Hanging fluvial valleys form at mouths of tributaries that are unable to incise as quickly as the trunk stream. Although hanging valleys at tributary mouths are uncommon, in very rapidly eroding ranges, such as the Himalaya, they can attain heights of ∼1 km and display mean channel slopes exceeding 30°. Given a hypothesis based on bed load–saltation erosion for how such features form, this study addresses the question of why hanging valleys are not more common and what limits their growth. We implement a numerical model of bed load–saltation erosion for a tributary junction experiencing base-level fall to explore conditions that may lead hanging valleys to form or subsequently degrade owing to climate variability. We find that increased frequency of bed load mobilization and enhanced bed load supply can drive the degradation of hanging fluvial valleys. Moreover, when trunk aggradation overtops the hanging valley, the knickpoint tends to be removed during subsequent degradation of the alluvial surface. Channel narrowing, increased bed load size, or bed load supply each can inhibit the formation of hanging fluvial valleys. Under steady hydrological and sedimentologic conditions, bed load–saltation models predict hanging valleys could be quite common where main stem erosion is rapid. The frequency and magnitude of changes in climatically modulated sediment loads in natural systems, however, typically overwhelm factors that promote hanging valley formation without such bed load variability. Because rates of channel aggradation commonly outpace rates of hanging valley formation, we propose that hanging valleys tend to be preserved only in regimes where trunk-stream erosion exceeds several millimeters per year.