The pilot of the amphibious airplane departed on a personal flight and performed several water landings on a lake. Upon leaving the area of the lake for the return flight to the departure airport, he lowered the landing gear (deviating from his standard procedure of lowering the landing gear while in the airport traffic pattern). Upon reaching the airport, and realizing there was still daylight remaining, he elected to perform one more water landing on a nearby river. Approaching the river, he lowered the flaps but neglected to verify the correct position of the landing gear and touched down on water with the landing gear extended. After touchdown the airplane nosed down and came to an abrupt stop resulting in substantial damage to the fuselage. The pilot stated that there was no preimpact mechanical malfunctions or failures or of the airplane that would have precluded normal operation.