0 7)" .,.C NOTICE : This material may be protected by copyright MICHAEL F. STEPHEN `aw (Title 17 U .S . Code) . DONN S. GORSLINE Sedimentary Aspects of the New River Delta, Salton Sea, Imperial County, California ABSTRACT: Located at the southern end of the Sediment mineralogy reflects the composi- Salton Sea in Imperial County, California, the tion of its source material--Colorado Delta de- New River Delta has a subaqueous extent tritus. Organic carbon is characteristically high greater than 25 km2. The accumulating deltaic (5%, maximum) in fine marsh clays, mudflat, sediments are supplied by the New River at a and prodelta clays and low in delta front and rate of 500 x 106 kg suspended sediment distributary coarse silts and sand (0.58% annually. New River drains 6,500 km 2 over its minimum) . 150 km length . Climatic factors exert considerable control on Because of its unusually small size, the New deltaic configuration and the subaerial extent River Delta provides an excellent model for a of the New River Delta has been severely re- detailed analysis of deltaic facies distribution duced by rising water levels . The delta has ac- and relationships . Subaerial deposits compris- cumulated sediment at an average rate of ing distributary channel, levee, interdistribu- slightly over 1 cm/yr for the past 66 years . When tary subaerial flat and crevasse deposits are the the total volume of sediment in the delta, 0.024 most varied. Subaqueous deposits are largely km3, is compared with river discharge, it is prodelta clay and delta-front fine silt. apparent that most of the sediment is trapped Sediment size distribution is related to dis- by the delta . tributary patterns . Sand and coarse silt charac- Factors common to most modern deltaic mas- terize distributary channel and proximal delta ses such as abundant plant remains, high mica front environments . Lateral gradations from content, elongate sand bodies, highly organic sand to clay or sand to interbedded silt and clay prodeltaic deposits, and laminated sands over are common as one moves from distributary muds can be used in recognition of ancient del- channels to interdistributary areas. Vertically, tas. All of these features are manifest in the basal prodelta clays grade upward into silty fades of the New River Delta and indicate that delta front facies which either are overlain by similar balances between sediment input and interdistributary and marsh clays and silts, or local energy levels will produce similar by sands of distributary channels . sedimentary patterns regardless of scale . INTRODUCTION widths ranging from a few hundred meters to 2 The New River Delta, located at the southern km. The total area of influence is about 25 km2. end of the Salton Sea, Imperial County, Califor- Because the valley has been an important ag- nia (see Figure 1) is the product of the cata- ricultural area since the beginning of this cen- strophic flooding of the Salton Sink by the Col- tury, the water levels, rates of evaporation and orado River in 1905-1906 . The delta in 1972 had a water budget of the sea and its tributary streams subaerial length of approximately 3 km and have been recorded in detail. As a result, the history of the factors affecting delta develop- Stanton, 1930 ; Dibblee, 1954; Allison, 1964; ment are known for the entire period of deposi- Biehler et al ., 1964; Walker, 1967) . More recent tion. The delta provides a scale model of deltaic students of the area (see in Henyey and Bischoff, processes in an area unaffected by tides and 1973) have shown that this entire structural with low wave energy . The study was initiated province is intimately related to the movement in order to determine the sedimentologic simi- of the North American and North Pacific crustal larity or difference between this small feature plates in middle to late Tertiary time . and the equivalent lobes produced by single dep- Although much remains to be done in precise ositional cycles in large deltas of major rivers . If research, the sediments of the upper Gulf of the small delta is similar to the large ones, then California adjacent to the Colorado Delta record the results of contemporary investigations can changing rates and sources of sediments that be extrapolated to identify ancient deltas with must be a product of recent shifts in the Col- confidence that the differences in scale of indi- orado River's course (Byrne and Emery, 1960; vidual deltas will not affect the resulting charac- Van Andel, 1964 ; Thompson, 1968) . Work is in teristic assemblages of facies. progress by Bischoff and Henyey on the chemi- cal evidence from cores in the northern Gulf that will aid in documenting the late changes . Arnal (1961) has shown that the