Lake Manix, in south-central California, was the terminal basin of the Mojave River until the late Pleistocene, when it drained east to the Lake Mojave Basin. Based on new field observations, radiocarbon ages, and soil development, we propose modifications to previously published hypotheses on the timing of the last 543 m above sea level (masl) highstand of Lake Manix, the timing of the first discharge eastward, and the time required to cut Afton Canyon between the two basins. Subtle beach barriers, wave-cut scarps, and lagged beach gravels indicate that Lake Manix reached highstands between 547 and 558 masl at least twice prior to its previously known 543 m highstands. Properties of soils formed on beach barriers at 547–549 masl compared to soils on dated deposits suggest an age of older than 35 cal ka for this highstand. Calibrated radiocarbon ages for three lacustrine highstands at or near 543 masl are ca. 40–35 ka, 33–30 ka, and 27–25 ka. Lake Manix periodically discharged down a drainage presently located on the north rim of Afton Canyon at 539 masl. Soil development estimated from multiple buried soils within fluvial deposits and overlying fan deposits suggests that discharge was coeval with or somewhat older than the 547–549 m highstand, and that fluvial aggradation in this drainageway was followed by a period of relative landscape stability and episodic burial by alluvial-fan deposits. Strath terraces below these highest fluvial deposits, but above the canyon rim, record initial incision of the Lake Manix threshold. Surface and soil properties indicate that they are latest Pleistocene to early Holocene in age, similar to the previously studied strath terraces that are inset well below the rim and below the basal lake sediments. We suggest that the higher straths above the rim formed no earlier than ca. 25 cal ka. We interpret the soils, stratigraphy, and fluvial landforms in the canyon to indicate relatively rapid incision of Afton Canyon to the depth of the bedrock floor of Lake Manix, followed by intermittent, gradual bedrock incision.

ABSTRACT Lake Coyote, California, which formed in one of five basins along the Mojave River, acted both as a part of the Lake Manix basin and, after the formation of Afton Canyon and draining of Lake Manix ca. 24.5 calibrated (cal) ka, a side basin that was filled episodically for the next 10,000 yr. As such, its record of lake level is an important counterpart to the record of the other terminal basin, Lake Mojave, following the draining of Lake Manix. We studied lake and fluvial deposits and their geomorphology and identified five principal periods of recurring lakes in the Coyote basin by dating mollusks. Several of these periods in detail consist of multiple lake-rise pulses, for which we identified specific fluvial deposits that represent the Mojave River entering the basin. The pulsed record of rapid lake rise and decline is interpreted as switching of the Mojave River between Lake Coyote and Lake Mojave. A composite lake record for both basins shows nearly continuous lake maintenance by the Mojave River from 24.5 cal ka to ca. 14 cal ka. One potential gap in the lake record, ca. 22.7–21.8 cal ka, may indicate either temporary river routing to yet another basin or a dry climatic period. The Mojave River discharge was sufficient to maintain at least one terminal lake throughout most of the Last Glacial Maximum and deglacial periods, indicating that paleoclimate was moist and/or cool well into the Bølling-Allerød and that the lake records may not be sensitive to variations from moderate to high discharge. Nuances of lake-level changes in both the Coyote and Mojave basins are difficult to interpret as paleoclimatic events because the current chronologic control on lake levels from nearshore deposits does not provide the necessary precision. Mojave River avulsion leading to flow to Coyote basin may have been influenced by rupture on a dextral-oblique fault. Earliest post–Lake Manix stream deposits of the Mojave River leading to the Coyote basin are faulted, and most subsequent streams were confined to the downthrown fault block. This fault rupture and possible enhanced river routes to Lake Coyote, rather than Lake Mojave, are bracketed by dated beach deposits to the period ca. 20–19 cal ka. Later, headward erosion through the fluvial plain by the Mojave River eliminated flow to Coyote basin after ca. 14 cal ka and completed incision of the plain after ca. 12 cal ka.