Double Cone Quarterly
Winter Solstice 2001 -- Volume IV, Number 4




Sediment Yield Variations in
the Northern Santa Lucia Mountains


Barry Hecht
1


Episode: An event which suddenly changes the source of, and processes which, deliver sediment and other watershed products to a stream system, and from which the stream system gradually returns to the conditions normally encountered.

This paper is a quasi-technical discussion of variation in sediment yield from watersheds within the northern Santa Lucia Mountains. The paper is modified from its original form as published in the guidebook for the Spring 2000 field trip of the Peninsula Geological Society 2. The scientific study of natural phenomena requires a systematic naming convention that serves the important purpose of assigning universal language reference to same-type phenomenon observable world-wide. As such, this paper contains scientific terms, mostly from the voluminous and arcane language of Geological Science. As deemed appropriate, some words of the 'language' of the geologist, words such as geomorphology 3, are referenced to footnotes that contain definition of the word. The intent is to enable the lay reader to more fully understand the concepts of the science of geology, and implications of the concepts of the science, to the northern Santa Lucia Mountains. I have relied primarily on the widely acclaimed Glossary of Geology published by the American Geological Institute, Third Printing, as an authoritative foundation for the definitions.

Finally, this paper is submitted to the Ventana Wilderness Alliance, Double Cone Quarterly, with the permission of the author, Barry Hecht.

Robert Zatkin, Geologist



Overview

The quantity of sediment derived from stream channels and slopes of the northern Santa Lucia Mountains may affect numerous, interconnected landforms, and aquatic and riparian biotic habitat, in the lower reaches of watersheds through which the sediment is transported. The effects manifest from sediment moving through a watershed may include:

  • Stability of stream channels 4.
  • Levels of land inundated during floods along the larger streams.
  • Distribution of riparian vegetation.
  • Aquatic habitat associated with streams.
  • Changing quantities of sediment supplied to beaches.
  • Changing quantities of sand available for transport onto the continental slope and the deep sea canyon network immediately offshore.

Sediment yields 5 vary considerably through space and time in the northern Santa Lucia Mountains. Understanding this variability is one key to usefully reconstructing events of the recent past and anticipating channel, beach, and offshore dynamic changes likely to occur in the near future.

Present knowledge of the amount and rates of sediment transported through watersheds in the Northern Santa Lucia Mountains is based mostly on the following types of approaches and published field-based research.

  • A limited number of measurements of sediment being transported during storms (Matthews, 1989; Hecht and Napolitano, 1995).

    Note: Names and dates in parenthesis at the end of a sentence reference a paper by the given author(s) and the date of publication. Complete citations for the references are located at the end of this paper in the section titled References Cited.

  • Measurements through time of sedimentation rates in three reservoirs located in the northern Santa Lucia Mountains (Woyshner and Hecht, 2000).

  • Miscellaneous observations made by biologists and engineers in the course of evaluating biotic habitat in stream channels, the stability of stream channels through time, or the potential for flooding along reaches of specific streams.


These approaches to estimating quantities of sediment, and rates of sediment transport through time, have been developed during the past thirty years. Prior to about 1970 the absence of such data, and the analysis of the data, precluded developing even initial meaningful assessments of sediment yields

Sediment originating in the northern Santa Lucia Mountains is transported in approximately equal proportions as suspended sediment 6, and as bedload sediment 7 (Kondolf, 1982). This approximate equal distribution of sediment transported by these two mechanisms stands in contrast to many other central California coastal streams in which bedload is frequently 10 to 20 percent of the sediment in-transport in channels supplied by large watersheds.

The predominant types of sediment transported in stream channels of the northern Santa Lucia Mountains are derived from granitic parent material of the Salinian block 8 or crystalline-metamorphic parent rock 9. These parent materials often weather to relatively coarse sands, normally transported as bed load. A recent analysis of portions of the sediment retained in San Clemente Reservoir located on the Carmel River (Moffatt and Nichol, 1996) indicates that about 95 percent of the sediment is sand-primarily coarser than 0.25 millimeters in diameter.

