Merced River Restoration - Meadows and Wetlands

Royal Arches Meadow in Yosemite Valley.

 

River Restoration

• Overview of Merced River Restoration
• Restoration Projects - Riverbanks
• Restoration Projects - Meadows and Wetlands
• Merced River Restoration Study
• Study - Technical Findings
• Study - Documents

The large, moist, mid-elevation meadows and the riparian vegetation communities of Yosemite Valley owe their existence to river and groundwater processes that produce regular flooding and sustain high water tables, and past burning by American Indians and current prescribed burns that maintain open conditions for meadows. Yosemite Valley meadows and riparian habitats support rare and endemic species as well as an exemplary diversity of plant and animal species found in a variety of ecological niches.

Most meadow loss occurred from when Euro-American settlers came to the Valley in the 1850s until the 1940s. While some scientific studies have shown natural factors contributing to these changes, it is most likely a combination of human induced and natural changes, such as cessation of burning by American Indians, altered hydrology, domestic livestock grazing, public use of the meadows, wildlife herbivory, natural succession, and climate change. Prior to then California Indians conducted small, low-intensity surface fires for centuries to increase growth and yield of crops, aid in hunting and insect collection, and perform other functions.

 

Euro-American settlers eliminating anthropogenic fire had immediate effects, with a widespread establishment of trees in and around the meadows taking place after. Plowing, mowing, burning, and probably in some cases severe overgrazing, complicated the increase in tree cover to varying degrees, as did the clearing activities of the 1890s, 1930s and 1940s. Since then park managers have taken action to control conifer encroachment in meadows.

Alterations in meadow hydrology, almost always making meadows drier, have had an equally altering effect. Anthropogenic impacts to hydrologic flows in Yosemite Valley were both purposeful and inadvertent. For example, in 1879 Galen Clark ("Guardian of the Yosemite Grant"), used blasting methods to lower the level of the terminal moraine located just downstream of El Capitan Meadow in an effort to drain upstream meadows and enhance access to east Yosemite Valley. This action likely dropped the water table in El Capitan Meadow, making it more conducive for tree establishment. Ditching done to drain the meadows had the same drying effect, with roads built across meadows exacerbating the hydrological alterations. Most Merced River tributaries in Yosemite Valley were also channelized in part, altering the path of water that would naturally flow from cliff walls in a sheet or braided fashion across the meadows.

The effects of these actions, taking place over more than a century, are that an estimated 64% of the original meadow, open forest, and wetland habitat in Yosemite Valley has converted to forest since the mid-1800s. Infrastructure, development, and increasing rates of visitor use continue to influence the hydrologic regime, reducing the distribution and extent of connected floodplain, level and extent of meadow inundation, and the meadow extent.

 

Park staff removes a conifer in Leidig Meadow.

Restoration of meadows and wetlands seeks to counteract the following impacts:

• Conifer encroachment: In five of six meadows surveyed, tree seedlings were present in more than 10% of the study plots, indicating that the tree encroachment documented since 1870 continues. The extent of tree seedlings is highest in El Capitan and Stoneman Meadows (32% of plots contained seedlings), indicating that nearly one-third of meadow area in El Capitan Meadow and Stoneman Meadow has some degree of tree encroachment.

• Non-native species: Non-native species are common across all Yosemite Valley meadows, with the highest extent of non-natives found in El Capitan Meadow and Stoneman Meadow (as inferred from percent of plots with non-native plants present—92% to 96% of plots contained non-native species). Additionally, the mean cover of non-native plants is lower in saturated and inundated soils (by a factor of two to seven) compared with moist to dry soils. The distribution of non-native plants is strongly linked to water table depths in meadows, with a higher presence of non-native species in drier areas. Maintaining meadow water tables to promote areas of wet soil may be a means to sustaining native meadow vegetation composition.

 

Park staff removes a ditch in El Capitan Meadow.

• Altered meadow topography: Ditches and other human alterations to meadow topography, remnants of the past agricultural era, remain within Yosemite Valley meadows. Ditches were also constructed during NPS administration beginning in 1929 (they were often referred to as “moral ditches” to keep people from driving into meadows). Ditches increase drainage and lower natural water-table levels, favoring non-native meadow vegetation.

• Informal trails: Informal trails are social trails created by visitors or administrative users that are noticeable to observers and generally not managed directly by park staff, as opposed to formal trails that are mapped, periodically assessed, and regularly maintained. The 2014 Merced Wild and Scenic River Comprehensive Management Plan calls for the removal of informal trails that incise or fragment meadow habitat. Heavy visitor use and foot-traffic off of designated trails destroys vegetation throughout the meadow. This repeated use creates informal trails and soil compaction that reduces water retention and filtration capabilities of meadows, roots can no longer grow, and seeds cannot establish.

Informal trails have many harmful ripple effects in natural systems. Fragmentation has further effects on meadow hydrology, habitat quality, and soil moisture, and creates conditions ideal for the introduction of non-native species. Finally, trail corridors have also been shown to pose barriers for small mammals and other wildlife. Indeed, researchers investigating trampling impacts in Yosemite Valley meadows have found that meadow condition is frequently poor in heavily used areas, smaller areas are more prone to difficulties with recovery than larger areas, and visitor-created trampling has a significantly negative impact on vegetation and macroinvertebrate structure and diversity.

 

Left: National Park Service (NPS) employee planting native plants in Ahwahnee Meadow. Right: NPS employee placing decompacted soil by hand.

 

In recent decades, NPS staff manage public use of meadows and riparian zones within the Merced River corridor to minimize habitat fragmentation, maintain high ecological condition, and protect the integrity of streambanks to conserve ecosystem processes associated with meadow hydrologic and ecological function. The MRP includes a number of projects to enhance the condition of meadow and riparian areas in Yosemite Valley.

These projects include:

• Extensive removal of high priority non-native species in meadows and riparian areas.
• Boardwalks installed in Sentinel and Stoneman Meadows, substantially reducing the dense network of informal trails in these meadows.
• Fill removed in Sentinel Meadow from the site of a former movie house and dance hall (Pavilion Square), and natural meadow topography restored at the site.
• Ecological restoration in Cooks Meadow involving removal of a historic road (abandoned), filling in ditches, and restoration of natural meadow topography; and construction of a boardwalk across sensitive meadow habitat.
• Riparian habitat restoration at Lower River Campground, Housekeeping Camp, El Capitan Picnic Area, Devil’s Elbow, Sentinel Bridge, Swinging Bridge, Clark’s Bridge, North Pines Campground, and the Cascades Dam site after dam removal.
• Removal of infrastructure from meadows and riparian habitat, including actions to remove buried utility lines in meadows and replace them under existing roadways, removal of underground utility lines that cross the Merced River, and removal of utility lines and lift stations from riparian/riverbank areas.
• Remove informal trails in meadows where they fragment meadow habitat or cross through sensitive, wet vegetation communities. De-compact trampled soils, and use salvaged plants growing in trail ruts and local seed to revegetate area and consolidate multiple parallel trails. Overall, NPS will restore six miles of informal trails throughout Yosemite Valley.

 

National Park Service employee collects willow poles.