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Technical Memorandum

DATE: July 17, 2009 To: Neal Conatser, Assistant Engineer, Marin County Department of Public Works From: Michael Love P.E., Principal Engineer, Michael Love & Associates [email protected] / ph: 707-476-8938 / fax: 707-476-8936 Subject: Review of Background Information and Control Alternatives for Easkoot Creek, Stinson Beach CA.

Project Background

Easkoot Creek in Stinson Beach, California is a tributary to Bolinas Lagoon in western Marin County. The creek drains a watershed of approximately 1.59 square miles (FEMA, 1997) of steeply sloping hills that border the Pacific Ocean before flowing through the town of Stinson Beach and into Bolinas Lagoon. Due to a lack of channel capacity, the frequently overflows its banks between the Arenal Avenue and Calle de Arroyo crossings (Figure 1). Nearly half of this 2,000-foot channel reach is located within the National Park Service (NPS) Golden Gate National Recreation Area (GGNRA) and is adjacent to several parking lots.

The current-day stream reach of Easkoot Creek downstream of the GGNRA parking lots crosses under a series of streets, collectively referred to as the “Calles”. The Calles from Calle del Pinos to Calle del Arroyo frequently experience flooding from Easkoot Creek, compounded by occasional flooding from wave overtopping to the west. Regular out-of-bank flooding also occurs upstream of the Calles within the GGNRA parking lots and in the area of the Parkside Restaurant.

Currently, flooding within lower Easkoot Creek is exacerbated by sedimentation that reduces channel capacity. Sedimentation and the resulting decrease in channel capacity likely contributed to the out-of-bank flooding that occurred at the Park Entrance during a relatively small flow event on January 25, 2008. The out-of-bank flow traveled northwest through the parking lot and onto the wave slope through a gap in the dunes (Neal Conatser, Marin County Flood Control and Water Conservation District, personal communication).

Historically, sediment from lower Easkoot Creek was routinely and extensively dredged to maintain hydraulic capacity. In recent years, only limited dredging at specific locations adjacent to the road crossings has been conducted. Routine dredging of the channel is viewed as unsustainable due to cost and the regulatory restrictions associated with the presence of steelhead trout (Oncorhynchus mykiss), a Federally listed threatened species that resides in the stream.

Over the past 35 years, there have been numerous studies of flooding and flood control options for Easkoot Creek. Reports prepared by M. M. Sadjadi (1971) and by William Spangle and Associates July 17, 2009 Page 2

(1984) developed several alternatives to alleviate flooding. These studies recommended diverting or bypassing high flows directly to the ocean through a bypass channel or some type of overflow feature while maintaining “normal” flows in the stream channel downstream of the bypass point. Several potential bypass points were identified, with the preferred point located at the existing channel bend near the Parkside Restaurant. This overflow would cross through the existing GGNRA parking lot and/or picnic area before discharging onto the wave slope. Both studies indicated that sediment management would be a critical component of the bypass channel.

The recommendations of the two engineering studies, as well as the results from more recent studies of the Bolinas Lagoon watershed, were incorporated into recommendations for the Bolinas Lagoon Ecosystem Restoration Project developed by an interagency working group (Working Group, 2008). Recommendations that directly relate to Easkoot Creek included:

1. Investigate utilizing a portion of the Golden Gate National Recreation Area Stinson Beach Parking Lot as seasonal for Easkoot Creek 2. Conduct a qualitative sediment source analysis (human induced) of the Bolinas Lagoon watershed and seek remedies for problem areas 3. Improve floodplain access and remove sediment deltas along the eastern shore of Bolinas Lagoon consistent with flood protection

Though dated, the bypass flow recommendations of the two engineering studies merit further exploration. At the time the reports were developed, protection of native fisheries was not a noted concern. Additionally, more limited dredging and sedimentation in the channel associated with large flow events in Easkoot Creek has highlighted the need to develop a sediment management plan in conjunction with any proposed flow bypass alternatives.

In July 2006, the Marin County Flood Control and Water Conservation District (District), initiated discussions with NPS regarding various high-flow bypass alternatives that would be sited in the GGNRA to address flooding issues in lower Easkoot Creek. In addition to concerns regarding infrastructure and maintenance, NPS has expressed concerns regarding the creation of a bypass system that discharges floodwaters directly into the ocean. Of particular concern is the protection of the steelhead trout, which would be adversely affected if diverted into the ocean.

