A significant portion of impaired streams within the five states of EPA Region 6 (R6) include wadeable streams where water quality and physical habitat are degraded by changes in hydrologic regime due to watershed disturbances or land use changes.  Watershed disturbances which modify hydrologic characteristics, i.e. magnitude, duration, and velocity of storm flow, have a cascading effect on physiochemical and biological characteristics of the receiving waterbody (Figure 1).  Hydrological changes are commonly accompanied by water quality degradation due to increased pollutant loads to streams, i.e. nutrients, or by erosion of stream bed and banks, i.e. sediment.  

 Under Section 303 of the Clean Water Act, waterbodies are deemed impaired when the concentration of one or more pollutants exceeds water quality standards developed to protect the designated uses of the waterbody.  Designated uses, such as recreation or fish and wildlife propagation are directly protected by the implementation of supporting numeric or narrative criteria.  Restoring impaired streams calls for the review of applicable water quality standards or the development of TMDLs designed to be implemented over time.  

As Figure 1 illustrates, water quality and biological stream characteristics are indirectly affected by watershed and hydrologic disturbance.  Using water quality and biological criteria to drive higher-level (watershed scale) restoration in impaired waters can be problematic.  In these cases, assigning or revising a single-pollutant water quality criterion may not be the most effective method to achieve restoration or preserve stream features.

The use of flow duration curves is one tool available to analyze and document the hydrologic patterns of streams.  In its simplest form, a duration curve is a graph representing the percentage of time during which the value of a given parameter i.e. flow, is equaled or exceeded over a specified period.  For example, low flows are exceeded a majority of the time, while floods are exceeded infrequently.  Because the full range of stream flows is included, patterns in the FDC can be used to indicate hydrologic condition (wetness or dryness and flood or drought) and may help to identify important watershed processes (e.g. base flow and storm flow), key pollutant delivery mechanisms (point source vs. non point source pollution) and seasonal patterns.  This information allows resource managers to devise and implement hydrologic targets for watershed-specific protection measures. 

Information derived from FDC analyses may serve as the foundation for load duration curves, on which TMDLs can be based, as well as implementation and protection activities.  In 2007, EPA published a guide to aid watershed and resource managers in using flow duration curves to establish flow-based water quality targets for impaired waters (USEPA 2007). The guide describes basic steps needed to develop duration curves to identify loading capacities, load and wasteload allocations, margins of safety, and seasonal variations.