Groundwater can compound this problem by adding weight to the material above and reducing the bluff's resistance to shear. Groundwater seepage out the face of a bluff between different sediment layers acts as a lubricant between layers and further reduces the stability of the bluff. When this occurs the material in an upper layer may slide downward in a mass. Rain saturating a bluff can increase the rate of erosion in a similar fashion by adding weight to the bluff and decreasing the shear resistance of the shore deposit. Rain also produces runoff which can cause surface erosion creating gullies and ravines and steepening bluffs.
Erosion occurs in a seasonal sequence.
In the summer and fall, a steepening and undercutting of bluffs occurs as material is removed from the toe of the bluffs by wave action. Winter is a fairly static period because of freezing temperatures and lake ice. In the spring, however, frequent rains, storm runoff, increased groundwater levels and storm waves cause already steep, undercut bluffs to become unstable and mass wasting occurs. As long as wave energy carries material away from the base of the bluff and groundwater saturates the bluff to create an unstable situation, the bluff will never reach a point of equilibrium and will continue to recede. Thus, recession of the shoreline is continuous.