SI-4C (12/1/04 version)
Watershed-Based Options To Address Stream Integrity Impairment: Establishing Stream Integrity Targets
As noted in the overview, once a stream integrity problem is identified, the causes are defined, and predictions about future disturbances and associated in-stream responses are generated to define the "as is" stream integrity situation. Steps 6 and 7 define the "desired" stream integrity situation by identifying the ultimate channel form necessary for water quality attainment and determining if intervention is feasible to achieve the desired endpoint. Options and considerations for making these decisions are outlined below.
Ultimate Channel Form
Data for indicators of channel stability, stream classifications, and noting the current stage of channel evolution can be used to establish the gap between current conditions and those associated with a self-sustaining stream channel.
Particularly for entrenched channels, a productive approach to communicating the difference between the assessed "as is" channel morphology and the desired morphology, is to show an existing stream channel cross section plot overlain with the target cross section. An on-line
spreadsheet and instructions is available to illustrate these differences. Examples of the results of this type of assessment are available by clicking here.
Water Quality Attainment
Detailed information about the indicators used to evaluate the ecological aspects of stream integrity are available by selecting the applicable stream system subunit at this link. Available assessment results for each stream sub-unit can be accessed by clicking here. Desired states for individual watersheds are often expressed as a target QHEI value in TMDL Reports with habitat recommendations.
Feasibility of Intervention
Once stream morphology and biology assessment data are evaluated to compare "as is" to "desired state", there are many criteria that can be used to evaluate the feasibility of intervention to achieve the desired state and the intervention options available. Feasibility criteria may include, but are not limited to, local, state and federal regulatory requirements, desired land uses, short versus long-term water resource use goals, intervention cost-benefits, timelines for recovery, and ability to significantly contribute to achieving water quality standards. In the context of the NPS Management Plan, consideration of intervention feasibility criteria will focus on aquatic life use attainment.
Key considerations include the following:
- Regardless of the selected intervention, implementers must expect incremental change over time. Recovery potential, in shorter time frames with more manageable impacts, is greatest for channels that have been modified by straightening without being lowered.
- When stream channel/floodplain encroachment occurs, prospects of stream integrity recovery diminish;
- Intervention may be necessary to achieve QHEI targets recommended in TMDL Reports or local watershed action plans. However, habitat improvements will not achieve water quality attainment without a self-sustaining stream channel.
- Identifying and managing the causes of the problem(s), versus treating the symptoms of the problem(s), results in long-term cost-savings. For example, the natural formation of new floodplain is the aspect of stream evolution that takes the most time. However, without human intervention, lower energy channels do not easily recover active floodplains;
- Changing existing channel form is expensive (tens to hundreds of dollars/lineal foot). Therefore, active channel restoration is most appropriate on sites that are extraordinary for some reason. Passive channel restoration, through allowing and setting aside the areas needed for natural recovery has a long timeline for recovery of vertical stability. However, passive recovery of streams offers the most economical model for improving basic stream morphology.
- Areas where construction involving channel modification or re-location is planned, for reasons other than stream restoration, offer potential opportunities to improve stream morphology to achieve a sustainable stream channel.
- Several issues integral to the local land development process have
significant consequences for stream morphology. Guidance on this topic is available through The Center for Watershed Protection.