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Stream Assessment

Finger Lakes Winter
Snow clouds over the Watershed

     Considerable time is spent altering riparian lands or repairing stream channels. Very little effort is made to understand how riparian areas in the same watershed have responded to previous alterations and repairs.
     Stream classification provides a tool that allows better understanding and communication about stream systems. The objectives of stream classification include:

  • Prediction of a stream's behavior from its appearance.
  • Development of specific hydrology and geomorphology relationships for a given stream type.
  • Extension of data specific to a measured reach to other reaches having similar characteristics.
  • Providing a consistent frame of reference for communicating stream form and condition among the various people working and living along a stream. Rosgen, 1996, p. 3-3 .
         Dave Rosgen of Colorado, recipient of the 1993 Environmental Protection Agency Outstanding Achievement Award, has developed a stream classification system used throughout the United States. It is frequently modified to meet specific needs and extend local capabilities.
         The process of performing a Rosgen Assessment is divided into four levels. Much of the level I geomorphic characterization utilizes aerial photography and topographic maps. Riparian landforms are sorted into 11 valley types based upon the slope of the stream and topography of its valley. Rivers and streams are divided into 8 broad types on the basis of their channel and floodplain geometry. Individual level I reach classifications are checked by field examination of the reaches.
         The level II morphological description provides a much more detailed description based upon stream measurements at selected locations. Streams are further subdivided into 94 subtypes based upon degree of entrenchment, width to depth ratio, water surface slope, streambed materials, and sinuosity. Pictures of the Rosgen stream types may be seen in the Appendix.
         Of level II criteria, the width-to-depth ratio is the most sensitive and positive indicator of trends in channel instability. Rosgen, 1996, p. 5-2.
         Stream form and structure, as defined by Level II criteria, describe a stream's basic pattern. The level III assessment of stream condition:
  • Describes the potential stability of a stream, as contrasted with its existing condition.

    Streambank Failure

  • Determines the variance of a stream's existing condition from a reference baseline.
  • Provides guidelines for documenting and evaluating riparian vegetation, the pattern of channel deposits, channel obstructions, stream meander pattern, stream channel stability, and fish habitat.
         Careful documentation of the observations obtained in the Level II and III classifications permit the development of informed conclusions about stream condition.
         Level IV field data verification substantiates the extent and magnitude of stream channel adjustment processes indicated from collected stream and riparian area data. Such verification permits measurable extension of the stream classification technique to other areas having similar characteristics as determined from a Level III inventory. This approach blends both physical and biological function within a watershed context, providing a wide range of interpretations for natural resource management applications. Rosgen, 1996, p.6-3.
         The Level IV analysis includes measurements of stream sediment, stream flow, and stream stability. Trends over time are also monitored. This analysis allows evaluation of sediment transport relationships, prediction of the response of tributaries to changes in land use, and development of effective stream restoration activities.

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