General bank instability occurs when banks are too high or steep. If gravitational forces become larger than the resisting forces within the soil, mass failure (bank collapse) occurs. Since soils become heavier when they are wet, and since the pressure of water in soil voids tends to push the interlocking soil particles apart, general bank instability often follows prolonged rainfall, snowmelt, or high water. During high water, banks may become saturated with water, and drainage may lag the stream recession. Rapid drawdown of the stream therefore may trigger bank instability. Bank collapse may also occur when bank materials are changed (by cut or fill) or when bank angle is increased. Since bed lowering increases bank height, general bank instability often follows general bed degradation or headcutting. General bank instability may be manifest in various types of mass failure.

Photograph (courtesy of D. J. Hagerty) shows rotational slumping caused by rotational slip failure. Tilting of trees away from the channel and toward the top bank is an indicator of rotational failure. Rotational failures or slips occur along deep-seated, arc-shaped surfaces within high, cohesive banks with slope angles less than about 60 degrees. The bank line will have a scalloped appearance following failure.

Shallow slide failures occur in materials with low cohesion along planes parallel to the ground surface. Since the failures occur along shallow planes, the failure masses do not topple or rotate.

Slab-type block failures typically occur in cohesive banks with slope angles greater than about 60 degrees. They result in deep-seated failure blocks that slide or fall forward into the channel. Failure planes are parallel to the top bank, and tension cracking parallel to the bank is often observed in soils along the bank top prior to failure. The bank line will have a planar shape following failure. Slab-type failures are often triggered by toe scour.

Wet earth flows occur when soils become saturated to the point of flowing downslope as a highly viscous liquid. This condition can occur following prolonged rainfall, snowmelt, or rapid drawdown. Failures often leave lobate or jug-shaped voids along the bank, as circular or oval-shpaed zones of liquid soils flow out of relatively narrow notch-type openings. Dried mudflows and deltas may be found downslope of wet earth flows.