Spatial Variability

The data generated to-date indicates significantly greater temporal variability, which effectively masks sub-regional or watershed-by-watershed tendencies. In keeping with sediment yield patterns observed elsewhere in the world, limited information currently available indicates that the sub-arid portion of the northern Santa Lucia Mountain region, with a mean annual rainfall less than 20 inches, appears to yield more sediment per unit area than the sub-humid areas of the region, with a mean annual rainfall 20 to 40 inches.

An important mechanism for the apparently higher rates of sediment yield from the drier areas is mobilization of sediment stored in valley fill 10 by the down-cutting of the larger streams into the alluvium 11 deposits that veneer stream valleys (Williams and Matthews, 1983; Hampson, 1997). Higher topographic relief in the watersheds located in the wetter areas of the region contributes an unquantified, but greater volume of sediment to yields from the headwaters due to the effects of gravity on steeper slopes and greater rainfall quantities due to orographic lift 12, than watersheds with lesser topographic relief. An additional complexity is the difficulty in distinguishing the effects of climate from the effects of livestock grazing, or other human land-use practices, which tend to affect the drier areas to a greater degree than wetter areas.

The geologic material underlying a given watershed strongly affects the mechanisms, and presumably the rates, of erosion, and therefore sediment transport and sediment yield, from a given watershed. About one-quarter of the northern Santa Lucia Mountains are underlain by Tertiary sandstones and shales including shales of the Monterey Formation, or Mesozoic age Franciscan Formation and Great Valley Assemblage. There is little knowledge of sediment yields from these exposed rock types (Hampson, 1997; Matthews, 1989), therefore rates and processes may be best inferred from adjoining areas with similar erosional influences (Brown, 1973; Hecht and Enkeboll, 1981; Hecht and Kittleson, 1997-for Tertiary sediments; Brown and Jackson, 1973; Knott, 1976; Hecht, 1983-for the older rocks).

Short-Term Variability

The rates and processes of sediment transport vary between transport episodes in the northern Santa Lucia Mountains. In particular, sediment yields following large wild land fires can abruptly change sediment transport and deposition, and produce fundamental changes in the processes which move sediment (Cleveland, 1973, 1977; Jackson, 1977; Hecht, 1993). A relevant example of sedimentation being altered by wildland fire is the case of Los Padres Reservoir. During the winter following the Marble-Cone fire of July and August 1977, the volume of sediment in the reservoir which began filling in 1947, was effectively doubled (Hecht, 1981). Another example from the same fire is the east draining Arroyo Seco which aggraded 13 nine feet at the Green Bridge crossing upstream of Greenfield. The channel bed was gradually exhumed 14 during the following four to six years (Roberts and others, 1984). Further downstream from the crossing, the non-cohesive 15 banks of the Salinas River were destabilized during the ten years following the Marble-Cone fire, most notably during the storms of 1983.

The pulse of suspended sediment in the lower Salinas River generated by the Marble-Cone fire was several times larger than that the pulse transported by the record storms of January and February 1969--two of the regional floods of record. Other geomorphically significant, yet smaller, episodic events that generated large volumes of sediment have been attributed to large regional storms, landsliding associated with large-scale civil engineering works grading, and channel incision and instability (Kondolf, 1982; Williams and Matthews, 1983; Matthews, 1989; Hampson, 1997; Woyshner and Hecht, 2000).

It is difficult to evaluate sediment yields, or sediment storage, in the stream channels of the northern Santa Lucia Mountains without knowing the recent local history of fire, floods and other sediment producing events. For example, the storm of March 1995 fundamentally altered sediment transport and deposition, and channel stability on several regional streams, for example Cachagua Creek (Kondolf, 1995). Channel incision events occurred on Tularcitos Creek during 1983 and 1998 events which altered the entire Carmel River corridor downstream. At the campground in Pfeiffer Big Sur State Park, debris flows 16 generated following fires have complemented overbank flooding during 1995, which left atypical high mountain valley vegetation washed in from the watershed growing on the floodplain downstream from the mountain front (Jeff Norman, pers. comm.).

