7920

• UCREFRP Catalog Number: 7920
• Author: Van Steeter, M. M., J. Pitlick and B. Cress.
• Title: Aerial Photograph/GIS Analysis and Field Studies of the Grand Valley and Ruby-Horsethief Canyon of the Colorado River.
• USFW Year: 1995.
• USFW - Doc Type: \
• Copyright Material: NO

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4

AERIAL PHOTOGRAPHY/GIS ANALYSIS

Methods
Aerial photographs of both the Grand Valley and Ruby-Horsethief
Canyon were acquired. Photographs were chosen for 1937, 1954, 1968, and
1993. All photographs are black and white and 1:20,000 in scale. Photographs
from 1937 and 1993 were taken at moderate flows of approximately 270 ems
(9,500 cfs) at the state line gauge, and the 1954 and 1968 photographs were
taken at low flows near 70 ems (2,500 cfs), thus limiting comparisons between
photographs of similar flow. The outlines of specific features were digitized
using a computer aided design system (ACAD), and then areas were measured
with ARC INFO, a GIS software. Features included river banks, islands and
emergent bars, and side channel/backwater areas which are defined as all areas
other than the primary channel (Fig. 2). Our error estimates are::!: 2% for
water and island areas, and::!: 10% for side channel/backwater areas.

. islands
. side channels

0 500 meters
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0 1000 2000 feet

Figure 2. Map of main channel, islands, and side channel/backwater area