Turbidites and rip-up clasts in Elder Creek

Elder Creek, part of the South Fork Eel River watershed, lies in the Franciscan Formation found underfoot in most of the Northern Coast Ranges of California. The rocks here were deposited in marine environments when the Farallon slab was still subducting under the North American plate at this latitude. Subsequent uplift following the passage of the Mendocino Triple Junction has elevated these rocks out of the sea.

Clastic sedimentary rocks found in Elder Creek record information about their depositional setting. Grain size, lithology, and shape all provide clues about the energy of the flow and the time spent in transit, sorting and abrading. The vast majority of the rocks in Elder Creek are turbidites, formed from turbidity currents: dense slurries of sediment sloughing off the edge of the continent, rushing off the continental slope to final resting places in deeper, still waters. These currents are thought to be triggered by earthquakes, among other things.

Turbidites contain sand and pebbles that were rounded in terrestrial rivers prior to their arrival at the ocean. They also contain small clay-sized particles that fall out of the ocean water column (the long snowfall, in Rachel Carson’s words). As numerous currents are laid down over time, they create a rhythmic sequence of grain sizes, with a fining upward sequence recording stratigraphic ‘up’ (left to right in the image from the bed of Elder Creek below.)


Rhythmically bedded turbidite sequence (pebble to sand to clay size), Elder Creek. Photo credit: Jesse Hahm

Sometimes turbidity currents race over clay-sized mud deposits (shale). They pick up bits of the semi-lithified shale and carry them along. These shale bits are called rip-up clasts or intra-formational clasts. They are recognized by their darker color and angular shape, and are often much larger than the terrigenous sediment that surrounds them.

Rip-up (intra-formational) shale clasts in sandy matrix, Elder Creek

Rip-up (intra-formational) shale clasts in sandy matrix, Elder Creek. Photo credit: Jesse Hahm

Measuring stream discharge with the salt dilution technique

Measuring water fluxes in the Eel River Watershed is extremely important. We are in the midst of a multi-year drought and demands on the water supply for agriculture and rural use are only increasing.

An ongoing project at the Eel River Critical Zone Observatory is to improve existing stage-discharge relationships, to better document the amount of water flowing through the watershed. Stage refers to the height of water in the river, and discharge refers to the volume of water that flows by in a given time.

We can measure the stage with an automated system that makes use of pressure transducers, but knowing the discharge is complicated because of the ever-changing geometry of the river bed and the turbulent nature of flowing water. The approach to this problem is to develop an empirical relationship between stage and discharge across a range of stages, from low summer baseflow to high winter floods.


Salt dilution technique by David Dralle

Salt dilution technique by David Dralle

Here David Dralle is demonstrating the salt dilution technique to measure discharge on the South Fork of the Eel River, just downstream from Headquarters. A known volume of salt solution is added to a turbulent stretch of the river, and the increase in electrical conductivity is measured downstream, after the salt is well mixed into the flow. The more the salt is diluted, the higher the flow.

Dan Moore has written a very helpful series of articles on the use of the technique. For more information, see the intro to the series, published in Streamline Water Management Bulletin (http://www.siferp.org/sites/default/files/publications/articles/streamline_vol7_no4_art5.pdf)

Phil G encounters invaders in Hunter’s Pool, just downstream from Angelo


Invasive red swamp crayfish.  Photo by Phil Georgakakos.

Phil Georgakakos sent these photos yesterday of two invaders–the red swamp crayfish, Procambarus clarkii, and bullheads tentatively identified by Mary Power as black bullheads, Ictalurus melas.  (It may be a brown bullhead, check out the pectoral spine-if smooth, black, if barbed, brown bullhead, I. nebulosus).  Phil and I had seen red swamp crayfish in S.Fk. Eel just below its confluence with Ten Mile Creek near Laytonville last March (see previous post), and Sarah Kupferberg and I have found them upstream within the reserve later, but all the individuals we saw were dead.  Phil yesterday found the first live specimen–a male (see the claspers where its belly button would be, were it a placental mammal…).


Baby Bullhead. Photo by Phil Georgakakos


Photo by Phil Georgakakos

While we are concerned about these invaders harming native species in the Eel, I have to mention how cool bullheads are.  These catfish know each other individually through chemical recognition–fish respond very differently when “enemy” vs “friend” water is poured into their aquarium (google John Todd’s article in Scientific American).


Bullhead family. Photo by Phil Georgakakos.

Mother and Dad take care of the fry (the little black guys with adorable mustaches in Phil’s photo, above.), marching them around pool bottoms in little swarms as they learn to feed.   But what are they feeding on?  Bill Dietrich was just wondering if they, among other reasons, may be why we no longer see little native freshwater mussels, and are so worried about their failure to recruit in rivers along the North Coast…bullheads suck up their food from the substrate, and this might include newly settled mussels in depositional pool habitats…