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	Flooding in Tillamook County A Recurring Problem Tillamook County has experienced major flooding throughout its history. During the 1990s, severe and repeated flooding in the county resulted in millions of dollars in property damage. Flooding also affects water quality and increases sedimentation. As floodwaters rise and leave their stream banks, they come in contact with industrial, agricultural, and waste products. These products and the excess sediment swept up by floods spread with the rising water and are deposited in streams, the bay, and onto the landscape as floodwaters subside. Because of negative impacts to water quality, aquatic wildlife suffer as well. There are a number of contributing factors to Tillamook's flooding problem. Many occurred largely in the past (the Tillamook Burns in the 30s, 40s and 50s) or have increased gradually over time (continued development of floodplains and construction of levees to channelize stream), making modern floodplain management a challenge. Over time, human activities have changed natural water systems in the Tillamook Bay Watershed. Wetlands have been drained to recover the land for other uses. Levees have been built to keep rivers in rigid channels rather than allowing them to meander as they would naturally. Dikes have been built to restrict flow. These are the typical landscape changes that accompany human development of an area. But these landscape modifications have changed the water flow and drainage patterns in the watershed. They have put people, property, and waste products in the path of floodwaters and, ultimately, they have increased the impacts of flooding.   What Do Wetlands Have to Do with Flooding? Wetlands act like natural sponges, holding excess water when it rains, then releasing it slowly. You can learn more about the connections between flooding and changes in natural landscapes at the University of Wisconsin's The Why Files Web site. The image to the right shows how Tillamook's wetlands have shrunk over time. Comparison of the two maps shows that once-continuous wetlands in the area around the town of Tillamook have been fragmented. Tan areas on the maps are nonwetlands. Given the changes that humans have made in the landscape, it is not surprising that the 100-year floodplain (red stipples) in the current map includes these areas that were wetlands in the 1850s.   Click on the map to compare historic and current wetland extent in a 3-D view. Warning - Image size: 65.6 Kilobytes Where Is Tillamook's 100-Year Flood Plain? A "100-year flood" doesn't happen only once every one hundred years. The term refers less to a flood event than to a flood elevation. As the Tillamook community knows firsthand, the 100-year flood elevation may be reached by floodwaters more than once in a relatively short period of time. The 100-year flood elevation has a one percent chance of being reached or exceeded by flood waters each year. The "100 years" refers to the statistical frequency, or the chance, of that happening. Consider the example of putting 99 white marbles and one red marble in a series of bags. Each time a marble is drawn from one of the bags, the statistical chance of drawing a red one is 1 in 100, or one percent, but a red marble could actually be drawn more frequently according to chance. Similarly, flood waters may reach the "once in 100 year" elevation more frequently than that. Red areas on the map at left indicate those locations at or below the 100-year floodplain elevation currently designated by the Federal Emergency Management Agency (FEMA) for the Tillamook Watershed. One of the CCMP flooding actions is to update this map. The 100-year flood elevation is the standard used by the National Flood Insurance Program (and most federal and state agencies) for floodplain management. Learn more about FEMA and flood zone designations at the: FEMA Flood Hazard Frequently Asked Questions Web site. Image and Data Credits What's Next? See what we're doing to reduce flooding impacts.