Since 1996, GCSWCD has been utilizing and promoting the use of the Rosgen Stream Classification System. Developed after 30 years of stream monitoring by David Rosgen of Wildland Hydrology, the classification system is based on a number of delineative criteria associated with the stream's morphology. The Rosgen system is favored by the District and our partners at NYCDEP due to its use of measurable criteria and its consideration of the stream's sediment regime.
Why We Classify Streams
One of the first tasks GCSWCD completes when initiating a new stream corridor project, is the classification of the stream reaches. Classification is based on the "current" status of the stream and it provides several distinct benefits.
- Allows for effective communications between various disciplines, such as geologists, hydrologists and biologists working on stream management.
- Provides a consistent, replicable platform for integration of various stream resource inventories and assessments
- Assists with predictions of future stream behavior based on local knowledge of how different stream types respond to change.
Levels of Stream Classification
The Rosgen Classification System utilizes a series of 4 levels to classify streams, assess their current condition and to monitor future change (or document no change).
Level I - Geomorphic Characterization
Level I is a broad-level description of Rosgen’s major stream types. This description is based on general map and visual assessment of valley types; landforms; and the stream’s shape, slope, and channel patterns.
Level II - Morphological Description
Level II requires field measurements. This level assigns a number (1 through 6) to each stream type that describes the dominant bed material based on the reachwide pebble count and the particle size that 50% of the samples are equal to or smaller than.
Level III - Stream State or Condition
Level III is an evaluation of the stream condition and its stability; it requires an assessment and prediction of channel erosion, riparian condition, channel modification and other characteristics.
Level IV - Validation via Monitoring
Level IV is the verification of predictions made in Level III and consists of sediment transport, stream flow and stability measurements.
Natural Channel Design
Natural channel design is a method of stream restoration that attempts to create a stable stream channel that is capable of balacing flows and sediment loads. A naturally stable stream channel maintains its dimension, pattern and profile such that the stream does not degrade (erode) nor aggrade (rise). Naturally stable streams must be able to transport the sediment load supplied by the watershed. Instability occurs when scouring causes the channel bed to erode or excessive deposition causes the channel bed to aggrade. The Rosgen stream classification system categorizes streams based on channel morphology so that consistent, reproductible, and quanititative measurements can be made. This aids GCSWCD in developing stable natural channel design recommendations for our stream restoration projects.