The Watershed Physical Description

Overview

The Scotts Creek watershed is located on the North Coast of Santa Cruz County, California. It is thirty square miles of steep forested slopes, deep canyons with steep inner gorges, fertile narrow valleys, and grasslands. Most of the watershed is covered by forest. The creek system empties into the Pacific Ocean in the Monterey Bay National Marine Sanctuary at approximately 37° 02' 28" N and 122° 13' 50"W. Scotts Creek and its main tributaries Big Creek, Mill Creek, and Little Creek are perennial streams. Smaller tributaries such as Queseria Creek are usually perennial, whereas, Winter Creek and Archibald Creek are intermittent. There are numerous narrow gulches in which water flows during the wet season or, in some cases, only during precipitation events.

Rapid tectonic uplifting and down-cutting by stream erosion and landsliding has created canyons with inner gorges where steep slopes terminate in the creeks. The top of the watershed is at an elevation of a little over 2,600 feet above sea level. The elevation descends to sea level over a lateral distance of about seven miles (measured perpendicular to the shoreline). Parts of the canyons and valleys have narrow floodplains only a few hundred feet wide at most. Other parts have no floodplains at all. Relatively narrow ridges separate the drainages.

Upper Scotts Creek and all the major tributaries flow southwest directly towards the coastline. A ridge along the ocean shore turns Scotts Creek to the southeast and thus extends the length of the lower gradient reaches by about five miles. The tributaries are nearly perpendicular to lower Scotts Creek.

The creek channels are incised into the soils, alluvial sediments and landslide deposits of the valley floor. Rapid tectonic uplift combined with a “flashy” flow regime cutting through easily-scoured materials is likely the primary mechanism causing channel incision. The Scotts Creek watershed regularly experiences some of the highest rainfall intensities in the state. Flashy runoff from these rains drive a high rate of natural background erosion and frequent landsliding. The local bedrock is highly fractured, decomposes easily, and the resulting material is easily transported by flowing water.

Hydrological Character
The flow regime in the Scotts Creek watershed is naturally “flashy” in character. Winter storms come in off the Pacific Ocean laden with moisture and slam into the Santa Cruz Mountains. This area, particularly in the north coastal mountains of Santa Cruz County, has one of the highest rainfall intensity ratings on the west coast (Ranz, 1968). The resulting high flow stages transport large amounts of sediment and woody debris. A wide variety of types of landslides are triggered by super saturated conditions and intense rain. After the rainy season creek flows diminish steadily and predictably until the onset of the following rainy season.

Rainfall and Air Temperature

The mean annual precipitation is about 30 inches in the lower watershed (unpublished data from Big Creek Ranch 1955 to present) and 45 inches in the upper watershed (unpublished data from Lockheed Martin 1976 to present). The maximum and minimum rainfall year totals from the Lockheed Martin data are 106 inches in 1982/83 and 27 inches in 1976/77. Corresponding data from Big Creek Ranch are the maximum in 1997/98 of 71 inches and a minimum of 13 inches in1975/76. It is not unusual for rainfall totals to vary widely from one year to the next. During periods of high rainfall intensity, the upper watershed typically receives about twice as much rain as the lower watershed.

The mean annual air temperature is about 57 degrees ºF. From December through March, average temperatures range from highs in the 50s to lows in the 20s ºF. In the spring, average daytime highs are in the 60s with lows from 30 to 40 ºF. Summers often have cool morning fog that usually burns off around midday. Prolonged periods of marine radiating fog common to this region in summer can help to support base flows by reducing the rate of evapotranspiration from trees in this mostly forested watershed. Summer temperatures average highs from 75 to 95 ºF, and lows ranging from 40 to 50 ºF. Fall is characterized by warm days and cold nights with occasional early season rainstorms. Average highs range from 60 to 75 ºF, with lows ranging from 30 to 50 ºF (BBRSP 1999). The riparian zone is cool with temperatures typically 5-10º less than surrounding areas.

