In this paper, the water level fluctuations of eight Ethiopian Rift Valley lakes were analyzed for their hydrological stability in terms of water level dynamics and their controlling factors. Long-term water balances and morphological nature of the lakes were used as bases for the analyses. Pettit’s homogeneity test and Mann–Kendall trend analysis were applied to test temporal variations of the lake levels. It is found that the hydrological stability of most of the Ethiopian Rift Valley lakes is sensitive to climate variability. In terms of monotonic trends, Lake Ziway, Hawassa, Abaya and Beseka experienced significant increasing trend, while Ziway, Langano and Chamo do not. In addition, homogeneity test revealed that Lake Hawassa and Abaya showed significant upward shift around 1991/1992, which was likely caused by climate anomalies such as the El Niño / Southern Oscillation (ENSO) phenomena. Lake Abiyata is depicted by its significant decreasing monotonic trend and downward regime shift around 1984/1985, which is likely related to the extended water abstraction for industrial consumption.

Lake Hawassa is a topographically closed lake in the Central Main Ethiopian Rift Valley. The water level of this lake has been reported to dramatically rise without falling back to the original level. The cause of this rise is not yet sufficiently investigated and subjected to this study. This study argues that the general variability in the lake level and its resultant rise has significant linkage to the temperature variability at the Pacific Ocean. The linkage between water level dynamics and climate variability was analyzed through the application of diverse statistical techniques. It comprises the Mann-Kendall trend analysis to test monotonic variations over time; sequential regime shift index (RSI) to detect significant shifts in the mean values of time-series records of lake level; and coherence analysis to investigate the linear relationship between ENSO index and records of local hydrology. Despite the multiple rises and falls, the results of the trend analysis revealed that the lake level experienced a significant resultant upward trend with Mann-Kendall τ values of 0.558, 0.629, and 0.545 (at α = 0.05 and p < 0.01%) for monthly maximum, average and minimum values respectively. The sequential regime shift evidenced that most of the significant shifts coincide with the occurrences of ENSO events. Generally, the lake level tends to be high during El Niño and low during La Niña episodes. The typical examples are the coincidence of extreme historical maximum lake level to the strongest El Niño event of the century that occurred in 1997/98 and the lowest lake level record in the year 1975 with a strong La Niña year. The coincidence of climate regime shift in the Pacific Ocean in 1976/77 with an equivalent regime shift in the lake level is an additional confirmation for the possible climate-hydrology linkage. The likely involvement of anthropogenic factors (at least in modifying the effect of climate) is justified by the interplay between the non-trending rainfall and potential evapotranspiration and trending streamflow. The coherence analysis between 492 pairs of monthly step datasets of 3.4ENSO index and lake level changes is also found to have a significant linear relationship over frequencies ranging from 0.13 to 0.14 cycles/month or 1.56 to 1.68 cycles/year. This corresponds to a dominant average periodicity (coincident cycle) of about 7.4 months which is thought to be related to the time span of the two rainy season in the locality.

The Ethiopian Rift Valley lakes have been subjected to environmental and ecological changes due to recent development endeavors and natural phenomena, which are visible in the alterations to the quality and quantity of the water resources. Monitoring lakes for temporal and spatial alterations has become a valuable indicator of environmental change. In this regard, hydrographic information has a paramount importance. The first extensive hydrographic survey of Lake Hawassa was conducted in 1999. In this study, a bathymetric map was prepared using advances in global positioning systems, portable sonar sounder technology, geostatistics, remote sensing and geographic information system (GIS) software analysis tools with the aim of detecting morphometric changes. Results showed that the surface area of Lake Hawassa increased by 7.5% in 1999 and 3.2% in 2011 from that of 1985. Water volume decreased by 17% between 1999 and 2011. Silt accumulated over more than 50% of the bed surface has caused a 4% loss of the lake’s storage capacity. The sedimentation patterns identified may have been strongly impacted by anthropogenic activities including urbanization and farming practices located on the northern, eastern and western sides of the lake watershed. The study demonstrated this geostatistical modeling approach to be a rapid and cost-effective method for bathymetric mapping.

Gully erosion is a serious environmental issue in Lake Hawassa Sub‐basin. It has affected a large portion of the catchment, and a large amount of sediment has been transported into the lake. This condition needs to be addressed. However, before conservation techniques are implemented, the gully systems should be mapped, their extent and changes over time need to be measured, and their capacity to produce sediment from their catchments should be determined. The objective of this study were, therefore, to map and analyse the change in gully erosion and quantify the sediment loading rate and sediment yield from the active and connected gullies in Lake Hawassa Sub‐basin using object‐based image analysis (OBIA) with high‐resolution SPOT 5 satellite image of the years 2011 and 2020. This method was integrated with field observation for mapping gully features of the study area, in comparison with automatic digitization carried out with the help of eCogenition Developer Version 9.1 and ArcGIS tools. Overall accuracy and Kappa coefficient were determined and were found to be 85.2% and 0.81, respectively, for the image of year 2020 and 81.1% and 0.76, respectively, for the image of year 2011. Based on the OBIA method, the extent of gullies (in area) were found to be 63.5 km ² in 2011 and 79.9 km ² showing a rapid increase between 2011 and 2020 (an increase of 16.4 km ² (24.4%) in the 10 years considered). The later result shows that 5.53% of the area of the Lake Hawassa Sub‐basin is affected by gully erosion. The maximum gully density in the study area was found to be 589 km/km ² in 2011 and this increased to 884 km/km ² in 2020. The sediment loading rate from the Lake's catchment was found to be in the range of 12.62 to 38.59 ton per hectare per year. The sediment yield from the Lake's catchment was 8.83 to 27.02 t/ha/year. The total annual volume‐based sediment yield at the Lake generated from the gully was 2.39 million cubic meter considering sediment delivery ratio of 70% for fully connected gullies. This result shows that 0.21% of the storage capacity of the Lake was being lost due to sedimentation from the gully system every year. From the result by dividing the total volume of the sediment by the surface area of the lake, one can see that a silt thickness of 2.51 cm was being deposited in the Lake every year.