UNDERSTANDING CONTRIBUTIONS FROM EPHEMERAL RIVERS TO THE NORTH BASIN, LAKE TURKANA, KENYA

The North Basin of Lake Turkana, Kenya is a detritally-dominated, highly variable depositional environment that receives contributions from the perennial Omo River and smaller-scale ephemeral streams such as the Ileret and Tulu Bor Rivers. Only through understanding of the impacts of these two distinct fluvial styles can modern sedimentation in the North Basin be realistically characterized. Four short cores from the Ileret and Tulu Bor deltas were collected and analyzed. Data from these cores were used to document how ephemeral, flashy-discharge flooding events are preserved in the sedimentary record of Lake Turkana. In particular, littoral reworking of fluvial deposits was examined in detail to gain insight into how post-depositional processes alter the sedimentary record. The short cores are hypothesized to represent one flooding event, encompassing both deposition and locally reworking of sediment. During flashy-discharge events, reworking of previous deposits occurs and new sandy material is introduced to the lake. Detrital clays from Omo River’s sediment plume settle out of suspension for the majority of the year while the ephemeral rivers are dry. Pervasive wave action in Lake Turkana also reworks littoral sands, winnowing away fine material during periods of fluvial quiescence. This study shows that ephemeral rivers make significant, localized contributions to the basin and can have sedimentation rate of ~11.6 cm/yr or higher at the mouths of distributary channels. However, little of the original stratigraphy of this deposition may end up preserved in the sedimentary record at Lake Turkana, due to the high degree of lake level fluctuation and wave reworking. The localized nature of these depocenters makes lake-wide correlations difficult across the basins. However, by being able to identify and distinguish between localized deposition and larger, basin-wide processes like Omo River flood plumes, it may one day be possible to link records from the North and South Basins of Lake Turkana. Additionally, by connecting modern sedimentation to documented fluvial discharge events and climate fluctuation producing lake level rises and falls, older sedimentary records from the Holocene can be deciphered, giving valuable information about the history of Lake Turkana and the ecosystems that depend upon it.