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Measuring Trends in Channel Width
<br />The portions of the Green River investigated in this report include study reach 1 extending
<br />from 161 to 279 kilometers below Flaming Gorge Dam (near gauge 109261000 and including
<br />gauge 09307000 at 155 kilometers and 269 kilometers below the dam, respectively, and
<br />sediment sampling points A, B, and C located 164, 250, and 275 kilometers below Flaming
<br />Gorge Dam, respectively) and study reach 2 extending from 465 to 509 kilometers below the
<br />dam (including gauge 09315000, 465 kilometers below the dam), see figure 1. These reaches
<br />were selected to examine the effect of distance from the dam on channel changes.
<br />Black and white aerial photographs from five time periods were examined for reach 1: 1952,
<br />1963-64, 1974, 1978, and 1986. The 1952 and 1963-64 photographs represented
<br />pre-construction conditions, and the remainder represented post-construction conditions. The
<br />1952 photography covers only 51 kilometers of river, 39 kilometers at the north end of the reach
<br />and 12 kilometers at the south end. The incomplete 1963 and 1964 photography sets were
<br />combined to cover the entirety of reach 1 and designated 1964. Changes in channel morphology
<br />between 1963 and 1964 were assumed to be insignificant. Photographs differed in scale:
<br />1952-1:20,000, 1964 and 1986-1:24,000, 1974-1:31,680, and 1978-1:33,000. Photographic scale
<br />was verified by measuring distances between known points on the photographs and comparing
<br />them to distances on topographic maps.
<br />Reach 1 was divided into alluvial and geologically confined segments. This division was
<br />accomplished using aerial photographs, topographic maps, and aerial reconnaissance of the
<br />valley. Allowing for the discontinuous nature of the 1952 photography, three segments were
<br />analyzed for changes between 1952 and 1964 (predam): 161 kilometers to 193 kilometers below
<br />the dam-alluvial, 193 to 200 kilometers downstream from the dam--confined, and 269 to 279
<br />kilometers below the dam-alluvial. Four segments were analyzed for changes in channel
<br />morphology during 1964, 1974, 1978, and 1986: 161 to 193 kilometers below the dam-alluvial,
<br />193 to 238 kilometers downstream of Flaming Gorge-confined, 238 to 256 kilometers
<br />downstream of the dam-alluvial, and 260 to 279 kilometers below the dam-alluvial. Total
<br />alluvial and confined river lengths analyzed were 69 kilometers and 45 kilometers, respectively.
<br />Four photography dates were examined for reach 2: 1952, 1974, 1981, and 1987. The 1952,
<br />1974, and 1987 photographs were black and white, and the 1981 photographs were
<br />color-infrared. The photographs differed in . scale: 1952-1:20,000, 1974-1:34,500,
<br />1981-1:38,500, and 1987-1:26,000. The entire lower reach lies within an alluvial river segment.
<br />Each photograph was fitted with a mylar overlay on which the river channel was drawn. The
<br />river channel was defined by delineating the high water embankments and vegetated islands. A
<br />stereoscope was employed to assist in locating high water embankments. Following
<br />photointerpretation, the river channel was transferred to 1:24,000 scale overlays using a
<br />1 Gauge numbers given are those designated by the U.S. Geological Survey.
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