Fluvial Geomorphology

The Big River Estuary and immediate upstream area are comprised of:

A series of oxbows and old river channels in the floodplain;

Old river terraces next to the floodplain; and

Ancient sea terraces occurring between 200-300 feet in elevation (English 1979).

The estuary is the repository of watershed sediment carried downstream by the river and sand carried upstream by the tide. Therefore, estuaries are sites of active sedimentation. During floods, high tide waters mix with slow, silt laden river water in the estuary - resulting in sediment deposition. Large river sediment loads cause greater deposition in the estuary. Consequently an examination of geomorphic patterns in an estuary can reflect erosional processes occurring in the watershed (Marcus and Reneau 1981).

A 1981 study of the historic sedimentation in the Big River Estuary found that the estuary has experienced a rapid sedimentation of its channel and salt marshes since the advent of logging in the watershed (Marcus and Reneau 1981). The study mapped the distribution of vegetation along the estuarine channel and looked at historic photos as well. The estuary was also found to exhibit an unusual pattern of deposition. The most obvious indicator of accelerated sedimentation in the Big River Estuary was the occurrence of levees along the estuary channel. Levees form as silt laden flood waters are slowed along the edges of the channel. Coarser, heavier sediments settle out and form an embankment along tidal flats and estuary channels. The result is the storage of sediment in natural levees and on tidal flats.

Levees in the estuary extend along the channel to 1.7 miles above the river mouth and display a regular decrease in height. They vary in width from 40 feet in the upper estuary to 10 feet and less in the lower region. These levees record the transition in the estuary from primarily tidal influences (salt marsh and mudflat) to primarily river influences (floodplains).

The estuary channel has narrowed and the floodplain has grown at the expense of mudflat and subtidal areas as estuary banks have prograded. Blockage or reduction in tidal influence has occurred in the upper flats while a filling of sloughs and increase in mudflat height is found in the lower flats.

Marcus and Reneau used several examples to illustrate these estuarine processes:

A railroad system was used to transport logs to the estuary during the early logging. A log dump located 3.8 miles upriver served as a spur of the railroad where logs could be dumped directly into the water. This log dump is shown in a historic photograph taken in the 1920s as standing in open water (Jackson 1975). The border of Flat 8 sloped gently away from the water. Today the pilings of this log dump stand adjacent to Flat 8, bordered by a levee 4 ft. in height. The historic development of this levee records a major change in the hydraulic conditions of the estuary. Winter floods were not able to deposit enough sediment to build levees at the site of the log dump prior to 1900. Since the photograph was taken, levees have developed 2 miles further down the estuary. Once the logs were dumped into the estuary, they were rafted down to the sawmill at the mouth. To avoid stranding the logs on the tidal flats, rows of pilings were placed at the lower low tide line (Jackson 1975). Chains were stretched between these pilings and acted as a barrier to the floating logs. Presently in Flat 4, two sets of pilings occur, the outer one at approximately low tide line and the inner one trending back into the salt marsh. Two sets of pilings were installed during the logging operations before 1938 indicating that heavy sedimentation had extended the low tide line out into the channel, thus rendering the original set obsolete.

The filling of these tidal sloughs by sediment is demonstrated by the presence of several barges, buried in Flat 4. These barges were used for transport in the estuary. The barges are 42 ft. in width and were moored in the tidal slough, indicating the original slough was at least this wide. Presently the same slough is 7 ft. in width and the barge is buried adjacent to the bank.

CGS photo mapping found that within the Mouth of Big River PW, the main channel gained negative channel features between 1984 and 2000, due to accumulation of sediment that was visible in plan view in relatively small-scale aerial photographs (Figure 1). The year 1984 showed 2.8 miles of negative channel features; 2000 showed 5.3 miles of negative channel features, consisting of lateral bars and a few mid-channel bars. The length of negative channel features grew significantly from 18.5% (1984) to 34.7% (2000) of the length of the blue-line stream representing the lower mainstem channel in the Mouth of Big River PW.

CGS prepared an Engineering Geologic Resource Assessment for DPR in 2004. As part of their analysis, CGS identified stream channel conditions and sediment sources within the 7,315-acre Big River State Park. The Big River’s gradient through the park is approximately 0.0475 percent, making the Big River a Rosgen type C channel. CGS was able to separate the channel in the park into four different reaches based on changes in sinuosity (Table 1). CGS found that from the mouth of Big River to RM 6.7, tidal influences and estuarine processes appear to mask fluvial processes. Reneau (1981) estimated that over 100 feet of sediment has accumulated in the estuary by the mouth of Big River over the past 9,000 years - or approximately 3 millimeters of sediment per year. This is considered the natural background sedimentation rate in the estuary.

