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2025 Hazard Mitigation Plan Kittitas County, Washington When a river is restricted by development and confined to a single channel, the velocity of the water increases, and flood elevations increase. As the velocity increases, the water gains power and can erode its banks, scour the riverbed, and damage structures. Floodplain characteristics also affect the river's behavior during a flood event. Native vegetation helps slow flood waters, allowing it to soak into the ground or move gradually back into the stream channel. Established root systems of riparian vegetation help to control erosion by binding the soil. On the other hand, when vegetation is removed, water runs off more quickly, often taking valuable topsoilwith it. The overall effect of removing native vegetation can be an increase in debris, bank erosion, and bed load moved by a stream or river. Lawns and other non-native plantings do a better job than paved surfaces or bare soil, but they are not as effective as natural riparian and wetland vegetation. Furthermore, floodplains also help maintain water quality by filtering surface water runoff before it reaches a stream or river. Naturalfloodplains help keep ground water recharged, and summer low flows are often higher in streams with functioning floodplains because ground water recharge is higher. More water soaks into the ground because plant roots and soil-dwelling animals keep the soil porous.lae Geomorphology refers to the relationship between the shape and other physical characteristics of a river and the rocks and sediments of the valley in which it flows. The river creates its channel, which reflects the force of the flowing water and the material of which the bed and banks are made. Changes in watershed conditions can affect the amount of runoff and the amount and size of sediment that enters the river. Changes in runoff and sediment loading affect the river's behavior, including flood characteristics. 150 4.6.5.2, HazardLocation ln Kittitas County, the Yakima River is the principle hydraulic feature. lts basin covers 1,594 square miles of the County. The major Yakima River tributaries include the Cle Elum and Teanaway Rivers (all forks) and many creeks including, but not limited to, Cabin, Cole, Gold, Silver, Manastash, Taneum, Naneum, Wilson, Reecer, Mercer, Big, and Little. The hydrology of an area is largely affected by climate, topography, geology, and glacial history. Understanding the hydrology of the basin helps planners to estimate the frequency and magnitude of flooding.-151 Additionally, temperatures and precipitation shape the flood hazard potential in Kittitas County. The amount of snowfall and snowmelt runoff rates are critical in determining flood potential. Most flooding in the Yakima and Teanaway river basins follow periods in which large amounts of wet snow accumulate and are associated with rain on snow events during which runoff cannot percolate into the soil because the soil has been saturated or because the ground is frozen. The Yakima River's character changes in response to local geology as it flows downstream. Much of the river is braided, with interlaced channels and gravel bars and an active channel area; however, there are areas where basalt geology constricts the lateral movement of the river. All forks of the Teanaway River generally are constrained in their upper reaches. Moving downstream to the Teanaway River valley, the river is fairly channelized but has free lateral movement. 14e Kittitas County. (1996). Comprehensive Flood Hazard Management Plan. Retrieved from https ://www.co. kittitas.wa, us/u ploads/documents/pu blic- works/flood/documents/1996%20Kittitas%20Countv%20Comprehensive%20Flood%20Hazard%20Manaqement% 20Plan.pdf. 150 lbid. 151 lbid. Chapter 4: Hazard ldentification and Risk Assessment 124