Water usually flows much more in some seasons, creating periods of drought and floods at extremes. The pattern of water flow in a river is the “annual hydrograph.” Its shape depends on the rainfall and snowmelt patterns and shape of the above-stream catchment. Water flow management is often done with dams, straightening some river sections, snow surveys, retreatment of waste water, and desalinization of salt water if necessary. Water transboundary issues either in rivers or aquifers leads to questions of who owns the water and who is responsible for its purity; these have generally been resolved peacefully. Most water is used for irrigation or industry. Water is used industrially for thermal and hydroelectric power generation. It is often used in manufacture processing and cooling and recycled or released after use—increasingly after being purified before release. Recently, it is used in “fracking” oil shale. It is used in canal transportation, households and commercial enterprises, recreation, and ecosystem protection—where irrigation dams keep water flowing continuously in the stream for aquatic species. Water also is managed to avoid floods and avalanches.