Purpose of Memorandum

At the request of the District, Michael Love & Associates (MLA) has researched relevant background material and investigated the feasibility of providing adequate fish protection for juvenile steelhead for the previously proposed high-flow bypass system, as well as two additional alternatives proposed by the District. This memorandum also presents the results of readily available studies of sedimentation within Easkoot Creek and identifies additional considerations that might be incorporated into further feasibility studies and engineering design. This memorandum does not assess the flood flow attenuation benefits or actual costs or feasibility of constructing the alternatives

The three high-flow bypass alternatives for which the District requested MLA to assess protection of native fisheries are:

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1. Direct high-flow bypass channel to the ocean, as described in the William Spangle and Associates (1984) report.

2. Lateral weir/spillway to divert high flows into a with return to the existing stream channel and high-flow bypass channel to the ocean.

3. Floodplain/seasonal wetland restoration that overflows to the ocean, restoring historic processes that were associated with a wetland located at the site of the current-day Calles and GGNRA parking lots.

Review of Background Materials

Historical Geomorphic Conditions

Historic documents indicate that Easkoot Creek flowed into a seasonal wetland referred to as Willow Camp. The stream was also hydrologically connected through subsurface and potentially surface flow to a coastal lagoon referred to as Poison Lake or Willow Camp Lake (NPS, 2004 and Darren Fong, 2009 Personal Communication). During larger flow events, Easkoot Creek at Willow Camp occasionally overflowed to the ocean rather than flowing into Bolinas Lagoon. Poison Lake and adjacent wetlands were reportedly filled in the 1950’s to expand the parking lot for beach use (Spangle, 1984, NPS 2004). The Spangle and Associates (1984) report indicated that flooding issues along Easkoot Creek seemed to have increased after the filling of the area and suggested that re- establishing this flood bypass would alleviate sedimentation and flooding issues in lower Easkoot Creek.

The 1854 U.S. Coastal Survey chart and 1926 U.S. Coast & Geodetic Survey chart (with topography taken from earlier surveys) indicate that portions of Calles and GGNRA Stinson Beach parking areas were historically a wetland that received the waters of Easkoot Creek (Figure 2, Figure 3, and Figure 4). This wetland area appears to have been formed by coastal sand dunes on as many as three sides and uplands to the northeast.

The 1854 chart shows the area (approximately 20.5 acres) hatched distinctly different from the salt marsh and a mudflat to the northwest, indicating that it was a wetland or marsh (Tetra Tech, 2001a). The 1926 chart shows a similar area hatched as a wetland. Both the 1906 plat map (Figure 5) and the 1926 chart referred to this wetland area as “Willow Camp,” suggesting the marsh area was a seasonal freshwater wetland. A differently hatched area represents the extents of Poison Lake, historically in the present location of the GGNRA South Parking Lot (NPS, 2004, Darren Fong, Personal Communication).

On the 1906 plat map, much of the GGNRA parking lot areas are shown as an area of dense willows, with a ditched channel flowing northwest closely following the current-day channel alignment of Easkoot Creek. The ditch is shown ending at a willow stand near the current-day Park Entrance, with multiple flow arrows that suggest it drained into a willow wetland with no distinct channel. The map extents stop at approximately Calle del Pinos, adjacent to the Park boundary. The 1926 map shows a channel within the wetland that drains to Bolinas Bay. Historical accounts and the maps indicate that under normal flows, the creek followed an alignment from the wetland to Bolinas Lagoon similar to the present-day alignment (Tetra Tech, 2001b).

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Another channel is shown on the 1906 plat map, diverging from Easkoot Creek near the current-day Parkside Restaurant. This channel flowed through the wetland and dunes, presumably discharging into the Pacific Ocean rather than Bolinas Lagoon. The 1854 chart shows the dune field in this area narrowing to less than 80 feet wide, making it a possible location for breaching during high flows and large wave events. Recent studies have documented anecdotal evidence of this breaching prior to the filling of the wetlands (Tetra Tech, 2001b).

The low gradient and storage volume associated with the historical wetland would have presumably functioned as a depositional area for sediment, preventing coarse sediment from being transported further downstream to Bolinas Lagoon. The large size of the wetland would also have most likely stored floodwater and attenuated downstream flooding. Additionally, the breaching of the dunes in Willow Camp and possibly Poison Lake by during high flows would have reduced the amount of flow routed through the downstream Calles reach, potentially reducing the extent and duration of flooding.

