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......~ CHANNEL NARROWING BY VERTICAL ACCRETION, GREEN RIVER A c Figure 7. Matched photographs of the old ferry and cableway, approximately 9 kIn downstream from the town of Green River. (A) December 5,1911, photo looking west at ferry/cableway; (B) 1997 match of A; (C) 1911 photo looking east at ferry and cableway; and (D) 1997 match of C. Note that both the willow and saltcedar floodplain levels in D are not present in the 1911 photo. The photo source for A and C is U.S. Geological Survey archives. bankfull channel was about 125 m wide, and chan- nel width at this cross section did not change be- tween at least 1912 and 1928. During this period, floods and total annual streamflow were the largest that they have been during the twentieth century. Saltcedar had not yet established itself in the reach. Subsequent channel narrowing occurred dur- ing two different time periods that were separated by a period of channel stability. A first phase of channel narrowing at the present cableway oc- curred during a period of more than 8 years, and had begun by 1930 and ended before 1940. At the present cableway, the bankfull channel narrowed by about 4% from 113 to 108 m. Bankfull channel width was maintained be- tween 1940 and the late ] 950s. Thus, the adjust- ment of channel width to decreases in flood mag- nitude and total annual flow, that began in 1930, took place over a relatively short time period. Nar- rowing seems to have been primarily determined by these hydrologic changes, because saltcedar only began to be established in the reach at this time. The widespread proliferation of saltcedar occurred in the 1940s and 1950s in the study reach (Christensen, 1962) during a period when channel width was stable. Throughout this period of nearly 20 years, the difference between average bankfull width and the width of the low-discharge channel decreased from about 4.5 m to about 0.5 m (Fig. 15), and this decreasing difference repre- sents steepening of the banks, which could have been facilitated by increasing strength and density of saltcedar roots within the banks. Narrowing of the bankfull channel again began in the late 1950s and continues to the present. At the present cableway, the bankfull channel rapidly narrowed until 1962, and the channel has contin- ued to narrow slowly since then. These overall trends occurred elsewhere in the study reach; al- though we cannot prove that the periods of nar- rowing or stability were simultaneous everywhere. Rapid accretion occurred when flood dis- charges were less than 484 m3/s between 1959 and 1961. Saltcedar established itself on a newly emergent bar, and this bar persisted and was ag- graded by floods in 1962 and in later years. We cannot separate the relative effects of asymmetric distribution of shear stress at the present cable- way (Allred, 1997) from the increased resisting force of newly established vegetation in causing vertical accretion rather than bar scour on the right bank. Accretion has continued to the pre- sent, but the annual rate of accretion has declined, because only rare floods can now overtop this surface. Our data demonstrate that vegetation played some role in channel narrowing, consistent with Geolollical Societv of America Bulletin, December 1999 1765