Stream Flow

Watershed hydrography indicating base flow, peak flow, and flow-timing characteristics is not available as recent long-term measurements. Data were recorded at a USGS gauging station for the years 1921 - 1982. A mean peak flow of 275 cubic feet per second (cfs) in late January and a late summer base flow of less than 2 cfs was recorded. The gauging station was abandoned after it was severely damaged in the floods of 1982. Efforts to reinstate a gauging station on lower Scotts Creek are currently in progress. Wet-season rainfall totals can be used as an accurate predictor of summer flow volumes (Briggs, unpublished report, 2002). This linear relationship has been demonstrated to be reliable.

Flood History

Few records of early floods are found in the literature. Rainfall and flooding records must be specific to the watershed because of the spotty nature of intense rainfall in the local region. More recent flood information comes from long-time residents of the Swanton area and their personal recollections of past flood events. Major flooding events have occurred in 1940, 1955, 1982, and 1988. Prior to 1998, the flood of 1955 was considered to have been the most damaging, including some lives being lost. The 1998 floods exceeded the flow levels recorded in 1955, although there are no official stage records from either of these floods.

All major storm and flood events have resulted in dramatic changes in riparian vegetation and increased sediment transport and deposition. The age of the alders along the streams in the watershed date back to these specific flood events. Most of the mature alders date from the 1955 flood event, when major scouring of the stream course took place due to high flows and large amounts of woody debris transported. Aerial photographs from 1957 show that nearly all of the streamside vegetation of Scotts Creek from the confluence of Big Creek down to the ocean was stripped away by the 1955 flood. The riparian zone was free of vegetation over a width of more than one hundred feet, and in many reaches, considerably more. Multiple landslides and debris flows in the Little Creek canyon may have caused a temporary damming of the creek and an ensuing dam burst that resulted in a large flow out of the canyon. The impact of this is readily apparent in Little Creek on the 1956 set of aerial photographs.

Drought History

Significant droughts occurred in 1959, 1964, 1966, 1968, 1976, and 1977. During severe droughts the main stem of Scotts Creek will be dry between pools, but there is still evidence of interstitial flow through the creek bed sediments. Droughts are the direct effect of very low wet-season rainfalls, but the severity can be exacerbated by dry-season atmospheric conditions. Summer temperatures and the presence/absence of marine-radiating fog have a strong effect on base flows. High temperatures and strong insolation result in a high rate of evapotranspiration through riparian and upland vegetation. Much of the watershed area is heavily forested; thus this effect is stronger than for watersheds that include larger grasslands/coastal prairie. One acre of second growth (medium sized trees) managed redwood forest consumes 380,000 gallons of water per year (Briggs, 2001) while one acre of irrigated pasture consumes 70,000 gallons per acre and row-cropped vegetables need 140,000 gallons per year (California Water Resources Control Board). A high percentage of forest cover may result in lower creek-flow volumes during the dry season.

Historical and prehistoric evidence indicates that extreme droughts of long duration are not unusual for California (Benson et. al., 2002; Jones et. al., 1999; Stine, 1994; Weiss, 2002; Woodhouse and Overpeck, 1998).

Local Aquifer

Little is known about the aquifer(s) of the Scotts Creek watershed. The highly fractured bedrock (mudstone, sandstone, and granitic rock) act as an aquifer (or aquifers) as does the overlying alluvial and colluvial sediment. Lower Scotts Creek and nearby valley floor sediments appear to be quite thick, based on some water well drill logs. A well drilled on the Swanton Pacific Ranch near the train roundhouse (at Swanton Rd. 2.9 mile mark) hit a redwood log at 85 feet and reached the bottom of alluvial sediments at about 100 feet. Some wells drilled for agricultural irrigation in the lower Swanton Valley (at Swanton Rd. 2 mile mark) and a well recently drilled about midway along the valley floor (at Swanton Rd. 4.1 mile mark) reached the bottom of alluvial sediments at about 100 feet as well. A thick alluvial aquifer is likely to be fairly complex, composed of many different types of sedimentary deposits, all varying in thickness, size, porosity and permeability. It is not clearly understood to what degree water communicates between the bedrock and alluvial aquifers.