The filling of the wetland, channelization of Easkoot Creek, and resulting elimination of overflow directly into the ocean would be expected to initiate notable geomorphic response within lower Easkoot Creek. These alterations to the stream effectively removed the function of the wetland as a sediment trap, likely increased in-channel flows during storm events, and possibly lengthened the high-flow channel 1,900 feet by eliminating the ability of floodwaters to breach the dunes and discharge directly to the ocean. In addition to increased flooding, a predictable response to increased sediment load and channel lengthening is for the channel bed to aggrade (Lane, 1955).

Previous Studies of Peak Flow Hydrology and Floodplain Inundation

There are no long-term flow records for Easkoot Creek. Prior streamflow gaging activities are limited to sets of measurements recorded by the NPS and the District over short periods. None of them captured large flow events. Therefore, previous studies of Easkoot Creek estimated peak flows based on hydrologic modeling and extrapolation using flows from nearby gaged basins. MLA reviewed the hydrologic computations prepared for the project area in previous reports and the results are summarized in Table 1. Note that most of the studies estimated peak discharges at the confluence of Easkoot Creek with Bolinas Lagoon (drainage area of 1.59 mi2), rather than at the Park Entrance (drainage area of 1.3 mi2).

Sadjadi (1971) estimated peak flows for a 50-year event using the Rational Method, which is based on time of concentration, rainfall intensity, and land use. A peak 100-year discharge was computed using the Rational Method for the NPS as part of the Environmental Impact of the Easkoot Creek Restoration at Stinson Beach (Circa 2003). The Rational Method is intended for smaller watersheds, and may not be as accurate in a watershed the size of Easkoot Creek.

William Spangle & Associates (1984), performed a detailed hydrologic study using HEC-1 to compute the 1, 2, and 100-year flows. The actual computation method was not defined, but HEC-1 uses a more rigorous computational method than the Rational Methods, and considers land use, soil runoff properties, slope, and flow routing in computations of flows.

FEMA (1979, revised 1997) prepared an analysis of various peak flow return periods by preparing statistical and multiple regression analyses of six USGS gaged . These results were then applied to un-gaged basins, including Easkoot Creek to estimate the 10, 50, 100 and 500-year peak

Review of Background Information and Flood Control Alternatives for Easkoot Creek, Stinson Beach, California. Michael Love & Associates July 17, 2009 Page 5 discharges. The floodplain of Easkoot Creek was mapped as part of a FEMA Flood Insurance Study (FIS) in 1986, and the FIS was revised in 1997. The Flood Insurance Rate Map places all of the Calles, GGNRA parking lot, and properties adjacent to the Easkoot Creek road crossing at Arenal Avenue within the 100-year floodplain. Due to the topography, these areas are equally inundated during the 10-year flow.

Table 1. Previous estimates of peak flows and return periods for Easkoot Creek. Return Period

Source 1-year 2-year 10-year 50-year 100-year 500-year Sadjadi (1971) - - - 845 cfs - - (to Bolinas Lagoon) William Spangle & Assoc. 700 cfs 900 cfs - 2,000 cfs 2,300 cfs - (1984) at Bolinas Lagoon FEMA FIS 1986 (1997) - - 666 cfs 970 cfs 1,090 cfs 1,350 cfs (to Bolinas Lagoon) NPS EIR (circa 2003) - - - - 1,673 cfs - at Park Entrance

Flows reported in the FEMA FIS were used in this memorandum for the purposes of evaluating concept alternatives. Prior to further developing any alternative, it will be necessary to better predict peak flows in the Easkoot Creek to prepare an accurate assessment of future flood risks and project benefits.

Sedimentation

Lower Easkoot has been historically dredged to maintain channel capacity. The last large dredging effort in was 1987. Limited sediment removal from the top of bank was performed in 1997, 2006, 2007, and 2008. Most extensive of these removals was in 1997 and 2006. (Neal Conatser, Marin County Flood Control and Water Conservation District, personal communication). In recent years, only limited dredging at “glory holes” near bridge crossings has been conducted due to budgetary and regulatory restrictions. The out-of-bank flooding event in 2008, which likely resulted from sedimentation and the resulting decrease in channel capacity, highlights the need to address sedimentation as a component of a high-flow bypass project.

