Geography 360

GEOMORPHOLOGY

FALL 2006

RUSSIAN RIVER FIELD TRIP

Saturday, November 4, 9 to 3 p.m.

Meet at the So of Library Parking Lot, (Lot C) at 8:45 a.m.

Detail, Geologic Map of California, Santa Rosa Sheet, Koenig, 1963

Today we’ll take a look at examples of different channel types along the Russian River from Hopland to Healdsburg. Overall, recall that the Russian River is an antecedent river that was flowing through this land as the Coast Range gradually uplifted beneath it, allowing it to cut through mountains that it would otherwise have gone around. In the middle reach of the river that we’ll look at, the path of the river follows major faults, the Maacama fault in particular. Think about sources of sediment load, types and amount of sediment load, the influences of seasonality, and other controls on channel style.

We will travel up Highway 101 to Hopland, where our field trip will begin. As we go, notice how the landscape varies, from flat floodplain between Santa Rosa and Healdsburg, then through rolling hills, and down along the edge of another floodplain (Alexander Valley).

The east sides of these floodplains are bounded by strike-slip or transform faults. The Rogers Creek fault runs through Sonoma Mountain and ends at Santa Rosa. Continuing along the east side of the Cotati Valley north of Santa Rosa is the Healdsburg fault, which is a step-over from the Rogers Creek fault.

The surface we are driving across between campus and Santa Rosa is composed of Plio-Pleistocene alluvial fan deposits. As we drive north, we are traveling over Quaternary alluvium from the Russian River and its tributaries. As we pass north and east of Healdsburg, we cross over the Healdsburg fault, which at this location is cutting through Lower Cretaceous marine rocks with stringers of Mesozoic ultrabasic intrusive volcanic rocks.

Down the hill and into the Alexander Valley, we travel along the west side of the floodplain. Note that it has several levels, stepping down to the modern floodplain of the Russian River. The east side of this valley is marked by the Maacama fault. This is actually a fault zone, with many splinters all trending northwest-southeast.

As we reach Cloverdale, we leave the Cretaceous marine rocks associated with the Healdsburg fault behind and cross into Franciscan Formation rocks of more or less the same age. Here the river flows through a very unstable canyon cut through the Franciscan rocks. Note many landslides on both banks of the river, but especially consistently on the east side. See bare patches of soil/rock that look bluish green (serpentine) and common gray pine (digger pine) which doesn’t mind the toxic soils of serpentine. There are also outcrops of more resistant metamorphic rocks of various types. Just before we reach Hopland we are passing through a larger patch of the Mesozoic ultrabasic intrusive rock, and you may observe that the landscape becomes somewhat more stable, with lower, more rounded hills.

Our first stop will be at Feliz Creek, a small tributary of the Russian River. Here you will describe what you see in terms of channel geometry, size, sediment load, stability, channel landforms, roughness characteristics, etc. Note downstream of the bridge on the far side you can see layers of overbank sediments exposed by erosion during higher flow. How would you interpret the layers? How well rounded, well sorted is the bedload of this stream? What symbol does the topo map use to depict this stream?

Second stop will be near a bridge over the Russian River at Hopland. We’ll do rough measurements of width and estimate depth, note sediment load, riparian vegetation, sinuosity. How does this compare with Feliz Creek? Keep this channel reach in mind as we look at different parts of the Russian River downstream.

Third stop will be just downstream of Pieta Creek. During several large floods over the past 15 years, this creek has deposited large amounts of coarse sediment into the Russian River. This sediment input, combined with mass movements and input from Feliz Creek, has changed the nature of the channel somewhat. What do you observe that’s different here from the Hopland location?

Fourth stop will be just upstream of Squaw Rock. Here CalTrans has worked diligently to try to keep a freeway open under adverse circumstances. How have they solved this problem?

Fifth stop will be at Commisky Station Road. Here we will observe a rudimentary bridge over the river at the confluence of a small tributary from the west. This location has changed rapidly over the past few years, rearranging the location of the channel and gravel bars repeatedly during floods. During major floods the bridge is under water. Observe the far bank slightly downstream has been cut by high water. There is a benchmark at this location, according to the topo map. I haven't been able to find it.

LUNCH. Sixth stop will be at the USGS gaging station downstream from the old bridge. If conditions are good we’ll stop for lunch here. Note how the stilling pipes and measuring markers march down into the channel. The hydraulic radius of the channel at this location must be re-measured every few years because of changes in the channel bed. Do you think this is a good location for a gaging station? Why or why not?

Seventh stop will be under the highway bridge, built in the early 1990s I think. Note dry distributary channels. Do you think the bridge footings will be eroded or buried over time?

Eighth stop. From here we will travel along the old highway 101 on the west side of the channel. Stop eight will be at the location of the old bridge into Cloverdale. Here we’ll look at sand and gravel bars in the channel and how they build up and erode. How dynamic is this particular spot compared with some of the others you’ve seen so far?

BREAK After stop eight we will drive back toward Cloverdale and stop for snacks/bathrooms. Then we will drive back down the Alexander Valley toward Geyserville.

Stop nine will be next to a bridge over the River at Geyserville. Note dramatic changes in the river here compared with upstream. Why do you think the river is different here? What natural and/or human factors influence the channel here?

Depending on the time, we will travel on toward Healdsburg. Our last stop may be on the south bank of the river as it passes Healdsburg. How is the river different here compared with the Alexander Valley reach?

Here are a series of Discharge Diagrams for different reaches, upstream to downstream along the Russian River.

   
   
       
       


Last updated 11/3/06