Within the boundaries of the park, Big River meanders through a flat-floored valley with a bottom made up of alluvial sediment. About 85% of the river banks along the park are formed within this alluvial sediment, while the other 15% are eroded into steep valley floors underlain by colluviums or sandstone bedrock. The alluvial valley ranges from 275 to 1,400 feet wide at the confluence with Laguna Creek. The most common valley width within the park is 600 feet. The mainstem Big River is incised into the valley alluvium and has nearly vertical 3 to 20 foot high stream banks in most places. Banks are generally composed of weakly consolidated and uncemented silt and fine sand. When the stream banks exceed 6 feet in height, the banks slump and slough. Water levels in the late summer are between 3 and 6 feet in the tidal reach and between 6 and 20 feet in the upstream reaches below the valley flat (CGS 2004).

Old tidal mudflats and estuarine deposits of fine silt interlaminated with thin layers of peat have developed into salt marshes along the tidal reach. Upstream of this reach, mixed conifer forest has developed on valley flats composed of fine sand and silt deposited by the river. These valley flats are fluvial terraces and gently undulate to flat, slope down away from the channel, and usually contain a closed linear depression near the base of the adjacent hillslope. This is likely the result of natural levee formation (CGS 2004).

CGS (2004) examined silt lines on tree trunks on the fluvial terraces in the park. These lines are caused by regular inundation and have been described in historical accounts. Lines range from three to six feet above the terrace surface. Fritz (1923) described the lines in the Wonder Plot, a six-acre parcel of second growth redwoods set aside as a long term scientific experiment in redwood growth rates (Figure 2):

The site is on a high river bench of fine silt that is reported to be inundated once every four or five years. The mud line on the trees is in many cases 7 feet off the ground.


Another description of the same plot in 1945 states:

The sample plot, a square figure of one full acre, lies on a “flat” or a bench on the left bank of Big River…The soil is very deep silt, and, although about 20 feet above the bed of the river, is subject to inundation in occasional years. Mud lines on the trees indicate the level to have been 5 feet above the ground at the highest point of the plot.


CGS (2004) surveyed a cross-section near the Wonder Plot, at RM 8.7. The cross-section showed the level of the terrace surface to be 25 feet above the river bed, with silt lines on tree trunks at four to five feet above the ground surface. Thus the observations made by CGS in 2004 were very similar to those made by Fritz in 1923 and 1945. This indicates that the topographic relationship between the terrace surface and the active channel has not changed substantially in the past 80 years during the period of intense timber harvesting throughout the upper watershed (CGS 2004).

However, other aspects of channel morphology have changed. The 40 to 50 years of splash dam logging across the basin likely had an effect on stream channels. Channel clearing, artificial flooding, and battering by logs in transport likely greatly accelerated erosion and widened the width of the channel (CGS 2004). However, significant bed lowering along the lowermost reaches of Big River associated with splash dams is considered unlikely (CGS 2004). The very shallow gradient of the river inhibits stream power and the close proximity of the ocean provides ultimate control of base level. The remains of old log pilings and the foundation of a pier or log deflection wall within the channel from the early 1900s also support this idea. If bed adjustments had occurred in this section ofg the river, such old structures in the river would have been destroyed by annual and high flows over the past century (CGS 2004).

CGS (2004) examined the channel narrowing phenomenon documented by Reneau (1981) for most of the 20th century. They found that Reaneau had failed to discuss their findings within the context of “a river channel reclaiming itself after the multiple decades of channel clearing, splash dam flooding, and battering by logs in transport” (CGS 2004). CGS found it likely that channel conditions of the early 1900s documented by Marcus and Reneau (1981) were in fact an artifact of the splash dam era and represented a much wider channel. Therefore, channel narrowing seen since 1900 likely represents the channel re-adjusting to a more natural discharge regime (CGS 2004).

Figure 1. Map showing the relatively shallow gradient (<0.1%) of the lower Big River, where sediment appears to have accumulated between photo years 1984 and 2000. 

 

 

Table 1. Channel reaches of different sinuousity and possible geologic/geomorphic controls within the Big River State Park (from CGS 2004).
RM Reach Name Sinuosity Valley Flat/Flood
Prone Width (feet)
Most Commonly
Measured Widths (feet)
Possible Geologic and/or
Geomorphic Controls
0.0-2.2 Tidal 1.06 350:1,000 600 Drowned river mouth where tidal and estuary processes (sedimentation) dominate
2.2-8.0 Meanders 1.66 300:850 550 Incised antecedent meanders
8.0-10.6 Wonder Plot 1.18 400:1,400 600-800 General linear trend at ~N40W that parallels the structural grain of folding and faulting along California’s coastline
10.6-13.4 Woodlands 1.52 275:900 400-600 Return to antecedent meanders

 

Figure 2. Wonder plot experimental site.