Upstream floods are those affecting localized sections of a stream system and are caused by intense rainstorms and events like dam failures. They are typically short-term events. Downstream floods ' are those affecting large areas of a stream system and are typically caused by prolonged rain or extensive regional snowmelt. They typically last longer than upstream floods. Flood hydrographs and flood-frequency curves plot stream behavior versus time and are useful in assessing regional flood hazards.
Human activities aggravate flood hazards.
These include covering the land with impermeable materials, filling in floodplains, installing stonn sewers, and removing surface vegetation.