ABSTRACT
Lightning and hailstorms may be associated with tornadoes or flash floods, and periods of winter snowfall, which give rise to an avalanche hazard, may later swell rivers with spring meltwater, causing a seasonal flood risk. The scientific study of surface water flows and the physical characteristics of flooding is called hydrology. The stream hydrograph is a graphical measure of changes in discharge over time, as recorded or predicted for a given measuring station in the drainage basin. It can be drawn for individual flood events. Estimates of the magnitude and frequency of floods can be improved by dating the sedimentary deposits which they leave behind on the floodplain. Drought can be defined as a condition of abnormal dry weather resulting in a serious hydrological imbalance, with consequences such as losses of standing crops and shortage of water needed by people and livestock. In the usa mean annual snowfall increases northwards, in mountainous areas and towards the Great Lakes.
