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Gully expansion in the Ethiopian Highlands dissects vital agricultural lands with the eroded materials adversely impacting downstream resources, for example as they accumulate in reservoirs. While gully expansion and rehabilitation have been more extensively researched in the semiarid region of Ethiopia, few studies have been conducted in the (sub)humid region. For that reason, we assessed the severity of gully erosion by measuring the expansion of 13 selected permanent gullies in the subhumid Debre Mawi watershed, 30 km south of Lake Tana, Ethiopia. In addition, the rate of expansion of the entire drainage network in the watershed was determined using 0.5 m resolution aerial imagery from flights in 2005 and 2013. About 0.6 Mt (or 127 t ha<sup>−1</sup> yr<sup>−1</sup>) ofsoil was lost during this period due to actively expanding gullies. The netgully area in the entire watershed increased more than 4-fold from 4.5 ha in 2005 to 20.4 ha in 2013 (> 3 % of the watershed area),indicating the growing severity of gully erosion and hence land degradationin the watershed.  Soil losses were caused by upslope migrating gully heads through acombination of gully head collapse and removal of the failed material byrunoff. Collapse of gully banks and retreat of headcuts was most severe inlocations where elevated groundwater tables saturated gully heads and banks,destabilizing the soils by decreasing the shear strength. Elevatedgroundwater tables were therefore the most important cause of gullyexpansion. Additional factors that strongly relate to bank collapse were theheight of the gully head and the size of the drainage area. Soil physicalproperties (e.g., texture and bulk density) only had minor effects.  Conservation practices that address factors controlling erosion are the mosteffective in protecting gully expansion. These consist of lowering watertable and regrading the gully head and sidewalls to reduce the occurrence ofgravity-induced mass failures. Planting suitable vegetation on the regradedgully slopes will in addition decrease the risk of bank failure by reducingpore-water pressures and reinforcing the soil. Finally, best managementpractices that decrease runoff from the catchment will reduce the amount ofgully-related sediment loss

Morphological dynamics of gully systems in the subhumid Ethiopian Highlands: the Debre Mawi watershed