Episodic variability is sometimes caused and extended by two or more discrete unusual events occurring concurrently, or nearly concurrently. The high rates of sediment yield following the Marble Cone fire are likely related to a very large buildup in fuel load caused by a record snowstorm in January 1974 (Griffith, 1978). The numerous hardwood tree and shrub limbs which broke off during the snowstorm were thoroughly dried by a hard drought during 1976 and 1977 prior to the fires of July and August 1977. Indeed, 1976 and 1977 were the driest and third-driest years in a century of measuring rainfall at Big Sur. The winter of 1977 fire and 1978 sedimentation episode resulted from the sequential occurrence of snow, followed by drought, then lightning-induced wildland fire.

Sediment eroding during isolated or compound episodes can be rapidly removed from source areas and delivered to the lower alluvial reaches of the main stem stream, or to the near-shore marine environment. Factors promoting quick recovery of the sedimentary system and related in-stream and riparian habitat conditions are rapid curtailment of the source of sediment, such as the regrowth of vegetation following a fire, and maintenance of downstream channel stability along the river downstream. Resistant, boulder-lined channels will often recover quickly from a sedimentation event. Soft, non-cohesive stream channel banks may erode and retreat during rapid delivery of sediment, adding substantial volumes from the bank or bed storage of sediment to the event-related yield of sediment occurring far upstream. The additive yields of sediment from these primary and secondary sources can result in large accumulations of sediment in the lower alluvial reaches, or increased contributions to the adjoining marine continental shelf over a period of a few years. These marine accumulations of continental-derived sediment may potentially generating density currents in the marine environment which move down the continental slope in the direction of the abyssal plain. It may be that depositional sequences in the offshore canyons or abyssal plain are most likely to be generated or preserved following episodic events in the high-relief setting of the northern Santa Lucia Mountains.

Valley-Filling Events

The geological evidence points to a number of periods of valley-filling aggradation throughout the northern Santa Lucia Mountains. Multiple river terraces 17 are visible at elevations of 1,000 feet or more above sea level along the larger streams, such as the Carmel, Big Sur River, and Little Sur River. The terraces appear to be remnants of once-continuous and presumably coeval surfaces, now discontinuous and partially buried beneath alluvial cones or slopes of colluvial 18 deposition.

Kondolf (1982) showed that the flood of 1911, perhaps in combination with a smaller event in 1914, left deposits which now form much of the floor of the Carmel Valley. The flood(s) resulted in sedimentation typically two to 20 feet thick over much of the eastern half of the valley. No subsequent events have approached the elevation, magnitude, or extent of deposition of the 1911 and 1914 events. The river terraces visible high above the present-day valley floor may be eroded relics of comparable depositional epicycles in the past.

The existence of river terraces, alluvial benches 19, and alluvial fans are often ascribed to climatic change-typically to the drying stages of the fluctuations prevailing throughout the Quaternary 20. This mechanism may, or may not, play a significant role in the occurrence of valley-filling events. It might be held that this epicycle followed a period of protracted drying from the preceding glacial maximum, circa 15,000 years before present, when the Monterey Bay region was colder and wetter-conditions sufficient to locally support Sitka spruce woodlands (Adams, 1975). It is also possible that the valley-filling episodes are associated simply with peaks in sediment supply. Such peaks might be created by discrete events, such as massive erosion following wildland fires, or slope failures during earthquakes. The peaks may also be an artifact of the rapid topographic rise of the northern Santa Lucia Mountains--a phenomenon due largely to mountain building phenomenon associated with large scale faults. Another possibility is that very large sediment loads might be generated by temporary obstructions of the main channel, such as the debris flows of 1973 (Cleveland, 1977). Rapid erosion of large point bars 21 or previous generation(s) of terraces as rivers erode beneath the riparian trees which hold such features in place in less tectonically-dynamic settings 22, or by large failures of soils and regolith 23 as the river undercuts metastable 24 slopes not undercut for many years (Kondolf, 1995).

