Lakes and reservoirs typically have been considered synonomous; in fact, Hutchinson classifies reservoirs as lake type 73. Processes such as internal mixing, redox reactions, nutrient cycling, and primary production obviously occur in both lakes and reservoirs. The forcing functions or driving variables for lakes and reservoirs, however, may not be identical so the response of these two systems may be different. Regional considerations of the climatic, geologic and geographic differences between lakes and reservoirs indicate why our understanding and available predictive techniques for lakes should be tempered for the proper management or reservoirs. Lakes and reservoirs generally are distributed in different parts of the U.S. Most lakes occur in the glaciated portion of the U.S. while most reservoirs are located in the southeastern, central, southwestern, and western U.S. Geologic differences in these areas have implications for differences in the loading of dissolved and particulate constituents to lakes and reservoirs. Climatic differences such as precipitation-evaporation interactions also result in distinct areas. In the eastern U.S., precipitation exceeds evaporaton, water is plentiful and lakes are prevalent. In the western U.S., evaporation exceeds precipitation, water is scarce and reservoirs are prevalent. Geographic differences influence the management of lakes and reservoirs. Operation of reservoirs for hydropower, irrigation, industrial and public water supply can influence the response of the system to external and internal inputs. The distribution of lakes and reservoirs in conjunction with geologic, climatologic, and geographic patterns would imply potential differences in the limnological response of lakes and reservoirs. Proper management of our water resources requires that we consider these potential differences in the decision process.