Flooding at the Preserve
Highlights on Flooding at the Preserve
How Do Animals Cope With Flooding?The landscape changed dramatically at Jackson Bottom Wetlands Preserve during the final week of 2005. In mid December most of the ponds and marshes were frozen solid. Then drenching rain filled up the Preserve to the highest level in years. Thousands of waterfowl moved right in, and our phones started ringing with people asking, “What happens to the animals during a flood?” This is a very good question, although complicated to answer, so here is a short version. Rivers and their floodplains are connected as part of the same ecosystem. Over thousands of years the plants and animals living in these areas have adapted to flooding and have developed many different ways to survive. Trees such as cottonwoods and willows can survive highly saturated soils. Many of the animals simply move to higher ground. Birds fly to other areas in order to locate food and shelter, and many species take advantage of the flood waters. Waterfowl have more room to spread out in their search for food, and are more isolated from land predators such as coyotes and bobcats. Sparrows, finches and even a few warblers glean seeds and other food materials from the large mats of organic debris floating on the surface of the flood waters.
Hydrology of Flooding at the Preserve
The following table compares the water levels for different water events at the Preserve.
The following graph shows the water levels for the Tualatin River and the Gene Pool (a wetland marsh at the Preserve) during the winter of 2005-2006. Look at the peaks in the river water level. The water level usually crests about 2 or 3 days after major rain events. Note that the level of the river must rise to about 130 feet above MSL before overflowing into the Gene Pool.
The Significance of the Flooding at the PreserveJackson Bottom is part of the floodplain of the Tualatin River. After significant winter or spring rains (and other precipitation events), the Tualatin River overflows onto its floodplain, including Jackson Bottom. The wetlands at the Preserve actually helps with flood control by giving the river a place to go. As the water spreads out over the Preserve, the water slows down, dropping soil particles and other organic matter, which helps recharge the soil and cuts down on erosion along the river banks. Because the Wetland has been preserved as a natural area, flooding here is much more beneficial than harmful. For humans, it is a slight inconvenience because our trails are underwater, but it is truly fascinating to see the dynamic change in the Wetland during a flood. How Much Water Does Jackson Bottom Hold? During the floods of January 2006, Jackson Bottom held over 10 billion gallons of water! This is a rough estimate. How did we determine this? Water volume is often measured in units of acre-feet. One acre-foot is the quantity of volume of water that covers one acre to a depth of one foot. The floodplain of Jackson Bottom covers approximately 3000 acres. (The Preserve comprises 725 acres; Jackson Bottom also includes farmland.) A conservative estimate of the average elevation at Jackson Bottom is 130 feet above MSL. So, when the water level is 140 feet above MSL, the bottom is holding (3000 acres) * (140 - 130 feet) = 30,000 acre-feet of water. Since there are about 326,000 gallons in 1 acre-foot, at 140 feet above MSL, the Bottom has about 10 billion gallons of water! It's noteworthy that wetlands significantly reduce the damaging impacts of floods by holding all that water.
What is a 100-Year Flood?The notion of “100-year” flood causes considerable confusion. “Flood frequency” or “recurrence interval” is an estimate of the probability that a flood level will be equaled or exceeded in any given year. For example, a 100-year flood occurs on average once every 100 years and thus has a 1-percent chance of occurring in a given year. It is important to realize that 100-year floods can occur 2 years in a row.
Jackson Bottom Wetlands Preserve, Connecting
Water, Wildlife, and People.
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