Many houses and extensive public infrastructure have been built on deposits of the 1911 and 1914 floods in the Carmel River Valley. The scale of such valley-filling epicycles should be understood, if only in deference to the implications of such events to public safety. Such events also have a significant, albeit little understood role in the regional sediment budget 25 when considered in the context of the geologic time scale. Although perhaps better known from Southern California or the Wasatch Front of the state of Utah, the causes and implications of valley-filling events in a rapidly-uplifting region, such as the northern Santa Lucia Mountains, merit consideration and evaluation in many aspects of local geological investigations, and land-use planning at the local or watershed scale.

Conclusions

Sediment yields in the northern Santa Lucia Mountains region are variable spatially, with underlying geology, rainfall and relief as significant influences. Great variation of sediment yields in this region occurs over short term and geologic time frames. The sedimentary record preserved in reservoirs and Quaternary stream terraces within the region helps in understanding accumulation in the lower valleys and near-offshore environments during the recent geologic past. The episodes which generate sediment yields in the high-relief setting of the northern Santa Lucia Mountains may aid in understanding, and be better understood through, the turbidites 26 and other rock-types reflecting high-energy depositional environments 27 recorded in the Jurassic and Cretaceous age rock record preserved along the coast.

References Cited

Adams, D.P., 1975, A late-Holocene pollen record from Pearson's Pond, Weeks Creek Landslide, San Francisco Peninsula, California: U.S. Geological Survey Journal of Research, v. 3. no. 6, p. 721-731.

Brown, W. M., III, 1973, Erosion processes, fluvial sediment transport, and reservoir sedimentation in a part of the Newell and Zayante Creek basins, Santa Cruz County, California: U.S. Geological Survey Open-File Report, 29 p.

Cleveland, G.B., 1973, Fire + rain = mudflows Big Sur 1972: California Geology, v. 26, no. 6, p. 127-135.

Cleveland, G.B., 1977, Analysis of erosion following the Marble Cone fire, Big Sur basin, Monterey County, California: California Division of Mines and Geology Open File Report 77-12. 11 p.

Glysson, G. D., 1977, Sedimentation in Santa Margarita Lake, San Luis Obispo County, California: U.S. Geol. Survey Water-Resources Investigations Report 77-56, 15 p.

Griffith, J. R., 1978, The Marble-Cone fire ten months later: Fremontia, v. 5, p. 8-14.

Hampson, L., 1997, Sediment transport analysis, Carmel River near Carmel, Water years 1992 - 1995: Monterey Peninsula Water Management District Tech. Memorandum No. 97-03.

Hecht, B., 1981, Sequential changes in bed habitat conditions in the upper Carmel River following the Marble-Cone fire of August 1977, in Warner, R.E. and Hendrix, K.M., eds., California Riparian Systems: Ecology, conservation and productive management. University of California Press, pp. 134-141.

Hecht, B., 1983, Substrate enhancement/sediment management study, Lagunitas Creek, Marin County-Phase IIIb: Sediment transport and bed conditions, 1979-1982: H. Esmaili & Associates consulting report prepared for the Marin Municipal Water District, 173 p.

Hecht, B., 1993, South of the spotted owl: Restoration strategies for episodic channels and riparian corridors in central California: Proceedings of the Society of Wetland Scientists, Western Wetlands Conference, March 25-27, 1993, Davis, CA. 18 p.

Hecht, B., and Enkeboll, R., 1981, Channel and substrate conditions, sediment transport, and alternative approaches for sediment management in Zayante Creek below the proposed Zayante Dam: H. Esmaili & Associates, Inc. report to D.W. Kelley, Aquatic Biologist and the City of Santa Cruz. 93 p.

Hecht, B., and Kittleson, G.A., 1998, An assessment of streambed conditions and erosion control efforts in the San Lorenzo River watershed, Santa Cruz County, California: Balance Hydrologics consulting report prepared for Santa Cruz County Environmental Health Division. 77 p.

Hecht, B., and Napolitano, M.N., 1995, Baseline characterization of sediment transport and sedimentation at the Santa Lucia Preserve, Monterey County: Interim report: Balance Hydrologics, Inc. consulting report prepared for the Santa Lucia Preserve. 16 p. + 3 appendices.

