Editing Flood (section)

===River flooding===
Floods occur in all types of [[river]] and [[stream]] channels, from the smallest [[Stream#Intermittent and ephemeral streams|ephemeral streams]] in humid zones to [[Arroyo (watercourse)|normally-dry channels]] in arid climates to the [[Amazon River|world's largest]] rivers. When overland flow occurs on tilled fields, it can result in a [[muddy flood]] where [[sediment]]s are [[Erosion|picked up by run off]] and carried as suspended matter or [[bed load]]. Localized flooding may be caused or exacerbated by drainage obstructions such as [[landslide]]s, [[Ice jam|ice]], [[debris]], or [[beaver]] dams.

Slow-rising floods most commonly occur in large rivers with large [[Drainage basin|catchment areas]]. The increase in flow may be the result of sustained rainfall, rapid snow melt, [[monsoon]]s, or [[tropical cyclones]]. However, large rivers may have rapid flooding events in areas with dry climates, since they may have large basins but small river channels, and rainfall can be very intense in smaller areas of those basins.

In extremely flat areas, such as the [[Red River of the North|Red River Valley of the North]] in [[Minnesota]], [[North Dakota]], and [[Manitoba]], a type of hybrid river/areal flooding can occur, known locally as "overland flooding". This is different from "overland flow" defined as "surface runoff". The Red River Valley is a former glacial lakebed, created by [[Lake Agassiz]], and over a length of {{Convert|550|mi|abbr=on}}, the river course drops only {{Convert|236|ft|abbr=on}}, for an average slope of about 5 inches per mile (or 8.2 cm per kilometer).<ref>[https://www.dnr.state.mn.us/watertrails/redriver/index.html "Red River of the North State Water Trail"]. Minnesota DNR. Accessed 8 September 2024</ref> In this very large area, spring snowmelt happens at different rates in different places, and if winter snowfall was heavy, a fast snowmelt can push water out of the banks of a tributary river so that it moves overland, to a point further downstream in the river or completely to another streambed. Overland flooding can be devastating because it is unpredictable, it can occur very suddenly with surprising speed, and in such flat land it can run for miles. It is these qualities that set it apart from simple "overland flow". 

Rapid flooding events, including [[flash floods]], more often occur on smaller rivers, rivers with steep valleys, rivers that flow for much of their length over impermeable terrain, or normally-dry channels. The cause may be localized [[convective precipitation]] (intense [[thunderstorm]]s) or sudden release from an upstream impoundment created behind a [[dam]], landslide, or [[glacier]]. In one instance, a flash flood killed eight people enjoying the water on a Sunday afternoon at a popular waterfall in a narrow canyon.{{Citation needed|date=April 2024}} Without any observed rainfall, the flow rate increased from about {{convert|50|to|1500|ft3/s|sigfig=2}} in just one minute.<ref name="Hjalmarson">{{cite journal|last1=Hjalmarson|first1=Hjalmar W.|date=December 1984|title=Flash Flood in Tanque Verde Creek, Tucson, Arizona|journal=Journal of Hydraulic Engineering|volume=110|issue=12|pages=1841–1852|doi=10.1061/(ASCE)0733-9429(1984)110:12(1841)|bibcode=1984JHydE.110.1841H }}<!--|access-date=2015-07-24--></ref> Two larger floods occurred at the same site within a week, but no one was at the waterfall on those days. The deadly flood resulted from a thunderstorm over part of the drainage basin, where steep, bare rock slopes are common and the thin soil was already saturated.  

Flash floods are the most common flood type in normally-dry channels in arid zones, known as [[Arroyo (watercourse)|arroyos]] in the southwest United States and many other names elsewhere. In that setting, the first flood water to arrive is depleted as it wets the sandy stream bed. The leading edge of the flood thus advances more slowly than later and higher flows. As a result, the rising limb of the [[hydrograph]] becomes ever quicker as the flood moves downstream, until the flow rate is so great that the depletion by wetting soil becomes insignificant.