Purpose <p>To mitigate erosion, soil and water conservation measures have been introduced widely, with the ambition to reduce on-site erosion rates and catchment sediment yield. However, the success of such measures has been questioned, and often lacks a scientific basis. This is especially true in Ethiopia where gullying has been reported to worsen because of the implementation of soil and water conservation programmes targeting sheet and rill erosion from cropland only. The current research therefore focuses on identifying the sources of sediment based on the dominant erosion processes at play.</p> Methods <p>This study was conducted in the Fota-Gumara catchment (211&#xa0;km²), situated in the Lake Tana Basin. The investigation was based on the analysis of fallout radionuclides (¹³⁷Cs and ²¹⁰Pb-ex) tracers, which are able to discriminate between topsoil and subsoil. Target material consisted of sediment deposits collected during eight rainfall-runoff events, which occurred in the early and late rainy season of 2023. The activity of ¹³⁷Cs and ²¹⁰Pb was measured using gamma spectrometry, while the source apportionment was based on the implementation of Bayesian Models BMM and MixSIAR.</p> Results <p>Our findings confirmed that subsoil was the major source of sediment. Both models showed consistent results, indicating that approximately three-quarters of the sediment originates from subsoils, contributing a median average of 73% and 81% according to MixSIAR and BMM, respectively. Both the inter-event and seasonal variability of sediment source contributions were relatively low.</p> Conclusions <p>We conclude that gullies should be a land management priority. Additionally, we demonstrated the validity of using fallout radionuclides for tracing sediment sources in this region of tropical Africa.</p>