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516 TOPPING ET AL: COLORADO RIVER SEDIMENT TRANSPORT, t ". 1130 UTAH ARI2llHA ~ 1120 PARIA R. LEES FERRY GAGE . USGSGAGH35TAT1ON o USGS WATER-OUAUTY STATION o PREOAM FUX)() DEPOSIT LOCATION 1 ! ~~ ~ ". GRAND GRAN) CANYON GAGE LCR NEAR CAMEfK)N GAGE , o ... GlEN CANYON DAM LEES FERRY GAGE , 50 Figure 1, Map of the Grand Canyon region showing the locations where the data presented and analyzed in this paper were collected. Numbers next to the locations of predam flood deposits indicate the river miles of these locations. of the bed [Leopold and Maddock, 1953; Brooks, 1958; Howard and Dolan, 1981; BurlcJwm, 1986]. Furthermore, seasonal de- creases in the predam upstream supply of sediment have been interpreted to cause coarsening of the suspended sediment during the annual snowmelt flood, resulting in inversely graded predam flood deposits [Rubin et al., 1998J, Since March 1963, Glen Canyon Dam has regulated flows on the Colorado River and has greatly diminished the supply of sediment in lowermost Glen Canyon and in Marble and Grand Canyons, Growing public concern over perceived degradation of the riverine environment downstream from the dam culmi- nated in the completion of the Glen Canyon Dam Environ- mental Impact Statement [U.S. Depal1mem of the Interior, 1995J. A major conclusion of this environmental impact state- ment was that the dam reduced the sand-transport capacity to a greater degree than it reduced the supply of sand, thus transforming a naturally net degradational system into one in which sand may accumulate over time, This conclusion was reached by applying concepts developed in the framework of classical alluvial channels with ample supplies of sediment and without a detailed investigation of the flow and sediment- transport processes that opera led in the potentially supply. limited predam bedrock-<:anyon river, The goals of this investigation are to determine the degree to which the predam Colorado River in Glen, Marhle, and Grand Canyons was supply-limited with respect to fine sediment (i.e., sand and finer material) and to quantify the major downstream sediment-related impacts of Glen Canyon Dam. In this paper we pursue these goals by first determining the degree to which the predam Colorado River displayed evidence of the four effects of sediment supply limitation. Then, we construct and analyze a sediment budget to determine if the predam and early postdam river in Marble Canyon and upper Grand Can- yon met the integral cons(raint of supply limitation, that is, to see if, and over what timescale, the river exported the fine sedimen( supplied to it. The work we present in (his paper draws heavily on both analyses of hisrorical flow and sediment-transport data col- lected by the U,S. Geological Survey (USGS) on both the Colorado River and its major tnbutaries and also on fieldwork that we have conducted in the posldam river. Our study reach extends from Glen Canyon Dam to the Grand Canyon gage (Figure I). For the purposes of this paper, Glen Canyon lies upstream from Lees Fel1)', Marble Canyon extends from river mile 0 at Lees Fel1)' to river mile 61.5 at the mouth of the Little Colorado River, and upper Grand Canyon extends from the mouth of the Little Colorado River to the Grand Canyon gage at river mile 87.4. We present and analyze historical flow and sediment-transport data that were collected by the USGS at five sites (four stream gages and one water-quality station). These sites are the Colorado River at Lees Fel1)', Ariwna (station number 09380000), herein referred to as the "Lees Fel1)' gage"; the Colorado River near Grand Canyon, Ariwna (station number 094025(0), herein referred to as the "Grand Canyon gage"; the Paria River at Lees Fel1)', Ariwna (station number 09382(00), herein referred to as the "Paria River Lees Feny gage"; the LitUe Colorado River near Cameron, Ariwna (station number 09402(00), herein referred to as the "LCR near Cameron gage"; and the Little Colorado River at Cam- eron, Ariwna (station number 094012(0), herein referred to as the "highway 89 bridge at Cameron." 2. Definition of Sediment Supply Limitation A complete definition of sediment supply limitation in a fluvial system requires: (1) precise specification of the grain sizes for which the system is supply-limited and (2) the time- scale over which the system is supply-limited for these grain sizes. In this sense a reach of a river will be supply-limited with respect to a certain sediment grain size over a chosen timescale if, over tha( timescale, the river has the capacity to transport more of that size class of sediment than is supplied to it. According to this definition most rivers are expected to exhibit some degree of supply limiration. A second implication of this definition is (har sediment supply limit:ltion does not require