SUSTAINED REWORKING OF HYDRAULIC MINING SEDIMENT IN CALIFORNIA: G. K. GILBERT'S SEDIMENT WAVE MODEL RECONSIDERED

Hydraulic gold mining in the Sierra Nevada, California (U.S.A.) generated large volumes of sediment in the 19th century. Probing indicates that vast mining sediment volumes remaining in the lower Bear Basin are twice previous estimates. Topographic surveys document several meters of mining sediment erosion in the basin following a 1986 flood. The continued storage and mobility of sediment in the basin indicate increased sediment loads over pre-mining levels and call for reevaluation of Gilbert's symmetrical sediment wave model. In basins where channel storage is substantial, a sediment wave should be right-skewed in respect to time due to protracted sediment releases from easily eroded, unconsolidated deposits. Citation: James, L. A. 1993. Sustained reworking of hydraulic mining sediment in California: G. K. Gilbert's sediment wave model reconsidered. Zeitschrift für Geomorphologie, N. F. ; Suppl.-Bd. 88: 49-66. [This version is reformatted and is missing some figures. ] Introduction The long-term storage of sediment is of interest to engineers and geomorphologists because it has great bearing upon both sediment budgets and channel morphology. In addition, it is becoming increasingly clear that sediment storage represents a potential reservoir of toxic wastes that can be reintroduced to the environment during periods of channel erosion (GLYMPH and STOREY 1967, PHILLIPS 1986). The importance of sediment storage to sediment budgets is immediately apparent when the sediment mass balance equation is examined in its most elemental form: I = O + δS (eq. 1) where I is sediment input, O is sediment output, and δS is change in storage (DIETRICH & DUNNE 1978, SWANSON & OTHERS 1982). Many denudation studies assume that δS=0, although analyses of sediment delivery ratios have long indicated that this is not a rigorous approach (MANER 1958, ROEHL 1962, GLYMPH 1975, TRIMBLE 1975, 1977, DENDY & BOLTON 1976). In fact, the storage of sediment in channel systems is a critical component of the sediment budget over various time scales and accounts for sediment delivery ratios both less than and greater than unity (ANDERSON 1975, MEADE 1982, 1988, BERGSTROM 1982; WALLING 1983, 1988, KNOX 1989). Sediment storage is particularly important in studies of modern sediment yields due to accelerated rates of production by human activity (DOUGLAS 1967) and the high proportion of this sediment that is stored on floodplains. On Holocene time scales, large volumes of fluvial sediment storage resulted from land use changes in Europe (MACKLIN & LEWIN 1986) and in post-Columbian North America (KNOX 1972, 1977, TRIMBLE 1974, COSTA 1975). On very short time scales, storage has been related to channel morphology and sediment budgets in urbanizing channels (WOLMAN 1967), to seasonal sediment budgets on the Amazon River (MEADE & OTHERS 1985), and to exponential decreases in sediment loads in channels of experimental watersheds (SCHUMM, MOSLEY, & WEAVER, 1987:94). These realizations call for a reevaluation of a classic model of the transport of episodically introduced fluvial sediment. Gilbert's Sediment Wave Model The renowned geomorphologist, G. K. Gilbert, published a brilliant and comprehensive analysis of the nature, extent, and behavior of historical sediment in the Sierra Nevada in which he advanced a conceptual model of sediment transport as similar to a water wave: 'The downstream movement of the great body of debris is thus analogous to the downstream movement of a great body of storm water. The debris wave differs from the water wave in . that part of its overflow volume is permanently lodged outside the river channel. ' (GILBERT 1917:30) This model was derived primarily from time series plots of channel low flow bed elevations (Fig. 1). Gilbert (1917:36,46) understood the importance of sediment storage to the attenuation of sediment waves and considered most of the valley and canyon deposits of the mountains to be temporary, even coarse material (p.28). The permanent deposits he described were largely in low gradient portions of the lower Sacramento River, in bays near San Francisco, and near the mines (Gilbert 1917:27,46). Gilbert predicted the rapid depletion of most mining sediment stored in the mountains other than that which could be regarded as permanently stored: "After the lapse of, say, 50 years the annual tribute to the streams from both [piedmont and upland valley] deposits will have become so small that what then remains may be regarded as permanent. " (GILBERT 1917:67) [Figure 1. Gilbert’s (1917) sediment waves. ]