The South Fork Eel River meanders irregularly across the western part of the reserve. Lacking a floodplain, the channel is bounded by banks of bedrock, boulders and gravel, and contains very little woody debris. Bordering the channel are flights of bedrock terraces with mantles of coarse alluvium, evidence of former floodplains. These terraces create the only level land in an otherwise rugged terrain of narrow ridges and steep balleys. Surrounding slopes may exceed 50 percent. elevations range from 378 meters (1,240 feet) near headquarters to 1,290 meters (4,231 feet) at Cahto Peak.
Underlying most of the reserve are greywacke sandstones and mudstones of the Franciscan Complex, with derivative soils from the Josephine and Hugo series. Continued uplift and incision by the Eel River and its tributaries created the steep topography.
The South Fork Eel River runs north, nearly parallel to the Pacific coastline for most of its length. Recent studies suggest that the river’s downsteam reach may be uplifting as much as ten times faster than the headwaters area near the reserve. The channels of the upper river and its tributaries are dominated by bedrock and boulders with pronounced bedrock terraces. Ongoing research in Elder Creek (Seidl and Deitrich 1992) documents three flights of bedrock terraces adjacent to the channel. The channel bed is punctuated by two noticeably oversteepened reaches, or knickpoints. In the steepened upper reaches of Elder Creek, debris flows deliver large wood and boulders to the channel, creatinmg a series of cascades and pools. In these steep channels, debris-flow scour is hypothesized to be the primary mechanism of incision.