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Overbank flow width is very erratic, and not representative of bankfull channel width 100 99 98 z 0 97 ~ 96 > w ....J w~ 95 >- E c:~ <( 94 c: I- iD 93 c: <( 92 91 90 .. C!l> _. .0 ... e. .. .. o 20 ~ ALLRED AND SCHMIDT /r ,~' e-~'Laji9!Lbjl! _ y..~/ , . €I " ell ..", I bar I 1I3'Ia\iOtl-----. __ ,'\.o~.,.e_-...-- _~ " ~"::' ., , , . C!> " I e ,- , . .- ... _-., . <1>111> _. "".",,<IiD .. ....- .. . ... . . .. _.... ... ... ... ........ __ m. @. _.0 ....._ .......-..... __ 'CIlIiVGe ..., .~~ ... II> . . ...., . .... _ .- ........ -.111> ........G@ @... 40 60 80 100 120 140 Width near bankfull discharge is an accurate representation of active channel width . .. . -. ..... .. . ... 0 ...... e... ..-. ., eO .... o . eO. .,.. .: ..... ...... CD. ... ... .. .. ....... ........- Width at intermediate and low discharge is sensitive to topography of bed deposits DISTANCE (m) Figure 2. Schematic illustrating the importance of using the proper range of flows when determining bankfull channel width from discharge measurements. For example, the magnitude of the 2-yr recur- rence flood decreased by about 30% after 1930, and decreased by an additional 15% after 1963 (D.S.lnteragency Advisory Committee on Water Data, 1982) (Table 2). Thus, the 2-yr recurrence flood at Green River is now 45% less than it was between 1895 and 1929. The duration of the spring snowmelt flood is also shorter than it was in the beginning of the twentieth century (Fig. 3c). All flood frequencies were calculated using a Bulletin 17B Log Pearson Type III distribution. Effective Discharge The hydrologic changes that have occurred during the twentieth century have also de- creased the magnitude of those discharges re- sponsible for transporting the most suspended sediment through the study reach, when aver- aged over a period of years. We calculated the effective discharge for the three time periods described above by multiplying flow duration data for each period times the total suspended load transport relation computed by Andrews (1986). We did not compute the effective dis- charge for the period between 1957 and 1962, because this was a period when channel form was adjusting relatively rapidly. We assumed, 1760 as did Andrews (1986), that the sediment trans- port relation at the present cableway has not changed with time. Although Andrews (1986) had demonstrated that the post-Flaming Gorge Dam effective dis- charge curve had shifted toward lower discharges, our calculations demonstrate that the decreased magnitude of floods in the 1930s and 1940s caused a large decrease. in effective discharge prior to any large dam construction in the basin (Fig. 4). The modal value for the periods 1894 to 1929 and 1930 to 1957 are 1077 and 675 m3/s, re- spectively, which is a decrease of more than 37%. The further reduction in flood magnitude after dam completion in 1962 decreased the modal value to 494 m3/s, and there was a secondary modal value of 166 m3/s. Channel Bed Behavior The mean elevation of the thalweg at the pres- ent cableway has not changed appreciably since 1930, although the Green River annually scours and fills (Fig. 5). Historically, the cross section at the cableway scoured as discharge increased and filled as the annual flood receded. The magnitude of scour and fill may have decreased after 1962, however, the frequency of discharge measure- TABLE 1. DATES AND SCALES FOR AERIAL PHOTOGRAPHY OF THE GREEN RIVER NEAR GREEN RIVER, UTAH Dates of Approximate scale Mean daily Photo photographs of photographs discharge source (m3s-1) 7/7/38 1:12875 12100 National Archives 8/15/52 1: 16500 5820 CFSA - SLC, Utah' 10/2/52 1:16500 2220 CFSA - SLC, Utah' 6/16/62 1:16500 18700 CFSA-SLC. Utah' 8/15/85 1 :17 300 3310 CFSA - SLC, Utah' 6/13/93 1 :8875 14500 CFSA - SLC, Utah' 6/14/93 1:8875 13700 CFSA - SLC. Utah' 'Consolidated Farm Service Agency, Salt Lake City. Comments Negatives were enlarged to scale shown Negatives were enlarged to scale shown Negatives were enlarged to scale shown Geological Society of America Bulletin, December 1999