Channel Characteristics

The channel morphology appears to be largely controlled by creek bank tree failures and movement of woody debris through the system, landslides, and high flow events. Tree falls and the creation of log jams result in changes to the streambed profile and to the distribution of sediments. Channel widening occurs where the creek erodes away its banks in order to bypass the obstruction. Deep pools are often scoured under log jams. Gravel bars that have become embedded with sand over the years mobilize, resulting in the resorting and grading of sediment particle sizes. Gravel bars are often redeposited as clean, well-sorted gravels and sand drops out of suspension and bed-load transport of sand slows where the flow velocity slows.

Landsliding (including debris flowage) puts large quantities of sediment into the creeks nearly instantaneously. Since slides mostly occur during storm events, the associated high creek flows can transport the bulk of these sediments and distribute them through the system rapidly. The resultant high levels of aggradation in some reaches can cause acceleration of bank erosion and tree failures, thus leading to a broadening of the channel.

An increase in channel width after the 1998 flood was measured in a cross section survey on Scotts Creek. A baseline survey (longitudinal profile and cross sections) was performed during the summer of 1997. The high flows of the following El Nino winter, caused by extremely high rainfall intensity, were believed to have brought about changes to the Scotts Creek channel. A re-survey (longitudinal profile and cross sections) was conducted during the summer of 1999. The findings revealed negligible effect on the overall channel profile, but noted that a widening of the channel had occurred at many of the survey locations. Bank erosion was obvious in some places and more subtle in others. Some of the stakes marking the locations of profiles had been scoured out and washed away while others had been buried under as much as three feet of sediment. An increase in the accumulation of large woody debris was noted. Localized scour and fill were evidenced by changes in the locations of pools, riffles, and glides, but no net aggregation or degradation was made apparent by the survey. Material eroded from the banks was assumed to have been deposited on the floodplain. It was concluded that Scotts Creek has a high competence for detaching and transporting sediment during high flows. The above-described surveys on lower Scotts Creek and the summary conclusions were by Cal Poly graduate student Brooke Akers (unpublished report, 2000).

Salt Marsh

The 32-acre wetland at the mouth of Scotts Creek includes a tidally-influenced marsh dominated by tule rushes (predominantly Scirpus californicus). This property is owned and managed by Cal Poly. The causeway and bridge of Highway 1 (constructed in 1939) have a combined length of 1120 feet. This structure separates the marsh from the beach and is owned and managed by the California Department of Transportation (Cal Trans). The beach is owned and actively managed by the County of Santa Cruz. The bridge is centered between the bedrock bluffs to the north and the bedrock/dune to the south. The opening under the bridge is 120 feet wide with three central abutments that are each 22 inches wide. Cal Trans has been exploring the idea of replacing this bridge with the intent of improving safety and, (under the directive of the California Coastal Commission), perhaps, restoring the beach/marsh interface to a more natural state.

The bridge, and levees (constructed by the Army Corps of Engineers) directing Scotts Creek in a straight line through the marsh, altered the natural functioning of the wetland. Prior to these alterations, the creek channel was more sinuous through the marsh and the creek outlet was at the north end of Scotts Creek beach. Strong seasonal northwest winds built sand dunes to the south and would have predominantly kept the creek mouth pushed towards the north end. The active beach/marsh interface would most likely have been about 500 feet wide.

Laguna de Las Trancas

This ridge top pond/wetland is at least 50,000 years old (Sarna-Wojcicki (USGS). It has been dated through the use of tephrochronology (dating volcanic ash deposits) performed on core samples. Its location on a ridge top and local landforms suggest that the topographic depression creating the pond was formed by the head scarp of a large rotational landslide/slump. The pollen record in pond sediments reveal some surprising information about the pre-history flora of the region. A core from the pond studied by Adams in 1981 recorded sedimentation back to 30,000 years before present. This core revealed that forest cover was predominately Monterey pine (Pinus radiata) and grand fir (Abies grandis) while redwood (Sequoia sempervirens) did not occur as the dominant forest species until 11,000 years ago at the outset of the Holocene or Recent Era (Adams, 1981).

