Paleohydrology repeating? Regional hydrological change may lead to an overflow and cross-mixing of an alkaline and a freshwater lake in East Africa

Lake Bogoria and Lake Baringo, Rift Valley, Kenya Starting around 2010, several of Kenya’s Rift Valley lakes experienced significant rises in water levels. There is presently fear of an ecological catastrophe should the water levels of the alkaline Lake Bogoria continue to rise and subsequently overflow into and cross-mix with the freshwater Lake Baringo, a hydrological situation witnessed ∼8 000–10 000 years ago. In the last decade, data reveals significant meteorological and hydrological changes in the study region. Average annual rainfall has more than doubled, with substantial increases in variability. Despite a decrease in the number of rainy days within this period, intense rainfall days have increased by 318 %. Relative to the maximum water level observed in 2020, a 0.7 m increase in lake level would be sufficient for Lake Bogoria to reach the spill point located at about 1000.2 m, beyond which an overflow into Lake Baringo begins to occur. In 2023, the lake level declined by 1.5 m, resulting in a decreased risk of overflow of ∼20 %. Results indicate that mean annual rainfall after 2010 was high enough to provide an average annual flow of 500–1000 L/s from Lake Bogoria towards Baringo if lake levels were at spill point elevation. • Lake levels in the Rift Valley of Kenya have been rising since 2010. • Alkaline Lake Bogoria risks overflowing into freshwater Lake Baringo. • Annual rainfall has doubled since 2010; intense rain days surged 318% despite fewer overall rainy days. • Lake Bogoria neared overflow in 2020, needing only 0.7m rise; risk varied 16-77% across scenarios. • 2023 saw 1.5m level drop in lake levels, easing concerns by reducing overflow risk to about 20%.