Geology

Topography
The Reserve is located in the foothill portion of the Big Chico Creek watershed. The property is irregular in outline and extends a maximum of 4.5 miles in a North/South axis and 2.0 miles East/West. The narrowest part (where Big Chico Creek first enters the property) is only 0.5 mile East/West. Elevations range from 700 feet where Big Chico Creek exits the property to 2160 feet at the northeastern property line. The eastern approximately half of the property lies within Big Chico Creek Canyon with a fairly narrow floodplain and terraces, then walls rising 1000 feet in about 0.5 mile, often including vertical cliffs. The western half can be described as occupying the top of a gently tilted plain dissected by small headwater streams. The south (lower) end of this plain is about 1700 feet, dissected in spots to 1000 feet. The north (high) end is about 2000 feet, dissected to 1800 feet. Soils are relatively shallow, bedrock is exposed in many places, and surface boulders are scattered over much of the area.

tuscan cliffsGeomorphology
Geologic processes in the Reserve are dominated by the stream dissection of gently sloping layers of rock to form steep canyons. The upper portions of the Reserve are in the Tuscan Formation, about 4 million years old, composed of layered ash, volcanic mud flows (lahars) and patches of alluvium. Some of the lahars are very hard and form cliffs along the sides of the canyon. Beneath the Tuscan Formation lies the Lovejoy Basalt, about 15 million years old, very resistant to erosion and often forming cliffs.

Below the Lovejoy is the Chico Formation, about 75 million years old. It is sedimentary rock composed primarily of sand and contains numerous marine fossils. Relative hardness and permeability of the rock layers have a major influence on hydrology and vegetation within the Reserve. Mass wasting, landslides, and slumps have been major contributors to the current morphology of the canyon. Since the Chico Formation is much softer than the Lovejoy Basalt, as the stream carved its canyon it undercut the Lovejoy, allowing huge slabs and boulders of basalt to tumble down into the creek to be left on the eroded surface of the sandstone after the creek meandered away. As the canyon deepened, the hard Lovejoy formed cliffs high up the walls. Pieces of the basalt, ranging from small cobbles to large boulders, have fallen off the cliff faces to form talus slopes on the gentler surface of the Chico Formation. Occasional huge slabs have calved off the Lovejoy cliffs to generate debris flows that fanned out across lower terraces. Between the Lovejoy and the Tuscan lies a layer about a foot thick of tuff from a volcanic ash fall. Water seeping through the tuff weathers it to clay, forming a slip surface that lets masses of Tuscan slump off the top of the Lovejoy. Consequently, the floor and lower walls of the canyon are often covered with deposits of material from above, and the Chico Formation is rarely visible except in parts of the active stream channel. The Lovejoy basalt appears to be intermittent so that in some parts of the reserve the Tuscan Formation rests directly on the Chico Formation. In areas where the Lovejoy Formation is present, Chico Canyon is narrower and the creek bed is steeper.

Reserve lands from the top of the Musty Buck Ridge west consist of gently sloping ridge tops of exposed Tuscan dissected by headwater streams from Sycamore and Mud Creeks. Some cliffs are present in the deeper canyons, but landslides have been much less important than in Chico Canyon.

Soils
The soils from the top of Musty Buck Ridge west are classified as moderately fine-texture loam and are generally very shallow. These soils are derived in place from weathering of the parent Tuscan. Bedrock is exposed in many areas and surface boulders are abundant. Soils within Big Chico Canyon may have generated on alluvium, landslide debris, talus, or local chunks of any of the parent rock types. Accordingly, they are extremely variable and can only be classified on a site-specific basis.

Hydrology and Water Supply
Big Chico Creek originates from a series of springs, at an elevation of about 5,400 feet, northeast of the City of Chico on the southwest flanks of Colby Mountain and flows 45 miles from its origin, crossing portions of Butte and Tehama counties, to its confluence with the Sacramento River, at an elevation of 120 feet, west of the City of Chico. The watershed also includes three smaller drainages to the north: Sycamore, Mud, and Rock Creeks. Closest to Big Chico Creek is Sycamore Creek, which originates at around 1,600 feet and is a tributary to Mud Creek. Mud and Rock Creeks, further north, originate between 3,600-3,800 feet. Mud Creek drains off Cohasset Ridge to the south, flowing 26 miles to its confluence with Big Chico Creek. Rock Creek drains the north side of Cohasset Ridge and flows 28.5 miles before it joins Mud Creek. Median discharge for Big Chico Creek in Bidwell Park about three miles downstream of the Reserve is 175 cubic feet per second (cfs) winter and 30 cfs summer. Since the stream reach between the Reserve and the stream gage experiences a great deal of ground infiltration in summer, median summer flow within the Reserve is probably greater than 30 cfs. The BCCER also contains intermittent tributaries of the Sycamore and Mud Creek sub-basins of the Big Chico Creek watershed and numerous springs and seeps. Fresh water at the Henning Ranch house is from a year round flowing spring.

Climatic Conditions
The Reserve is located in an Interior Mediterranean Climate that is defined by its moist, cool winters and hot, dry summers (Critchfield, 1974). Yearly precipitation is extremely variable but averages about 40 inches, most of which falls as rain from November to May, although further upstream in the watershed colder winter storms can deposit large amounts of snow. Until this snow pack melts, usually by late spring, it adds to the regular base flow of Big Chico Creek. The temperatures for the area can drop well below freezing during the winter and commonly rise above 100°F during July and August.