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""l '" CHANNEL NARROWING BY VERTICAL ACCRETION, GREEN RIVER A 1938 B 1993 . Flood plain . Persistent Island III Secondary channel D Main channel Figure 12. Loss of secondary channels, from geographic infonnation systems (GIS) analysis of aerial photographs. Map A shows an island com- plex in 1938. Map B shows the same area in 1993. Note the nwnber and extent of secondary channels that are no longer part of the active chan- nel in 1993. The outside margins of several floodplain areas have been converted to agricultural lands and are no longer included as part of the floodplain. able to settle out in the shallow low-velocity en- vironment of the inundated floodplain. The con- tinuum of processes through the lower two zones leads to the classic upward fining sequence com- monly identified in riverbank deposits. As aggra- dation proceeds, moderate floods no longer inun- date this surface. Upper Zone-High Elevations. In the upper zone, the deposit surface has been elevated to the point where only large floods can overtop it. The turbulence generally associated with these large floods is competent to carry coarser sand in sus- pension, which builds the floodplain at times of inundation. When large floods do overtop the surface, rapid deposition of coarse sand occurs as flow velocity is quickly reduced by vegetation. The fining upward sequence of zones 1 and 2 is thus disrupted by increased proportions of sand in zone 3. The thickness of these zones depends on the sequence of floods. On regulated rivers with large dams, intermediate floods are typically reduced but the frequency of very large floods is not changed. Thus, regulated river floodplains have the potential to have thick upper zone deposits because the frequency of moderate floods that build the intermediate zone deposits is restricted by the dam. Assessing Historical Channel Change The evolution of channels has been described throughout the world. The vast majority of these Upper zone Sand unit-1984 thick beds 4m Thin silt layer over ripples Figure 13. Photograph of the of sand Top of 1983 deposit stratigraphy within the excavation near the present U.S. Geological - Sand unit-1983 Survey cableway at Green River, Utah. The two organic layers were Intermediate zone the exposed ground surface for silt and/or sand several years between flood events. Upper organic unit-1973 The thick sand layers deposited 3m Lower organic unit-1965 during the large floods of 1983 and with root crowns 1984 are visible above the upper Lower zone organic unit. This figure also illos. bar sands Sand unit-1960-1965 trates the presence of three differ- ent zones within the deposit, each fonned by different processes that Silt-sand unit result in an upward coarsening of Germination root crowns the deposit. 1959 2m Sand unit-1958 Geolol'ical Society of America Bulletin. December 1999 1769