Assessment Components

Overview

The watershed story of the Salt River Basin is one of hydrologic manipulation, human disturbance, and resultant systemic dysfunction. Salt River was the main vector of trade and transportation in the days of early settlement on the Eel River, and supported a shipping industry from 1860’s until the early part of the 20th century. What was once a tidally influenced slough that measured 200 feet across and 15 feet deep at Port Kenyon (River Mile 4.1), is now a marsh without a narrow, poorly defined channel. The historic channel of the Salt River is clogged with sediment eroded from its Wildcat Ridge tributaries. The sediment is covered with invasive vegetation and as a result, adjacent roads, houses, and pastures on the delta are flooded annually causing damage to both private and public infrastructure. There are several factors that prohibit the Salt River from functioning in a natural manner:


Sediment, from upslope and stream bank erosion, enters the mainstem from the tributaries in large volumes and deposits in the Salt River channel; reducing the channel capacity;

The reduction of tidal influence caused by the construction of levees and tide gates in the western delta, has lessened the systems ability to clear sediment deposits from the channel;

Extensive channelization and realignment of the Salt River tributaries have dramatically changed the discharge and sediment delivery equation as well as the drainage network;

The problems of flooding and sediment deposition have been made worse by the prolific growth of instream vegetation;

In-channel bedload transport from periodic Eel River floodwaters has practically been eliminated by levee construction in the eastern delta.

The fundamental energy problem with Salt River today is that it has received sediment faster than it has been able to naturally exhaust it because channel cleansing forces have been practically eliminated. Since settlement, Salt River Delta residents have been faced with repeated high water events that have resulted in the loss of property and capital. There have been numerous attempts to control flood events on the delta that have indirectly perpetuated annual flooding. The residents in the Salt River Basin are now seeking a solution on how best to restore the Salt River into a hydrologically and ecologically functional river while maintaining socio-economic needs. This restoration effort is supported by a myriad of public entities.

Location / Geography

The Eel River’s broad floodplain between the Pacific Ocean and the Eel’s confluence with the Van Duzen River, nearly fourteen river miles upstream, comprises the Eel River Delta, which is part of the Lower Eel Assessment Basin. The Salt River drainage forms a subbasin of the Lower Eel Assessment area, and consists of the southern side of the Eel River Delta and the Wildcat Hills catchment area. The Salt River Subbasin is 31,260 acres, approximately 49 square miles, and is located in Humboldt County, California, approximately fifteen miles south of Eureka. The city of Ferndale is the major population center within the Salt River Basin. Access to Ferndale is from Highway 101 and across the Eel River at Fernbridge via Highway 211.

Geology

The Eel River Delta lies in tehe Coast Range Province in Northern California. The valley is formed by the Eel River Syncline (Li 1992). The Eel River Delta is a depositional land formation that was created as alluvial materials were delivered from upslope sources to the ocean. The Eel River Delta is underlain to great depths by unconsolidated alluvial sediments originating from a complex system of Franciscan type rocks including mudstone, siltstone, claystone, and sandstone (McLaughlin, et al 2000).

The Wildcat Range is formed of a group of sedimentary rock called the Wildcat Formation Group. Wildcat Formation pertains directly to those sediments in the mountains south of Ferndale, but has since become established in literature as a loose term for Tertiary sediments of Humboldt County of suspected Pliocene age (Ogle 1953). The Wildcat Formation Group is subdivided into five categories and is highly susceptible to erosion and large scale landslides, particularly the Rio Dell Formation (Table 1).

Table 1. Wildcat geologic formation group. (USGS Fortuna Quad, OFR 85-1SF).
Wildcat Formation Name Description of Formation
Pullen Mudstone, diatomaceous mudstone, and local sandstone; highly sheared. 
Eel River Mudstone, siltstone, and sandstone; unit is sheared in places
Rio Dell Interbedded mudstone, semiconsolidated sandstone, and siltstone; landslide failures along the interface between sandstone and mudstone beds are common, particularly along the coast.
Scotia Bluffs
Fine-grained, massive semiconsolidated sandstone with minor amounts of siltstone, mudstone, and pebbly conglomerate
Carlotta Non-marine sandstone, conglomerate, and claystone gradationally overlying Scotia Bluffs Formation; deposits are poorly consolidated.


