Land Use Influences on RGA Scores and River Channel Widths
Land use impacts the health and morphology of the rivers within a watershed. Urbanization is by far the most forceful change in land-use to affect the hydrology of an area. Agricultural land use also impairs stream health and morphology. The increased amount of sediment, nutrients, and pesticides are characteristic of agricultural land use. Amplified sediment inputs alter the flow and channel characteristics of the stream, while increased amounts of pesticides and nutrients have the potential to be devastating to aquatic ecosystems. The Wallkill River and Papakating Creek have similar drainage areas, stream order (5th), climatic conditions, and bedrock lithology, and are adjacent to each other, allowing them to be paired in a space-for-time experiment testing the response of rivers to land use changes.
The health of six sites along the Wallkill River and seven sites along the Papakating Creek was evaluated using Rapid Geomorphic Assessments (RGA). RGA’s appraise the health of each site on a qualitative scale by evaluating primary bed material, bank protection, degree of incision, degree of constriction, erosion, stream bank instability, vegetative cover, bank accretion, and stage of channel evolution. The higher the score, the worse condition the stream is in. The percentages of urbanized, forested, and agricultural land within the drainage area of each site were determined through the use of geospatial analysis from 2007 USGS data.
Historic stream widths were obtained from the New Jersey Department of Environmental Protection’s (NJDEP) iMap geographic information system (GIS). Three time periods: 1930, 1995-7 and 2007 were used to identify changes in stream width. Sites were selected based on the visibility of the riverbanks and landmarks such as bridges, allowing them to be identified at different years. The 2007 aerial photo set was taken during the winter so streams have visible banks in almost all areas. Five width measurements were taken at each selected site and averaged to provide a single measurement for the period.
Figure 1: The percentage of urban land use within the drainage is of each site compared to the RGA score at that site. As the percentage of urban land increases, the RGA score increases signifying poor stream health.
Figure 2: The amount of impervious surfaces compared to the RGA score. As the acres of impervious within the drainage area increases, the RGA score increases. This high RGA score represents poor stream health.
Figure 3: The percentage of agricultural land use within the drainage area of each site compared to the RGA score at that site. As the percentage of agricultural land increases, the RGA score also increases. The high RGA score shows that as the percentage of agricultural land within the drainage area increases, the health of the stream deteriorates.
Figure 4: The percentage of forested land within the drainage area of each site compared to the RGA score at that site. As the percentage of forested land in each drainage area increased, the RGA score decreased. This decrease in RGA score represents better stream health.
Figure 5: The drainage area of each site compared to river's change in width over the past 77 years. The increase in urbanized land within the Wallkill watershed has caused sites with greater drainage areas to experience a greater impact resulting in a greater change in width.
Figure 6. Hydrology team performing total station survey.