Most of the Eel River watershed is underlain by sedimentary rock of the Franciscan Assemblage (or Complex), whose rocks date back to the Late Jurassic (161–146 million years ago).
The Franciscan is part of a terrane, or crustal fragment, that originated at the bottom of the Pacific Ocean.
Beginning several million years ago, tectonic forces shoved the Franciscan assemblage against the North American Plate, pushing up the Coast Ranges.
The Eel River basin is among the most seismically active areas in California, especially in the north (the river empties into the Pacific only several miles north of Cape Mendocino near the Mendocino Triple Junction, which marks the northern end of the San Andreas Fault and produces frequent earthquakes due to the juncture here of three tectonic plates).
In the western and northern portions of the Eel River watershed, soils eroded from the Franciscan assemblage are often sticky, clayey and highly unstable, creating a high risk of landslides.
This soil is often known as "blue goo" because of its gray-blue texture and its tendency to slip when saturated.
Further inland and south, soils are well drained, although landslides are nevertheless common because of the high rainfall and steep slopes.

Because of the mountainous topography, the only flat land in the Eel River watershed is along the larger river valleys, where stream terraces have been formed, and in the estuary region near the mouth of the river.
Terraces were formed due to a combination of the high sediment load of the river system, and the rapid regional rate of geologic uplift (up to 13 feet (4.0 m) per 1000 years, as measured at Scotia Bluffs).
The Eel's behavior of down cutting its own sediments has caused it to flow in a deeply incised channel, which can generally contain all but the largest floods.
The younger mountains in the north may be uplifting at a rate ten times faster than the headwater regions further south and east, which consist of much older rock.

The Eel River has the highest per-unit-area sediment yield of any river of comparable watershed size in the continental U.S., excluding those fed by active glacial or volcanic sources.
The estimated annual sediment load is 16 million short tons, or an average of 4,458 tons per square mile (1,720 tons/km²).
Flooding events have a large effect on the average amount of transported sediment: high water in the years 1969, 1983 and 1998 caused an annual sediment load 27 times greater than that of normal years.
Among rivers of the contiguous United States, only the Mississippi River carries more sediment to the sea (the Colorado River historically transported more than the Eel as well, but most of its sediment is now trapped by dams).
However, both the Mississippi and Colorado have lesser sediment yields relative to their drainage areas.
Most of the Eel River watershed is underlain by sedimentary rock of the Franciscan Assemblage (or Complex), whose rocks date back to the Late Jurassic (161–146 million years ago). The Franciscan is part of a terrane, or crustal fragment, that originated at the bottom of the Pacific Ocean. Beginning several million years ago, tectonic forces shoved the Franciscan assemblage against the North American Plate, pushing up the Coast Ranges. The Eel River basin is among the most seismically active areas in California, especially in the north (the river empties into the Pacific only several miles north of Cape Mendocino near the Mendocino Triple Junction, which marks the northern end of the San Andreas Fault and produces frequent earthquakes due to the juncture here of three tectonic plates). In the western and northern portions of the Eel River watershed, soils eroded from the Franciscan assemblage are often sticky, clayey and highly unstable, creating a high risk of landslides. This soil is often known as "blue goo" because of its gray-blue texture and its tendency to slip when saturated. Further inland and south, soils are well drained, although landslides are nevertheless common because of the high rainfall and steep slopes.
Because of the mountainous topography, the only flat land in the Eel River watershed is along the larger river valleys, where stream terraces have been formed, and in the estuary region near the mouth of the river. Terraces were formed due to a combination of the high sediment load of the river system, and the rapid regional rate of geologic uplift (up to 13 feet (4.0 m) per 1000 years, as measured at Scotia Bluffs). The Eel's behavior of down cutting its own sediments has caused it to flow in a deeply incised channel, which can generally contain all but the largest floods. The younger mountains in the north may be uplifting at a rate ten times faster than the headwater regions further south and east, which consist of much older rock.
The Eel River has the highest per-unit-area sediment yield of any river of comparable watershed size in the continental U.S., excluding those fed by active glacial or volcanic sources. The estimated annual sediment load is 16 million short tons, or an average of 4,458 tons per square mile (1,720 tons/km²). Flooding events have a large effect on the average amount of transported sediment: high water in the years 1969, 1983 and 1998 caused an annual sediment load 27 times greater than that of normal years. Among rivers of the contiguous United States, only the Mississippi River carries more sediment to the sea (the Colorado River historically transported more than the Eel as well, but most of its sediment is now trapped by dams). However, both the Mississippi and Colorado have lesser sediment yields relative to their drainage areas.
Eel River (California)