Sediment Sources to Easkoot Creek Easkoot Creek drains steep mountain slopes located in the Mount Tamalpais State Park and a portion of the GGNRA area. Geology of the mountains consists primarily of the Franciscan complex, an amalgamation of sandstone, shale, greenstone and other metamorphic rocks that are highly prone to landslides (Cal. Department of Parks, 1980). Correspondence between Marin County and California Department of Parks indicates that landslides have been active since at least 1952 and likely will continue to occur, contributing sediment to Easkoot Creek (Marin County Department of Public Works, 1982; and California Department of Parks and Recreation, 1982). A letter prepared by Marin County Department of Public Works in June of 2008 (Marin County, 2008) notes “a massive active landslide in the upper reaches of the creek (on Mount Tamalpais State Park Lands)…” A similar letter from a resident available in the Stinson Beach Community Archive (Posadas, 2006) indicates that landslide debris in Tamalpais State Park was present in Easkoot Creek.

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The resident expressed concern that it may block the Highway 1 bridge at Easkoot Creek. In their 1982 correspondence, the Park investigated several slide area and recommended several actions. The Park determined that action on their property would set costly precedents and unacceptable landscape management practices in Parks, but offered cooperation with channel maintenance in Easkoot Creek.

Tetra Tech, as part of the Bolinas Lagoon Feasibility Study, prepared a sediment budget that both quantitatively and qualitatively assessed sediment sources and supply from various watersheds surrounding Bolinas Bay (Tetra Tech, 2001b). Overall, they found that 86% of erosional features were shallow streamside landslides and that the primary source of sediment in Easkoot Creek is a result of mass wasting processes. The study found that the average annual sediment delivery in the Bolinas Watershed over a 50-year period is 233 tons/year-km2. Assuming Easkoot Creek is subject to the same average sediment delivery rate, approximately 783 tons of sediment per year (approximately 480 cubic yards per year) would be delivered to Easkoot Creek at the Park Entrance over a 50-year period.

Aggradation in Lower Easkoot Creek MLA examined previously surveyed profiles of the channel to identify changes in elevation of the channel bed. The County surveyed an extensive thalweg profile in 2007, from downstream of Calle de Arroyo to the Highway 1 crossing. Comparing the 2007 profile to the 1979 channel profile provided in the FEMA FIS shows the channel has aggraded 1.5 to 2 feet between Calle del Pradero and Arenal Avenue, while experiencing slight degradation near the Highway 1 crossing (Figure 6).

Profile information provided by the GGNRA for the channel between the Pedestrian Bridge and the downstream Park Boundary suggest the channel bed remained relatively unchanged between 1999 2004 (Figure 7). Between 2004 and 2006, the channel bed aggraded an average of 1.7 feet. Changes in the channel profile within the limits of the GGNRA may be influenced by a channel and floodplain restoration project constructed in 2003 (Garcia and Associates, 2004). However, because channel aggradation extends from downstream to Calle Del Arroyo to upstream of Highway 1 (Figure 6), the aggradation is likely due in part to geomorphic factors beyond the affects of the restoration project, including watershed sediment supply.

Comparison of the two profiles suggest in lower Easkoot Creek is associated with episodic events, with a period of relatively low sedimentation between 1997 (when dredging reportedly stopped) and 2004, and a period of high sedimentation between 2004 and 2006. This would be consistent with sediment supplies from watersheds dominated by mass wasting processes.

Evaluation of a channel cross section upstream of Calle del Pinos from the 2007 County survey suggests that this aggradation has reduced the bankfull channel depth from more than 3.6 feet in 1979 (Sadjadi, 1979) to 1.6 feet as of 2007. Current bankfull flow capacity at this cross section is approximately 60 cfs (assuming uniform flow conditions). Note that this is not necessarily the flow that infrastructure becomes inundated, but when flow spreads onto the floodplain along the eastern bank. Both the Sadjadi (1979) and Spangle (1984) engineering reports identified that the capacity of Easkoot Creek was approximately 400 to 500 cfs before out-of-bank flooding began. These flow capacities were estimated when the channel was undergoing routine dredging. The decrease in channel capacity resulting from sedimentation within the channel appears to have substantially reduced the channel capacity, which would be expected to increase the frequency of out-of-bank flooding.

