35 Surface Processes and Deposits (Posters)

Saturday, 26 July 2003
1:30 PM - 4:30 PM

HYDROLOGY, BIOLOGY AND SEDIMENTOLOGY OF A FRESHWATER WETLAND IN A SEMI-ARID ENVIRONMENT, LOBOI SWAMP, KENYA

Paper number: 35-19
Program: Technical Sessions
Time: 1:30 PM – 4:30 PM

Authors

Abstract

Loboi Swamp is situated near the equator on the western fault-bounded margin of the East African Rift. The freshwater wetland developed during the late Holocene on a low relief alluvial plain probably from subsidence associated with extensional tectonics. The swamp is ~ 2 km2 and is groundwater fed by springs emanating from the bordering grid fault system. Spring waters are ~35°C, with pH ~6.4-6.9 and the water compositions suggest that the sources are dominated by meteoric water with little contributed by deep re-circulating fluids. The climate is semi-arid. P is ~700 mm/yr on the valley floor and 1200mm/yr in the adjacent highlands; ET is estimated to be ~2500 mm/yr. Rainfall patterns vary with semi-annual monsoon rains and El Niño and La Niña periods (5-7 years). An irrigation scheme at the Swamp is utilizing wetland water for agriculture.

The modern swamp is dominated by Typha domingensis Pers. (~80%) and Cyperus papyrus L. (20%). The stratigraphy revealed in a soil pit and 8 piston cores (1.5-4 m long) records the formation of the wetland ~500 yrs B.P. Basal sediments are flood plain (sandy silts) comprised of a buried Inceptisol that is capped with a peat. The peat is thickest (1.5 m) in the spring-proximal area near the faults and thins to 0.30 m in the spring-distal areas. Siderite concretions in the flood plain silts reflect the Fe-reducing conditions that developed as the surface was submerged and developed a wetland. Stable isotopes record the abrupt shift from predominantly C4 vegetation to C3 vegetation. The diatom record also suggests a rapid change to wetter conditions.

Soils and paleosols developed on the periphery of the wetland reveal evidence for dramatic fluctuations in hydrologic budget, as indicated by formation redoximorphic features during wetter phases, and vertic (shrink-swell) features during drier phases. The pollen record shows a prevailing dry climate despite the development of wetter conditions at the valley bottom. The combined records of hydrology, biology and sedimentology of this wetland in an arid region suggests an abrupt change to wetter conditions due to tectonic event, rather than change in climate. The wetland is being maintained by groundwater sources, but may now be under stress due to recent anthropogenic impacts of land use change.