Altered Sediment Loads
Accelerated erosion from human sources has been described as the most significant cause of pollution in the Midwest region (Waters 1995). Poor soil erosion control methods during construction and other earth disturbing activities, operation of hydropower facilities, poorly designed and maintained road crossings in stream and wetland systems, and recreational activities on stream banks all lead to high sediment loads in aquatic systems. When sediments overwhelm a system, the geomorphology (pattern, dimension and profile) may be disrupted, leading to undesirable changes and further instability of the system (Rosgen 1996).
In Michigan, instream sediment tends to be sandy in nature. Excess sand bedload can cover and bury more productive gravel and cobble substrates. Productivity declines significantly as sand fills interstitial spaces in the coarser substrates, reducing habitat for macroinvertebrates. Sediment can also smother fish eggs or fry deposited on the stream bottom. Increases in trout production have been documented following removal of excess instream sand bedload (Alexander and Hansen 1982).
Excess sediment in riverine systems can cause a stream bed to aggrade, resulting in accelerated bank erosion and streams that are shallow and overly wide. In coldwater systems, these changes can lead to increased temperatures that negatively affect temperature-sensitive aquatic species. Once stream morphology becomes unstable, excess sediment load tends to convert meandering streams with pool-riffle complexes into long homogeneous runs. This subsequent loss of stream bed diversity can adversely affect many populations of aquatic organisms (Hynes 1970).
Dams often have an opposite effect on stream sediments. Water released from dams is free of sediment, because the impoundment has captured the upstream sediment load. These 'sediment-starved' conditions increase stream power (the ability of the system to move sediment, based on flow, sediment load and gradient). This increase accelerates erosion immediately downstream of the dam and causes excess sediments to accumulate at the first area of slow water further downstream, leading to stream aggradation. In sections immediately below a dam, excessive down-cutting can cause stream armoring due to decreased sediment load, resulting in reduced habitat.
Conservation Needs to Address Altered Sediment Load Threats:
Land, Water & Species Management
- Implement and enforce soil erosion prevention measures at soil disturbance locations
- Use best management practices for design and maintenance of road stream crossings, installing clear span bridges when possible
- Require run-of-river operations of dams to retain natural hydrography and prevent excess erosion downstream
Law & Policy
- Establish zoning ordinances which require native vegetative buffer zones along all water courses
Education & Awareness
- Educate recreationists about activities on stream banks that can lead to an imbalance of sediments in the aquatic system
- Provide financial incentives to private landowners to encourage soil conservation practices
Research, Surveys & Monitoring
- Continue to conduct research to determine the effects of runoff from agricultural landscape features on lowland and aquatic features
- Inventory and track erosion sites to prioritize remediation activities
- Monitor soil erosion prevention measures at soil disturbance locations
- Conduct research to determine long-term success of efforts to provide financial incentives to private landowners to encourage soil conservation practices
- Develop and test best management practices for design and maintenance of road stream crossings to reduce erosion