In general, a flood is a temporary condition in which a normally dry area of land or infrastructure is inundated by excess standing or flowing water. Floods can occur during any season and at any time. There are four principal sources of flooding that impact Thurston County:

1. River or Stream Riverine Flooding. Riverine flooding occurs when excess flow and volume of water crests a river channel’s normal capacity. Floodwaters consequently inundate areas within the river’s floodway, flood plain, and other low lying areas that may not be mapped as flood hazard areas. Flooding is a natural function of rivers, with their effects supporting productivity of wildlife and potentially increasing the fertility of farmlands within flood plains. Communities must balance the need to preserve the natural functions of floodplains with the need to protect property and human activities. Understanding how, when, and where to expect flood impacts is a first step in developing a mitigation strategy to minimize losses from floods and to protect the environment.
            
2. Groundwater Flooding. Groundwater flooding occurs when the water table raises due to persistent heavy rains. Areas of the county where an upper thin layer of permeable soils overlays an impermeable layer of hard pan is particularly vulnerable. As the ground absorbs more and more rainwater, the rate of infiltration of the stormwater is exceeded by accumulation of precipitation. Eventually the groundwater table rises as stormwater begins to pond on the surface. Since areas of high groundwater in the county are relatively flat, there is nowhere for flood waters to drain to. Flood waters can remain standing for several months, resembling ponds or lake like conditions. The Salmon Creek Basin experienced significant flooding in 1999, resulting in contiguous bodies of standing flood waters ranging from small puddles to over a hundred acres.
       
3. Coastal or Tidal Flooding. Spring tides, the highest tides during any month, occur with each full and new moon. When these coincide with a northerly wind piling water in south Puget Sound, tidal flooding can occur. Tidal flooding can also occur without the effect of storm surge. The tides can also enhance flooding in delta areas when rivers or creeks are at or near flood stage. Puget Sound marine flooding by itself does not produce major flooding in Thurston County. However, such flooding is expected to become more frequent and pose greater risks as sea level rise increases over the next 50 years and beyond. Downtown Olympia is at the greatest risk to coastal flooding and sea level rise.
              
4. Urban/Stormwater Flooding. Urban flooding occurs when excess precipitation is not readily absorbed by the ground, and stormwater runoff exceeds the ability of drain facilities’ capacity to safely convey and divert water. As a result, streets, parking lots, homes, and businesses may experience flooding. Excess water accumulation flowing off and over impervious surfaces from heavy rainfall or melting snow, over a short period, is the most common cause of urban flooding in the cities and developed areas of the county. Leaves, branches, snow or ice, and other debris that clog stormwater drains compound the problem. Other forms of urban flooding occur in residential neighborhoods constructed with insufficient stormwater conveyance capacity.