Assessing geomorphic change from increasing flood frequency and magnitude using stream gage data and aerial imagery

USGS's extensive stream gage network has significantly advanced our hydrologic understanding of watershed dynamics through the calculation of long-term streamflow measurements. These measurements include channel geomorphic properties through time such as channel area, stage height, bed elevation, and width. Several recent studies have used this data to assess changes in river dynamics in response to climate, land use, and hydraulic changes. To evaluate if gage-derived geomorphic data is appropriate for this type of analysis, we compiled river gage channel morphology data for 11 alluvial streams in New England, which have experienced increase flood frequency and magnitude in recent decades and compared this to aerial image-derived channel area change between pre-1975 and post-1975 data. Our preliminary results indicate that gage-derived geomorphic trends are not consistent with aerial image-derived trends. In locations with increased flood frequency and magnitude, gage data show no net aggradation and erosion for the region. However, aerial image analysis of these same sites shows consistent channel erosion since the 1970's. Other New England streams with steady or decreasing flood frequency and magnitude will be assessed to see if the observed trend will reverse or stay the same. In addition, this presentation will address the internal error estimation and methodology considerations. We believe this observed discrepancy between gage and aerial data is driven by preferential placement of gages in stable locations along the river. Dynamic river locations and in-channel storage zones, such as meander bends, islands, and sidebars, are excluded from gage derived data and are sensitive to changes in hydrologic function and sediment supply. For subtle changes in hydrology, gage-derived geomorphic measurements may be inadequate to quantify small alterations in river function and form.