Moisture-driven cracking triggers gully sidewall collapse: identifying critical thresholds in frozen Mollisols

• Threshold distances for crack initiation/growth near the gully sidewall were identified. • Crack depth develops before crack width; rapid widening starts after a critical depth is reached. • Crack depth has a greater influence on collapse than crack width. • Deep and shallow soil moisture control crack depth and width growth on gully wall. Gully erosion, accelerated by climate change, devastates agricultural land and soil resources worldwide. Soil cracking and gully sidewall collapse, influenced by variations in soil moisture and freeze–thaw cycles, accelerate gully expansion. However, the quantitative coupling between crack morphology (width and depth) and collapse, particularly under varying moisture–temperature conditions, remains poorly understood. This study develops a novel framework linking crack width and depth, and identifies critical thresholds that trigger sidewall collapse during the snowmelt period in Mollisols of China. High–resolution in-situ monitoring recorded spatiotemporal variations in soil moisture, temperature, and crack morphology across multiple depths (10–100 cm) and distances from the gully sidewall (0–80 cm). We identified a critical crack depth threshold (40–50 cm), beyond which rapid lateral widening commences, triggering collapse. Soil cracks predominantly developed near the gully sidewall (0–30 cm), with both crack width and depth decreasing with increasing distance from the sidewall ( p < 0.05). Greater average crack width (6.9 ± 3.9 cm) and depth (33.4 ± 11.5 cm) were observed on the left side compared with the right sidewall (width 1.8 ± 2.1 cm, depth 18.9 ± 7.4 cm). On the left sidewall, crack width and depth exhibited a strong exponential relationship (n = 570, R 2 = 0.89, p < 0.001). Crack depth increased more rapidly than crack width. Shallow soil moisture (0–30 cm) was primarily associated with crack width development, whereas deep moisture (60–100 cm) drove crack depth growth. Gully sidewall collapse was closely linked to crack depth growth, particularly during the rapid deepening phase in mid-April, with approximately 5 kg and 1.2 kg per day, on the left and right sidewall, respectively. These quantifiable thresholds provide a process-based foundation for predicting and mitigating gully expansion in agricultural landscapes under a changing climate.