This paper describes landslides and debris flows triggered by intense rainfall in a very small gully in New York State. The incised channel is 150 m long, and 500 m3 of material were removed in one event, amounting to about 1% of the total volume of the gully. Large amounts of very coarse material were transported from this tributary to the floodplain of the larger stream. Although such events are relatively infrequent, they are an essential link in the total system of sediment movement, including both the enlargement of the gully and the addition of coarse debris to the larger stream.
Long-term processes such as weathering and creep are important in making material available for transport in extreme events. Different parts of the sediment movement system function at different spatial and temporal frequencies, and the interaction of these parts may be masked in data on total basin output. It is suggested that more attention should be focused on subsystem interactions of this type in studies of drainage basin dynamics.