The Hurricane Agnes flood, the greatest ever recorded in the drainage basin of the Conestoga River, raised two significant questions: why was such massive flooding caused by such a relatively small total rainfall? and why did this catastrophic event cause only minor geomorphic changes in channels and flood plains? Total precipitation averaged only 8.5 inches in 54 hours. However, other factors-including high rainfall intensity in one 10-hour period, even distribution of rainfall throughout the basin, pre-Agnes saturation of the ground, the storm's arrival in June before crops were well established, and Man's shortsightedness-combined to produce exceptionally high runoff. The insignificant geomorphic effects of the storm also resulted from a combination of factors. The low stream velocity of flood waters associated with low gradients, a 15-20 times increase in stream widths and water depths rising 7-20 feet above floodplain surfaces minimized the water's erosive power. Also, the silt and clay loads of the flooding streams throughout much of the basin failed to provide effective abrasive tools for channel widening. Deposition on channel bottoms and floodplains was also minor because by the time the flood was ebbing, the suspended silt-clay load had diminished markedly and there was little sediment to deposit except at scattered localities possessing special characteristics. The effect of this flood with a several 100-year recurrence interval was to transport large quantities of fine sediment out of the basin but not to alter significantly the shape of channels or the contour of floodplains.