Jackson, L.E., 1977, Dating and recurrence frequency of prehistoric mudflows near Big Sur, Monterey County, California: U.S. Geological Survey Journal of Research v. 5, no. 1, p. 17-32.

Knott, J., 1976, Sediment discharge on the upper Arroyo Grande and Santa Rita Creek basins, San Luis Obispo County, California: U.S. Geological Survey Water- Resources Investigations 76-64, 29 p.

Kondolf, G. M., 1982, Recent channel instability and historic channel changes of the Carmel River, Monterey County, California, M. Sc. Thesis, UC Santa Cruz, 120 p.

Kondolf, G. M., 1995, Channel processes and riparian habitat in the deltas of Los Padres and San Clemente Reservoirs, Carmel River, California: Review of MPWMD Technical Memorandum 95-01. Consulting report prepared for the Monterey Peninsula Water Management District, 16 p.

Matthews,W.V.G., 1983, Discharge and sediment load for tributaries to the Carmel River: Carmel River Watershed Management Plan Working Paper No. 5, Monterey Peninsula Water Management District report to the California Department of Fish and Game, 95 p.

Matthews, W.V.G., 1989, Evaluation of reservoirs sedimentation rates in the upper Carmel River Watershed: Tech.

Monterey Peninsula Water Management District, 1988, Memorandum 88-03. 16 p.

Moffatt & Nichol Engineers, 1996, San Clemente Reservoir dredging feasibility study, Carmel Valley, California: Consulting report prepared for California American Water Company, Monterey Division, multipaged.

Roberts, B.R., Shanahan, E., and Hecht, B., 1984, Salinas River study, River morphology and behavior: Anderson-Nichols & Co., Inc. consulting report to the Monterey County Flood Control and Water Conservation District.

Williams, J. and Matthews, W.V.G., 1986, The 1983 erosion event on Tularcitos Creek, Monterey County, and its aftermath: Proceedings of the California Watershed Management Conference, Nov. 18 - 20, 1986, West Sacramento, California, Univ. of Calif. Wildland Resources Center Report No. 11, p. 155.

Woyshner, M.R., and Hecht, B., 2000, Hydrology, channel stability, and sediment management in the Carmel River: Balance Hydrologics, Inc. contributions to the Draft EIR for the Seismic Retrofit of the San Clemente Dam prepared by Denise Duffy & Associates. Multipaged, In press.

Footnotes

1 Barry Hecht, Principal
Balance Hydrologics
900 Modoc Street
Berkeley, CA 94707
(510) 527-0727(ph)
(510) 527-8531 (fax)
bhecht@balancehydro.com


2 Peninsula Geological Society, Spring Field Trip 2000
Salinia/Nacimiento Amalgamated Terrane
Big Sur Coast, Central California
May 2000


3 Geomorphology - The study of the classification, description, nature, origin, and development of present landforms and their relationships to underlying structure, and of the history of geologic changes as recorded by these surface features.


4 Stability - The quality of permanence or resistance of a stream channel, slope or embankment, to failure by sliding, overturning, collapsing, or other prevailing condition of stress.


5 Sediment yield - The amount of material eroded from the land surface by runoff and delivered to a stream system.


6 Suspended sediment (suspended load) - That part of the total stream load that is carried for a considerable period of time in suspension, free from contact with the stream bed. It consists mainly of mud, silt and sand.


7 Bed load sediment - That part of the total stream load that is not continuously in suspension or solution. Bed load is the part of the total stream load that is moved along, near, or immediately above, the stream bed, such as the larger or heavier particles (boulders, pebbles, gravel) transported by short intermittent leaps, jumps, hops, or bounces from the channel bed (the latter is named traction or saltation).


