Goals of Stream Restoration

According to Fischenich (2001), the conversion of forests, farmland, wood lots, wetlands, and pasture to residential areas and commercial and industrial developments directly impacts stream and riparian corridors by:

  • Altering stream channels through straightening, lining, or placement of culverts.
  • Reducing riparian corridor width through floodplain encroachments.
  • Increasing sediment yield during development and increasing pollutant loading following development.
  • Displacing native riparian plant communities by invasive non-natives.

Indirect impacts of this urbanization include:

  • Greater and more frequent peak storm flows, and longer duration of stream flows capable of altering channel beds and banks.
  • Enlargement of the channel through incision and widening processes.
  • Decreased recharge of shallow and medium-depth aquifers that sustain base and low flows.
  • Increased stream temperatures and higher nutrient and contaminant loading.
  • Alteration of the channel substrate.
  • Reduction of stream system function.
  • Reduction of riparian corridor function.
  • Reduction of native wildlife species.

The goal of restoration is to mitigate these impacts by returning the ecosystem to a close approximation of its condition prior to disturbance (NRC, 1992). In the process, ecological damage to the resource is repaired, and ecosystem structure and functions are recreated. A committee of the National Research Council (NRC) has noted that meeting this goal is difficult in urban ecosystems because the basic hydrologic, geomorphic, physical, biological, and biochemical processes have been forever altered. Nevertheless, limited systematic actions can be taken to enhance the system, including:

  • Developing buffers that provide protection to existing habitats.
  • Enhancing surface water management with facilities specifically designed to reduce adverse hydrologic and geomorphic impacts, to improve water quality, and to protect fish and wildlife habitat.
  • Undertaking stream corridor enhancement and restoration activities that will remediate existing problems or prevent future problems.
  • Implementing regulations and taking management actions that are aimed at reducing future adverse impacts of development.

These actions follow the ideas proposed in The Nature Conservancy's (TNC) Freshwater Initiative (TNC, 2001).

Hydrologic Engineering Study Objectives and Outputs

Measures of achievement of stream restoration goals are not defined in USACE guidance or authorizing legislation with the same degree of specificity as the national economic development goal. Consequently, the role of a hydrologic engineering study cannot be defined as concisely. Instead, the information required from a hydrologic engineering study will depend upon the particulars of the actions and measures proposed.

Nevertheless, in a USACE stream restoration effort, the study team should agree upon conditions that are desired and indices for measuring the degree to which these are satisfied by a project. From this set, the hydrologic engineer, working cooperatively with other team members, can identify relevant information required. Typically, the information will be much the same as required for damage-reduction studies: peaks of specified AEP; and volume, duration, depth, and velocity of specified AEP. Water-surface profiles and inundated area maps for specified events may be required also.

For restoration projects in which performance with low-flow conditions is critical, other indices may be of interest. For example, flow, velocity, and depth-duration functions may be desired for assessment of impacts of runoff on habitat development. From these, for example, the likely depths and durations of inundation during prime growing season of grasses can be found.
A hydrologic engineering study for stream restoration planning must assess watershed and channel conditions both with and without proposed changes. This will provide the information necessary to measure the effectiveness of different restoration alternatives.

Authority and Procedural Guidance

The following authorities have been used by USACE offices to restore aquatic habitats and mitigate development impacts:

  • WRDA 1986, Section 206. This legislation directs USACE to carry out aquatic ecosystem restoration and protection projects.
  • WRDA 1986, Section 1135. This authorizes USACE to modify existing project structures and operations to restore environmental quality. This subsequently was amended to include restoration project areas that are outside USACE project lands, but which were impacted by the project.
  • WRDA 1974, Section 22. With this, Congress gave USACE general authority to provide assistance to States and tribal governments with planning for the development, utilization, and conservation of water and related land resources. Recent amendments have expanded this assistance to include ecosystem planning.
  • WRDA 1992, Section 204. This authorizes USACE to protect, restore, and create aquatic habitat, including wetlands, in connection with dredging for authorized federal navigation projects.

Procedural guidance for conduct of hydrologic engineering studies to support restoration activities is given in USACE EMs (Engineer Manuals) and ERs (Engineer Regulations) cited in earlier chapters of this document.