Indicators of Hydrologic Alteration in North Carolina Catchments: Small Ponds and Artificial Drainage

Alterations to stream hydrology, which include changes in stream geomorphology, are primary impacts of anthropogenic disruption. In North Carolina, hydrological alterations lead to environmental impacts through degraded ecosystems and water quality. In collaboration with the North Carolina Department of Environmental Quality, Division of Mitigation Services (DMS), the USGS South Atlantic Water Science Center datasets are proxy measurements of the extent of altered hydrology in riverine systems across the State of North Carolina. The datasets consist of an inventory and characterization of small scale (mostly agricultural) ponds and artificial drainages, which are both significant hydrologic modifications in the region. Ponds are impoundments that are often used for flood control, water supply, irrigation, and recreation. The impoundments constitute modifications to the stream network that fragment the aquatic habitat by limiting river network connectivity necessary for fish passage. A dataset was developed to quantify small ponds as proxy measures of barriers to flow in stream networks across the State. The USGS used a combination of the 2016 National Land Cover Dataset (NLCD) and the National Hydrography Dataset (NHD) to identify 105,560 small ponds that are less than 10 acres is surface area. The features are more refined than larger scale assessments such as those in the NHDPlus High Resolution and provide a starting point for determining mitigation strategies. Artificial drainage has major ecosystem impacts through the development of extensive ditch networks that reduce storage and induce large-scale vegetation changes. This has been a widespread practice of water table management for agriculture in Eastern North Carolina. However, these features are challenging to identify because the structure has been determined by non-natural factors. A dataset of open ditches was processed by calculating terrain openness (also called positive openness): a value based on a line-of-sight approach to measure the surrounding eight zenith angles as viewed above the landscape surface. The result from calculating openness with high resolution lidar-derived digital elevation models (DEMs) was then refined by masking natural water ways (stream valleys) and channels that are associated with transportation and urban areas.

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  "010:12"
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  "geospatial"
]