8 Salinian block - The Salinian basement complex (block), composed of granitic rocks coeval to granitics of the Sierra Nevada, is one of the two basement complexes that underlies the other types of rocks in the northern Santa Lucia Mountains. The other basement rock is the Franciscan subduction complex. A basement complex underlies the oldest identifiable rocks in an area or region. Both basement complexes outcrop extensively north and south of the Big Sur region. Within the northern Santa Lucia Mountains the Salinian basement underlies most of the Monterey Peninsula, and from the Carmel River south intermittently to Point Sur. These coastal outcrops of Salinian basement granitics trend northwest-southeast and form portions of the exposed core of the Santa Lucia Mountains. The Franciscan subduction complex exhibits limited outcrop from the base of the Santa Lucia Mountains at Point Sur west to the beaches, and south to approximately the Post Ranch. Owing the extensive outcrop within the northern Santa Lucia Mountains, Salinian granitic rocks are one of two primary sources of sediment transported through west draining watersheds; the other primary source is the Sur Series (see footnote 9).


9 Crystalline-metamorphic parent rock - The crystalline-metamorphic parent rocks belong to the Sur Series (or Sur complex), an amalgam of metamorphosed rocks including schists, marbles, and gneiss, that outcrop extensively in the northern Santa Lucia Mountains. These rocks are underlain predominately by Salinian basement, and are the second primary source of sediment transported through west draining watersheds.


10 Valley fill - The unconsolidated sediment deposited by any agent (stream, slope transport during rainfall producing runoff, wind) so as to fill or partly fill a valley.


11 Alluvium - A general term for unconsolidated detrital material deposited during recent geologic time by a stream or other body of running water as a sorted or semi-sorted sediment in the bed of a stream, or on the streams flood plain or delta; or, as a cone or fan at the base of a mountain slope.


12 Orographic lift - Precipitation that results when moisture-laden air encounters a high barrier or is forced to rise over it, for example the precipitation on the windward slopes of a mountain range facing a steady wind from a warm ocean--or a not so warm ocean.


13 Aggradation - The building up of the Earth's surface by deposition.


14 Exhume (Exhumation) - The uncovering or exposure by erosion of a preexisting surface, landscape, or features that had been buried by later deposition.


15 Non-cohesive - A soil that has relatively low shear strength when dry.


16 Debris flow - A type of landslide characterized by the rapid flow of debris of various types under various conditions. A high-density mudflow containing abundant coarse-grained materials and resulting from an unusually heavy rainfall event.


17 River terrace (stream terrace) - One of a series of level surfaces in a stream valley adjacent to, flanking, and more or less parallel to, the stream channel. Such terraces originally occur at or below, but presently above, the level of the stream, and represent the dissected remnants of an abandoned flood plain, stream bed, or valley floor produced during a former stage of erosion or sediment deposition.


18 Colluvium - Loose, heterogeneous, and incoherent mass of soil material or rock fragments deposited mainly by mass-wasting, usually at the base of a steep slope or cliff.


19 Alluvial bench - A broad gently inclined surface extending along, and outward from, the base of a mountain range. The bench is formed by the lateral coalescence of a series of separate alluvial fans.


20 Quaternary - The last two or three million years of time as discerned in the rock record.


21 Point Bar - One of a series of low, arcuate ridges composed of sand and gravel developed on the inside of a growing meander bend of a stream by the slow addition of individual accretions accompanying migration of the stream channel in the direction of the outer stream bank.


22 Tectonically dynamic settings - Geologic environments in which the crust, or surface, of the Earth contain features produce by Earth forces. The northern Santa Lucia Mountains, regions to the north and south, and much of the State of California contain tectonically dynamic settings as indicated by large-scale structural features such as faults and folds, and mountain ranges.


23 Regolith - The entire layer or mantel of fragmental and loose, incoherent rock material that nearly everywhere forms the surface of the land and overlies or covers the more coherent bedrock.


24 Metastable - The state of a system, in this case mountain slopes, that is table with respect to small disturbances, but is capable of change is sufficiently disturbed.


25 Sediment budget - An accounting of the sources and disposition of sediment s it is transported from its point of origin to its eventual exit from a watershed.


26 Turbidite - A sediment or rock deposited from a turbidity current. A turbidity current is a density current that occurs in water, air, or other fluid, that is caused by different amounts of matter in suspension. For example, a bottom-flowing current containing suspended sediment, moving swiftly under the influence of gravity, down the continental slope and spreading horizontally on the floor of the abyssal plain.


27 Depositional Environment - The environment in which sediments are deposited.




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