Stream Channel Integrity

Background

Stream channel integrity is most often threatened by increases in peak flows due to increased impervious surfaces and in sediment loads due to erosion and land disturbances within the watershed. Increases in peak flow rates during storm events alter the channel geometry by scouring the stream bed and eroding the stream bank, which results in wider and deeply incised stream channels. These channel modifications diminish the aquatic habitat conditions by slowing flow velocities and reducing the water depth during low flow periods. The bank erosion may also increase the sediment load downstream, which ultimately is re-deposited causing channel filling in lower velocity sections. Additional sediment contained in runoff due to erosion of disturbed soils along the shoreline or within the watershed may also alter the stream substrate by embedding cobble, gravel and rocks with a covering of silt and fine sediment. As stream beds become covered with fine sediment, the macroinvertebrate communities that once inhabited the stream bed are altered and become much less diverse, which, in turn, causes a ripple effect of change to the entire food web interactions of the aquatic ecosystem. Streams exposed to excessive sediment inputs are often impaired and support only a limited number of the most tolerant species of fish and aquatic life. These tolerant species are generally less desirable than the diverse aquatic communities sustained in non-impaired streams.

The primary means to minimizing these threats is by requiring proper erosion controls for terrain altering activities and requiring peak flow control for the smaller (e.g. two-year design storms) and larger storm events as development projects create more impervious surfaces. These requirements can be enforced both at the state and local levels. At the state level, erosion control and peak runoff control issues are addressed through the Alteration of Terrain (AoT) Program that is required for projects disturbing more 100,000 square feet (2.5 acres) or 50,000 square feet (1.1 acres) within a Shoreland Protection Zone. The existing AoT regulations and permit requirements are currently in the process of being updated and strengthened based on new information and recent policies adopted in other states. Timber harvesting and agricultural activities are not subject to the same permit requirements and are handled in a Permit-by-Rule process where general BMPs are required but are not reviewed on a project-by-project basis. One of the shortcomings of this Program is that NHDES does not have sufficient staff resources to adequately conduct follow-up site inspections and enforcement subsequent to issuance of permits. For this reason and the fact that many smaller projects fall below the permit requirement threshold, it is important for local communities to establish similar, if not more stringent, erosion and peak runoff control requirements as part of their zoning ordinances and subdivision and site plan review regulations. For local requirements to be truly effective in protecting stream resources, all communities within the watershed should have consistent regulations.

GOAL 4: The upper Merrimack River and its tributaries exhibit channels that are stable, self-sustaining, and capable of supporting diverse biological communities or biota.

OBJECTIVE: SC-1 – By end of 2009, through the use of existing data and visual observations compiled as part of the stream bank survey, develop a draft River Restoration Master Plan that identifies corrective measures to address identified stream channel integrity problems, and integrate these restoration objectives into the Management Plan.

Activities

• Conduct a visual survey of the entire designated river corridor using the White River Standard Operating Procedures to identify, photo-document, assess severity, and map existing stream bank erosion areas and areas with extensive streambed embeddedness.
• Use the regional river channel geometry equilibrium curves to assess and prioritize the most problematic areas and identify likely sources and causes.
• Compare and evaluate existing local subdivision and site plan regulations in UMR communities with respect to erosion and stormwater management. Develop a set of recommendations to update and improve local regulations in each community based on most current NHDES guidance and regulations.
• Develop a draft river restoration master plan that includes stream channel restoration design concepts and recommends measures and actions needed to reduce future channel degradation.
• Review methods and cost estimates for other river restoration projects in the region.

OBJECTIVE: SC-2 – By 2010, develop conceptual restoration plans for the three most significantly damaged stream channel sites, based on natural channel design principles.

Activities

• Finalize a river restoration master plan that includes concept designs, a description of the restoration action, and an estimate of funding needs to restore the three most significant and/or degraded stream channel sites.
• Coordinate with NHDES and other interested organizations such as Trout Unlimited to develop consensus on restoration design concept and to identify potential funding sources.

OBJECTIVE: SC-3 – By the end of 2010, secure sufficient public and private funding to restore the priority stream sites such that the restoration construction/implementation is accomplished by the end of 2011.

Activities

• Pursue and submit grant application forms to acquire necessary funds to accomplish restoration.
• Prepare construction design bid plans for contractor bid and selection.

Merrimack River confluence with Stirrup Iron Brook in Boscawen (Photo Provided by UMRLAC)