- Leslie Reid - Research Geologist
- Susan Hilton - Hydrologist
- Elizabeth Keppeler - Hydrologist
- Diane Sutherland Montoya - Geomorphologist
Caspar Creek Experimental Watershed Study
A Brief History of the Caspar Creek Watershed Study:
Streamflow and suspended sediment have been gauged continuously since 1962 in the 473-ha North Fork and the 424-ha South Fork of Caspar Creek on the Jackson Demonstration State Forest by the USDA Forest Service Pacific Southwest Research Station and the California Department of Forestry and Fire Protection.
In most earlier publications, the area of the North Fork above the weir is given as 484 ha and the South Fork as 424 ha. These values were obtained by determining the watershed divides using the 7.5 min USGS topographic map. When the North Fork study was initiated in 1985, the drainage area of the North Fork and each of its tributaries was obtained by ground surveys of actual field-determined topographic divides. The ridge-top road around the North Fork watershed sometimes diverts surface drainage from one tributary watershed to another, and occasionally out of the North Fork watershed. Since the study was principally focussed on stormflow and sediment, we decided to define the area of the watershed(s) as that which contributes directly to surface runoff. Consequently, if road drainage was directed out of the watershed, the area above the road was subtracted from the watershed area; if directed into the watershed, it was added to watershed area. Consequently, this exercise lead to an overall loss of 11 ha and the drainage area of the North Fork became 473 ha. If one wanted to consider the topographic area of the watershed and ignore the issue of road diversion, the watershed area is 484 ha. So, the appropriate watershed area depends upon the set of hydrologic assumptions one uses.
The watersheds generally have a southwest orientation and are located about 7 km from the Pacific Ocean and about 10 km south of Fort Bragg in northwestern California at 39o21'N 123o43'W. Topographic development of the area is youthful, with uplifted marine terraces deeply incised by antecedent drainages. The hillslopes are steepest near the stream channel and become more gentle near the broad, rounded ridgetops. About 35% of the slopes are less than 17o and 7% are steeper than 35o. The elevation ranges from 37 to 320 m.
The soils of the basins are well-drained clay-loams, 1 to 2 m in depth, and are derived from Franciscan graywacke sandstone and weathered, coarse-grained shale of Cretaceous Age. They have high hydraulic conductivities and subsurface stormflow is rapid, producing saturated areas of only limited extent and duration.
The climate is typical of low-elevation watersheds on the central North American Pacific coast. Winters are mild and wet, while summers are moderately cool and dry. About 90% of the average annual precipitation of 1200 mm falls during the months of October through April. Snow is rare and rainfall intensities are low.
From 1963 to 1967, streamflow and sediment was measured in both second-growth watersheds to "calibrate" the watersheds prior to treatment. At that time, the watersheds supported a 90-year-old second-growth forest composed of coast redwood (Sequoia sempervirons (D.Don) Endl.), Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco), western hemlock (Tsuga heterophylla (Raf.) Sarg.), and grand fir (Abies grandis (Dougl. ex D.Don) Lindl.). The watersheds contained an average of about 700 m3ha-1 of stem wood.
In summer 1967, a main-haul logging road and main spurs were built in the South Fork. The road right-of-way occupied 19 ha adjacent to the stream, from which 993 m3ha-1 of timber was removed. The first of three stages of logging began in the South Fork in 1971, during which 59% of the stand volume was selectively cut from 101 ha. In 1972, 69% of the stand volume was selectively cut and tractor yarded from an additional 128 ha. In 1973, 65% of the stand volume was selectively cut from the remaining 176 ha (Rice et al., 1979).
From 1985 to 1986, 67% of an 87-ha ungauged tributary was clearcut and cable yarded immediately upstream of the North Fork gauging station.
Logging began in the main study portion of the North Fork in 1989 and ended in 1991. The timber volume removed from the North Fork was intended to approximate the volume cut from the South Fork in the early 1970's, but clearcutting with cable yarding was used in the North Fork rather than the selective harvest with tractor yarding that had been used earlier in the South Fork. Upstream of the North Fork gauging station, 48% of the area was clearcut and 4% of the streamside protection zone was selectively cut. The size of clearcut blocks in the North Fork ranged from 9 to 60 ha and occupied 35% to 100% of individual tributaries. New roads, landings, and skid trails occupy from 2.1% to 7.0% of individual logged watersheds.
Three tributaries in the North Fork were left in an untreated control condition. Post-logging measurements continue in the North Fork and South Fork watersheds to the present.
Updated on October 10, 1997, by Bob Ziemer