Thomas Creek Watersheds

Results and Current Status

Streamflow responses to harvesting on Thomas Creek, South Fork were:

• Annual water yield increased about 2 inches or 45%. The yearly streamflow increases were generated by winter snowmelt or rain storms.
• Increases in peak flows occurred. Annual increases were about 3 ft3/s/mi2 or 65%.
• Water yield increases are usually attributed to increased snowpack accumulations and reduced evapotranspiration in the forest openings. Snow surveys did not measure any significant changes in snow water equivalents following the harvest, although more snow was measured in the small openings than in the adjacent partially-harvested stand.
• During high and average precipitation years, increases in water yield can occur without heavy reductions in stand density or the creation of large openings because the residual stand is unable to use all of the available water.

There was a concern that the timber harvest on South Fork would cause accelerated soil erosion. Studies showed:

• Overland flow and sediment delivery from severely disturbed and undisturbed sites were low and inconsequential. This was true even though measured sediment deliveries from severely disturbed areas (37 lbs/acre/yr) were higher than from undisturbed areas (5 lbs/ac/yr).
• Increased streamflow after treatment caused the natural channel adjustment process to accelerate, but the impacts on downstream areas were unknown.
• Mule deer, elk, and livestock benefitted from the harvested openings because of increased production of herbaceous species.
• Only minor, short-term changes in bird populations were reported.
• Habitat requirements of red squirrels (Tamisciurus hundsonicus) were evaluated and used in the preparation of a marking guide to minimize detrimental affects from timber cutting.

Ecologically-oriented studies were also conducted on the Thomas Creek areas before treatment, showing that before 1900 fires burned through South Fork at intervals of 22 years and that small, localized fires occurred frequently throughout the area. Fire suppression resulted in changes in stand structure and composition, a greater number of smaller trees, and more shade-tolerant species that normally would have been reduced by periodic ground fires. Other studies were used to characterize dwarf mistletoe infections by host, site, and stand characteristics and to describe species and stand growth of an old-growth mixed conifer forest.

Studies were also initiated during preparation of or immediately after the harvest. Determination of the effects of treatment on forest productivity and sustainability was based on common stand parameters such as species composition, stand structure and stocking, regeneration success, and growth of residual trees and stands. The Thomas Creek treatment produced a wide range of benefits while retaining many desired old-growth stand characteristics.

Current Status

Most of the watershed experiments were terminated between 1983 and 1986. However, the Apache-Sitgreaves National Forest continues to collect streamflow and precipitation measurements from the Castle, Willow, and Thomas Creek Watersheds as part of its hydrological monitoring program. The vegetation transects and points continue to be visited by USDA forest pathologists to determine changes in dwarf mistletoe infections over time. Additional inventories are being considered by scientists at the Rocky Mountain Research Station. Data from the watersheds have also been used by graduate students at the University of Arizona.