Mill and Boyer Creek Reservoirs

The Big Creek Power Company constructed a relatively large (for that time) hydroelectric power generating plant on Big Creek, beginning operation in 1895. This facility operated until 1948 when its flumes were destroyed by a forest fire. The power plant diverted large volumes of flow, sometimes nearly the entire flow, from Mill Creek, Boyer Creek, and Big Creek. A reservoir was constructed on Boyer Creek to divert water via a wooden flume to the power plant’s penstock. The reservoir is now in late stages of sediment infilling and eutrophication. Very little open water still occurs there. The Mill Creek dam (constructed for the power plant’s water supply) and reservoir (about 11 acres in area and up to 30 ft. deep) still exist and are maintained by the Lockheed Martin Space Systems Company. This company uses the reservoir for water storage.

Biological Setting

Forest/Grassland Setting

The watershed is dominated by forest. There are also substantial areas of grassland. Some of the grasslands are considered to be remnant native coastal prarie, others are of unknown origin. It is known that the local Native Americans employed a practice of burning that maintained open grasslands. It is not known to what degree that practice was used in the Scotts Creek watershed. Local historical knowledge indicates that forest cover in the watershed is currently of greater extent than at any previously known time. The diversity of forest types and tree species in the upper watershed is quite high. The inner gorges are typically dominated by redwood (Sequoia sempervirens) and Douglas-fir (Pseudotsuga menziesii). Some ridge tops exhibit chaparell-like assemblages in dry areas of rapidly draining bedrock. These zones are dominated by a variety of manzeneta, madrone (Arbutus menziesii ), coast live oak (Quercus agrifolia) and other Quercus species, and knobcone pine(Pinus attenuata.  Part of the Ano Nuevo stand of Monterey pine is located in the watershed. This is the northernmost of the three endemic and native stands of Monterey pine. The riparian forest below the steep inner gorges is dominated by red alder (Alnus rubra). Bay laurel, big leaf maple, box elder, various willows, and creek dogwood are also abundant in this zone. The lowermost riparian forest just above the marsh is dominated by willow.

The Flora

Scotts Creek Watershed is home to over 600 native plant species and subspecies, which comprise more than 10% of California’s flora. Of the state’s 173 plant families, 88 are represented in the watershed (West 2002). About 1,200 specimens (in the form of pressings) from the watershed are catalogued at the Cal Academy of Sciences and the Jepson Herbarium. These specimens were collected by Roy Buck PhD (U.C. Berkeley), Randy Morgan (Research Associate, California Academy of Sciences), Grey Hayes PhD (U. C. Santa Cruz). These three scientists are all contributors to the new Jepson Manual. Their collective experience in the Scotts Creek watershed represent seventy years of expert scientific observation. Expert amateur botanist James West has a lifetime of experience observing plants in the Scotts Creek Watershed. He has contributed greatly to the knowledge of the watershed’s flora. A narrative description by James West of the watershed from a botanical point of view can be found on this website at “A Botanist’s View of the Scotts Creek Landscape.”

The Fauna

A complete inventory of the fauna of the watershed has not been undertaken. Lists of some groups are fairly complete and were compiled by experts with specific knowledge and substantial experience in the watershed. Specific knowledge that can add to these lists is welcomed. If you have observed a species in Scotts Creek watershed that is not included the lists please contact us. If you have or know of any substantially complete list of a fauna group for Scotts Creek watershed that does not appear on this website please do contact us.

The Lists

Scotts Creek Flora

• Comprehensive Native Plant List - Native A
• Plant list Nonnative

Scotts Creek Birds

• Explanation of list
• Bird list

Scotts Creek Amphibians and Reptiles

• Description of lists
• Amphibian list
• Reptile list

Scotts Creek Mammals

• Description of list
• Mammal list

Scotts Creek Fish

• Fish list

Scotts Creek Butterflys

• Description of list
• Butterfly List