Yosemite National Park has had 11 winter floods since 1916 that have caused substantial damage to property. All of these floods took place between November 1 and January 30. The largest floods occurred in 1937, 1950, 1955, and 1997 and were in the range of 22,000 to 25,000 cubic feet per second, as measured at the Pohono Bridge gauging station in Yosemite Valley. These floods were caused by warm winter rains falling on snow at elevations up to 8,600 feet (e.g., Tuolumne Meadows), partially melting the accumulated snow pack. The 100-year floodplain is the area that is inundated by a 100-year flood, or the annual peak flow that has a 1% chance of being equaled or exceeded in any given year. Prediction of the 100-year floodplain is necessary in order to comply with Executive Order 11988 (Floodplain Management) and with the NPS Floodplain Management Guideline. In order to predict the 100-year floodplain, it is necessary to perform a flood frequency analysis of the nearest gauging station data to determine the flow rate of a 100-year flood. This flow rate, along with topographic cross sections, is used by models to predict the inundation (or floodplain), flow velocities, and inundation depths of a 100-year flood event. The accuracy of these predictions is higher for areas near gauging stations, for areas with gauging stations that have been operating for many years, and for areas with more precise topographic cross-section data.
Floodplain Characteristics of Yosemite Valley, El Portal and Wawona
When the 1997 flood overran the park's bridges, receding waters left trees and other debris tangled in the railings. NPS Photo.
The floodplain of the Merced River in Yosemite Valley is well developed in some sections, such as in meadow areas in Yosemite Valley. In other areas the floodplain is lacking due to narrowing of canyon/valley walls, such as the gorge, or incision of the channel into moraine deposits, such as west Yosemite Valley moraines. In Yosemite Valley, the character of the floodplain varies in different locations because of local hydraulic controls. From Clark’s Bridge to Housekeeping Camp in the east Valley, the Merced River floods areas outside the main river channel with shallow swift flows that cut across meander bends. Near Yosemite Lodge and downstream to the El Capitan moraine, flood waters back up against the dense vegetation and tend to be deep and slow (low velocity). From the El Capitan moraine downstream, the river channel is steeper and confined in the narrow river canyon, the floodplain is narrow, and flow velocities are high.
The broad, well-developed floodplain that occurs in Yosemite Valley between Housekeeping Camp and the El Capitan moraine serves many hydrologic functions, including dissipation of flood water energy as water spreads out over the flat, expansive plain. The meadows in Yosemite Valley occur primarily in the floodplain and are maintained and rejuvenated by periodic floodwaters. The roads across Stoneman, Ahwahnee, Cook’s, Sentinel, and El Capitan Meadows have varying degrees of influence on the function of the floodplain.
The river channel in El Portal is narrow and steep, though less steep than in the gorge segment immediately upstream, and flow velocities are very high. The river channel can shift laterally during large floods.
In Wawona, an elongated alluvial valley, the river meanders less than in Yosemite Valley, but the river channel can shift laterally during large floods. Development in Wawona has altered the floodplain. Surface water diversions affect the Wawona floodplain through reduction of the water table during dry periods such as drought and in the fall before the onset of winter rains. Water diversion is governed by the Wawona Water Conservation Plan, which includes provisions for reduction and/or cessation of withdrawals when stream flow drops to critical levels.
Frazil Ice Flooding
Waterfalls in the park occasionally produce a late winter and early spring phenomenon called frazil ice at the base of the fall. Small ice crystals develop in turbulent super-cooled stream water when the air temperature suddenly drops below freezing. These ice crystals join into slush and become pressed together as more crystals form. Frazil ice lacks the erosional force of regular stream ice, but it can cause streams to overflow their banks and change course. Frazil ice sometimes reaches a depth of more than 20 feet along Yosemite Creek at the Lower Yosemite Fall Bridge. A 1954 flow of frazil ice completely filled the streambed of the creek and covered the footbridge near Lower Yosemite Fall with many feet of ice. More recently, a frazil ice event covered the Yosemite Falls footbridge on February 27, 1996.
Non-Flood Alterations of the Floodplain
Although floods are significant to ecosystems because they can induce large changes in channel morphology and the floodplain landscape, low stream-flow characteristics are also important. Low stream flow during the summer can affect the surrounding floodplain as riparian and wetland communities undergo a drying phase. Diversion of river flows for human consumption can upset this normal balance and induce further reduction of riparian communities and destabilization of stream banks. Prior to 1985, potable water in Yosemite Valley was produced almost entirely from surface water diverted from the Merced River upstream of Happy Isles. It is estimated that up to 54% of the low stream-flow discharge may have been diverted for park facilities. This practice has been terminated in Yosemite Valley, and all potable water is now taken from groundwater wells; however, water continues to be drawn from the South Fork in Wawona to augment groundwater supplies.
Development in Floodplains
Several campgrounds were severely damaged in the 1997 flood. Photo by Steve Thompson/NPS.
Executive Order 11988 (Floodplain Management) and the NPS Floodplain Management Guideline provide guidance for the protection of life and property in conjunction with natural floodplain values in the National Park System. This guidance applies to both existing facilities and proposed facilities, and requires the National Park Service to avoid locating facilities in floodplains if alternative locations are feasible. Where no alternative exists, and with a formal statement of findings, properly mitigated facilities can be located in floodplains. Each action (or facility) is assigned to one of three classes, depending on its use, and each class has a different regulatory floodplain. Actions of a given class can occur within the regulatory floodplain if properly mitigated. The regulatory floodplain for Class I actions, such as administrative facilities, residential areas, warehouses, and maintenance buildings, is the 100-year floodplain. The regulatory floodplain for Class II actions, such as medical facilities, emergency services, schools, irreplaceable records, museums, and fuel storage areas, is the 500-year floodplain.
Excepted actions are exempt from the NPS Floodplain Management Guideline if risks to human life and property are studied and then minimized or mitigated through design. Examples of excepted actions are bridges, flood control facilities, picnic areas, trails, roads, day-visitor parking facilities, and campgrounds. If a non-exempted action is proposed, a formal statement of findings is required. The statement of findings includes a description of the site-specific flood risk, describes why the action must be located in the floodplain, and describes how the action will be designed or modified to minimize harm to floodplain values or risk to life or property.