1. A method of assessing a riparian area, comprising: mapping a physical extent of a potential riparian area (PRA) along a stream using first imagery having a first resolution; determining a first percent corresponding to basic functional groups of vegetation within the PRA using the first imagery; determining a second percent corresponding to basic functional groups of vegetation within the PRA, using second imagery that is contemporaneous with the first imagery and has a second resolution that is lower than the first resolution; determining a Normalized Difference Vegetation Index (NDVI) for the PRA using the second imagery; setting an NDVI threshold value for riparian vegetation in the PRA using the NDVI determined from the second imagery; converting the NDVI threshold value to a corrected NDVI threshold value using a computer system by: a) comparing the first percent with the second percent to approximate a corrected NDVI threshold value; and b) repeatedly adjusting the corrected NDVI threshold value until the second percent, modified by the corrected NDVI threshold value, approximately equals the first percent; and determining a proper function and condition (PFC) assessment of the PRA by applying the corrected NDVI threshold value to imagery comparable to the second imagery and corresponding to multiple years.