Earthquakes
The Eel River Delta is located upon a complex tectonic setting near the junction of three crustal plates known as the Mendocino Triple Junction (MTJ). The MTJ is where the Pacific and the Gorda Oceanic plates meet the North American plate. The cascadia subduction zone (Csz) is an area just offshore where the Juan de Fuca and the Gorda plates are under thrusting beneath the North American Plate. The Csz originates at the Mendocino Triple Junction and extends north through Oregon and Washington running parallel to the Pacific Northwest coast line. The complex tectonic structure contributes to a high concentration of earthquakes in the north coast region. The Salt River area has experienced hundreds of earthquakes of significance (M ≥ 4 on Richter scale) in the past 120 years (USDA 1993).

For example, on April 25, 1992 a 7.1 magnitude earthquake, the Cape Mendocino earthquake, occurred near Petrolia, (USGS 2004). The earthquake had a very energetic aftershock sequence, including two large strike-slip earthquakes (both magnitude 6.5) and caused measurable coastal uplift of 4 to 5 feet on Cape Mendocino and a two foot tsunami was measured in Crescent City as a result of the earthquake (Carver, et al. 1994). The center of Ferndale was severely impacted with significant damage to buildings, but Ferndale citizens were fortunate as there were no major injuries reported.

Landslides
The potential for landslides in the Wildcat tributaries is high due to a number of factors including unstable geological structure, high rates of tectonic activity, climatic interactions, and current and historic landuse practices. The dominant factor that contributes to the high potential of landslides in the Wildcat tributaries can be attributed to the unstable nature of the Wildcat Formation Group, particularly the Rio Dell formation.

Subsidence
The Eel River Delta is affected by both tectonic uplift and subsidence, which has resulted in slight changes in elevation. Subsidence is the process where land sinks and results in a drop in elevation. The Eel River Delta subsidence is typically triggered by tectonic activity. Subsidence is distinguishable in estuarine soil profiles because a sudden submergence in the land forms an abrupt and sharp contact between overlying inter-tidal mud or sand and buried rich organic matter creating an obvious stratification (Li 1992). Essentially, the mouth of the Eel River suddenly dropped in elevation five times in the past 2000 years; however, the change in elevation is not occurring equally across the delta. An average subsidence rate has been calculated, 1mm/ year in the northern area, and 3.6 mm/year in the southern area (Li 1992). Over time, the unequal subsidence in the Eel River Delta will dictate the stream channel’s location as it migrates to the lowest elevation areas.

The impaired condition of the Salt River and its aquatic resources are partly due to the unstable nature of these geologic factors and consequently the Wildcat tributaries dominated by loosely consolidated sedimentary rock formations, high frequency of earthquakes, and the potential of tectonic uplift and subsidence. These conditions and processes ensure an ongoing background of high sediment yield to Salt River, which has been further exacerbated by human caused disturbance regimes since European contact.

Climate

The climate of the Redwood Creek basin varies from moderate seasons along the coast to the more extreme seasons common to the higher inland areas. The predominant influence on the climate in the lower basin, extending some ten to twenty miles inland, is moist marine air, which moves inland by prevailing onshore winds. Fog is a dominant climatic feature along the coast, generally occurring daily in the summer and not infrequently throughout the year. This oceanic influence has a greatly moderates the climate of the coastal areas over most of the year. Temperatures in the coastal region of the Redwood Creek basin vary only slightly, with a seasonal difference of only 10–15°F. For example, mean temperatures at Redwood Park are 47°F in January and 59°F in June.The Eel River Delta or ‘foggy bottoms’ is influenced by coastal fog throughout the year and is one of the cloudiest areas in the country (Stokes 1981). The dry season is marked by considerable fog or low cloudiness that clears by late morning or early afternoon. The amount of sunshine increases greatly with distance inland from the coast. The weather of the coastal Salt River Basin can be characterized by a definitive rainy season with mild fluctuations in daily and annual temperature. The rainy season lasts from October through April, accounting for about 90 percent of the annual precipitation (USDA 1993). Average annual precipitation in Ferndale is 44.2 inches per year (USDA 1993). The dry season lasts from May through September. Temperatures on the delta are moderate and the annual temperature range is one of the smallest in the lower 48 states (Stokes 1981). Average annual temperature is 52°F. The Eureka weather station reports a mean low temperature of 41.2°F. September is the warmest month on the Eel River Delta with an average daily high of 67.5°F.