Why did an ancient basin suddenly experience debris flows outbreak and migration in recent 50 years?

Tongde Basin, situated on the east edge of Tibetan Plateau, has accumulated substantial deposits of pebble-sand. The uplift of the Tibetan Plateau has initiated incision in the upper reaches of the Yellow River, subsequently causing incision in tributaries like the Daheba River within Tongde Basin. This transitioned Tongde Basin from a sedimentary to an erosional context from 150,000 years ago. However, debris flow gullies began to develop along Daheba River since the 1970s and migrate upstream fast. The relationship between the location, outbreak time, and migration of debris flow gullies was analyzed using dendrogeomorphic dating techniques. The results indicate that the distribution of debris flow gullies is concentrated in the lower reaches of Daheba River, and the frequency of outbreaks is high. The distribution of debris flow gullies in the middle reaches is relatively small, and the frequency of outbreaks is low, but there is an accelerating trend. The distribution characteristics and frequency of debris flows are closely related to the depth of incision of the debris flow gullies and the thickness of the gravel-sand-layer. In the middle and upper reaches of Daheba River, the river incised through the pebble-sand-layer, resulting in exposed bedrock. The slope of the pebble-sand layer is steep but difficult to support large heights. The slope of the bedrock is gentle but can support a large height. Therefore, the gravel-sand sediment layer is prone to instability and continues to provide a source of debris flows under the action of river incision. The migration of debris flow gullies is related to the potential volume changes of unstable pebble-sand layers caused by the river incision. • Dendrochronological reconstruction indicates upstream migration of debris flows. • Slopes detachment drives debris flows migration. • Funnel-shape and tensile cracks of pebble-sand layer is unable to support high slopes.