Measuring Changes in River Geometry Using Aerial Photographs and Field Surveys: Narraguagus and Sheepscot Rivers, Maine
We study channel geometry of the Sheepscot and Narraguagus rivers in coastal Maine. We work in mainstem reaches, where channel gradient is generally <0.5% and bankfull width is 10-30 m. We used georeferenced historic aerial photographs (from 1940-1946) and recent digital orthophotoquadrangles (DOQs from 1996-1997) to make measurements at 100 m intervals along channel thalwegs. We also surveyed selected reaches of both rivers, at the same locations used in our aerial-photograph comparison, measuring bankfull and wet (active- channel) widths. From aerial-photograph analysis, the Sheepscot River exhibits modest narrowing over time, with 2 of 5 subreaches showing statistically significant change (2-7 m). The West Branch of the Sheepscot River also shows a statistically significant narrowing trend in 4 of 6 subreaches. The subreach with the most pronounced change (mean aerial-photograph width from 42 m in 1940 to 19 m in 1996) resulted from a dam removal. The Narraguagus River shows modest widening from 1944-1946 to 1996, with 2 of 6 subreaches showing statistically significant change (1-8 m), although this trend may be the result of high flow stage in 1996. We are also analyzing the influence of riparian-corridor land-use change on the observed trends in both watersheds. Comparison of field measurements of wet width in 2006-2007 with the 1996-1997 DOQs produced statistically indistinguishable results in 3 of 4 subreaches, in spite of higher flow stage during the aerial surveys. This suggests that aerial-photograph analysis provides a reasonable and inexpensive means to measure active- channel width in these systems. Our ongoing research includes comparing field and aerial-photograph channel geometry measurements in the Maine rivers with those from the Jacquet River in New Brunswick, Canada.
AGU Fall Meeting Abstracts
- Pub Date:
- December 2007
- 1825 Geomorphology: fluvial (1625);
- 1855 Remote sensing (1640);
- 1856 River channels (0483;