Channel alteration
Altered river and stream reaches are those that are specifically identified as canals or ditches. These represent areas where the hydrography, flow, and water quality may be highly altered compared to natural conditions.
Networks are characterized by the percent of their total network length that is in altered river and stream reaches.
Highly altered
Rivers and streams altered by artificial channelization to create canals and ditches may have a lower variety and quality of in-stream habitat due to differences in hydrography, flow, water quality, and other factors compared to natural stream channels. Barriers with more highly altered upstream networks may contribute less natural habitat if removed.
Less altered
Rivers and streams with lower amounts of artificial channelization to create canals and ditches may have a wider variety of higher-quality in-stream habitat. Barriers with less altered upstream networks may contribute more natural habitat if removed.
Methods:
- Stream and river reaches are identified as altered where they:
- are specifically coded by NHD as canals or ditches
- fall within a waterbody specifically coded by NHD or other data source as a reservoir or one that has "reservoir" in its name, or a waterbody specifically coded by the National Wetlands Inventory (NWI) to indicate anthropogenic alteration (e.g., excavated)
- have significant overlap with a 10 meter buffer around NWI riverine areas that are coded to indicate anthropogenic alteration (e.g., diked/ditched)
- The total length of the network is calculated
- The total length of unaltered reaches within the network is calculated
- percent unaltered = 100 * (unaltered length / total length)