record of changing salinity following the formation of the Salton Sea can be read in the characteristics of the microfaunal assemblages in the modern sed- iments . Blake (1854) reiterated Indian accounts of great floods which forced tribes to flee the valley floor and move up into the surrounding moun- tains. A few radiocarbon dates for calcareous lake deposits (R . H . Merriam, personal com- munication) indicate that a lake filled the basin as recently as 700 years ago . However, no men- tion of any lake was made by explorers (Kino, 1701, as cited in Byrne and Emery, 1960) . The agricultural value of the southern Impe- rial Valley, referred to as the Salton Sink before 1905, was recognized as early as the 1850's by Blake (1854) . But not until 1900 was work begun to irrigate the basin using water diverted from Figure 1. Location map of the New River Delta and the Colorado River (see in Brown, 1923 ; Sykes, drainage area (after Dibblee, 1954) . Alamo River 1937). Inasmuch as the Colorado River had sent (AR) and Whitewater River (W) also drain into the its flood waters over its banks and into the Sal- Salton Sea . ton Sink for many years along the New River and Alamo River channels (Chase, 1919), it was felt diversion could be accomplished easily, History of the Salton Sea especially because the natural gradient into the Salton Basin is greater than toward the Gulf of The geologic record shows that the Colorado California. An artificial gap was excavated into River has alternately deposited its load into the the levees of the main river channel to lead water northern and southern portions of the great by canals to the Basin . Brown (1923, p. 11) notes: structural trough of which the Salton Sink and "Several unusual floods that occurred in the Gulf of California are part (Buwalda and the spring of 1905 greatly widened the arti- NEW RIVER DELTA, SALTON SEA 269 ficial breach in the river bank made to admit into the sea and 3600 x 10 6 m3 a year of water is extra water into the canals . These floods imported by canals for irrigation (Anon ., 1970) . also carried out the dams built to seal off this Average subsurface flow into the Salton Sea is inlet. .During the high water season of this estimated to be 60 x 10 6 m3 a year. Surface of the year, much water flowed through the canal lake in 1970 was about 70 m below sea level, and and over its banks and wasted into the basin maximum depth was about 15 m . at the bottom of the Salton Sink, where it The land surrounding the lake exhibits the formed the beginning of the present Salton typical physiographic features associated with Sea. Strenuous efforts were made to dam arid regions . Huge alluvial fans spread out from up the intakes, but flood followed flood, the surrounding mountains, sand dunes occur and one after another the structures were in the northern areas and along the western carried away . margin of the lake. Badlands carved in soft clays The Salton Sea grew day by day . as the and sandstones of the Mecca and Indio hills are whole flow of the Colorado River was pour- present along the borders of the basin. Arroyos ing through an opening now hundreds of dissect the surrounding desert flats . Vegetation feet wide. The water on its way . .ate great is sparse. canyons in the soft silt of the valley . These Temperature, precipitation, and evaporation canyons are followed by the present chan- values for the Salton Sea region are characteris- nels of the New and Alamo Rivers ." tic of mid-latitude deserts . Of these, the most The breach was finally closed in November, significant to our study are the rainfall and 1906, and the history of the New River Delta evaporation rates . starts from that date . Local records of water flow Rainfall ranges between a maximum monthly and suspended sediment discharge have been value of 17 mm precipitation in December to a maintained virtually from the date of initiation. trace recorded in June . Average annual rainfall over a 73-year period is 81 mm. Physiography and Climate Evaporation at a rate of about 1780 mm per Situated in a huge northwest-trending top- year (Blaney, 1955) is recorded with a maximum ographic and structural depression, and at Indio of 422 mm for the month of July . flanked by faults in the San Andreas zone, the New River Location, Length and Discharge Salton Sink's surface elevation is approximately 85 m below sea level. The depression is bounded The Imperial Canal, which branches from the on the north and west by the Peninsular Ranges Colorado River, and the Paredones River are which include the San Jacinto, Santa Rosa, tributaries of the New River at its initiation along Vallecito, and Laguna Mountains . To the east of a natural divide on the Colorado River Delta .