factors. Therefore, we may expect urban streams to have decreased stream health compared to natural streams. Stream restoration projects are underway in urban streams all over the country to try and make urban streams more physically similar to natural streams, with the goal of increasing the health of these streams.
The Mud Creek study provides an interesting way to assess the ability of a stream to recover from the effects of urbanization. At Upper Mud Creek the stream is surrounded by a suburban neighborhood in a medium sized city and shows classic features of the “urban stream syndrome.” The creek then flows downstream into a forest that has been protected for almost a century and appears to have many attributes of natural streams at Lower Mud Creek. Mud Tributary is completely enclosed within the protected forest and has experienced no effects of urbanization. By comparing all three sites we are able to assess not just whether Lower Mud Creek is healthier than Upper Mud Creek but also whether Lower Mud Creek has fully recovered from the effects of urbanization so that it has stream health comparable to that of Mud Tributary.
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By Christine Muth, Leslie Brinson, and Emily Bernhardt.
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