Natural or human-caused changes to any of these characteristics can impart water quantity and quality. For example, the removing of vegetation by fire or through development will change the storage and infiltration characteristics of a watershed. Replacing natural vegetation with streets, parking lots, and roofs will increase the quantity of water runoff, resulting in a larger quantity of water in a stream or river, often increasing the chance of flooding. As water moves, it dissolves or carries many different materials. Streams, rivers, or lakes provide a fingerprint of the activities that take place in the watershed.
Floods usually result from events such as a large amount of precipitation or the failure of a dam.
The volume of water in a natural flood is a function of the size of the watershed, the amount of precipitation within that watershed, and the ability of the watershed to dissipate the water. The size or magnitude of a flood is described by a term called recurrence interval. On average, a 100-year flood would occur every 100 years. There is, however, a one percent chance that a 100-year flood could happen during any year. Also, as more information is available concerning the flow regime of a stream or river, and as changes to a watershed occur, the recurrence interval can change.