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Feasibility of Proposed Alternatives

Current regulatory requirements, including the listing of steelhead trout under the Federal Endangered Species Act, necessitates protection of native fisheries as part of any project impacting the fish bearing portions of Easkoot Creek. Steelhead trout are rainbow trout with an anadromous life history component. They spend the juvenile portion of their life cycle in fresh water. Prior to entering the ocean, they undergo a smoltification process to adapt to saltwater. Steelhead trout spend their adult life in the ocean before returning to freshwater streams and to spawn. If a steelhead trout is entrained into diverted flows of Easkoot Creek and swept into the ocean before smolting, the fish will not survive the exposure to salt water. Therefore, flood control alternatives that include a flood bypass channel designed to divert high flows directly to the ocean may require measures to protect juvenile steelhead trout.

Findings of numerous studies of the project area indicate that any flood control alternative considered for lower Easkoot Creek needs to address management of sediment. The Sadjadi (1979) and Spangle & Associates (1984) engineering reports found that the lower Easkoot Creek channel had a dredged capacity of approximate 400 to 500 cfs, which is less than the 10-year flood event. Due to sedimentation, the channel may currently have a capacity as low as 60 cfs, which is less than a 1-year flow event. The loss of the historic wetland function as a flow detention, flow diversion, and sediment detention area may have increased both flows and sedimentation in the Calles reach of lower Easkoot Creek.

The following sections present a conceptual description of three high-flow bypass alternatives, with a discussion of each alternative’s potential impacts to steelhead trout and potential sedimentation issues. Specific sedimentation issues identified with each alternative are discussed in general in a subsequent section.

Alternative 1: Direct High-Flow Bypass Channel To The Ocean

Alternative 1, as described in detail in the 1971 Sadjadi Report and 1984 report by Spangle & Associates, uses a flood bypass channel that discharged onto the wave slope within the GGNRA (Figure 8). The reports presented various channel configurations constructed of box culverts, earthen, concrete and rip rap channels.

Potential Impacts to Steelhead Trout The likelihood of fish entering a high-flow bypass channel during high flows is substantial given that the vast majority of streamflow would be diverted to the ocean. In other applications, such as water supply diversions, protecting fish and preventing entrainment is typically accomplished using fish screening technologies. Figure 9 shows a typical arrangement for a streamside screened diversion. The water depth in the stream in which flow would begin to be diverted, the rate of diversion and the depth over the screen is typically controlled by a weir, gate, or spillway located behind the screen.

Fish Screening Criteria Both NOAA Fisheries (1997) and California Department of Fish and Game (CDFG, 2000) have fish screening criteria that apply to both adult and juvenile steelhead. The criteria from the two

Review of Background Information and Flood Control Alternatives for Easkoot Creek, Stinson Beach, California. Michael Love & Associates July 17, 2009 Page 8 agencies are functionally equivalent. The feasibility of screening diverted flows as part of a flood- flow bypass channel on Easkoot Creek was evaluated based on these criteria.

The agencies recommend placing screens streamside, along the bankline at the point of diversion, when feasible (Figure 9). If the screen is located within the bypass channel, then a juvenile bypass system must be included that has an entrance near the screen and safely returns fish back to the stream channel. There are numerous hydraulic criteria associated design of fish bypass returns that are beyond the scope of this memorandum.

The approach velocity and sweeping velocity are standard design parameters for screens. The approach velocity is the component of the water velocity perpendicular to the screen face. For screening of juvenile steelhead, the approach velocity should not exceed 0.33 ft/s if the screen is equipped with a cleaning system (i.e., brushes). To satisfy this approach velocity criterion requires a minimum of 3.0 ft2 of submerged screen per 1.0 cfs of flow diverted. For screens with no cleaning system, the CDFG design approach velocity criteria is one-forth of 0.33 ft/s to allow for the screen to remain within criteria with up to three-fourths of the screen clogged with accumulated debris. Therefore, if no cleaning system is included in the design, 12 ft2 of submerged screen is required per 1.0 cfs of flow diverted.

The sweeping velocity is the water velocity component parallel and adjacent to the screen face. Higher sweeping velocities move fish past the screen more quickly and reduce the amount of debris accumulation on the screen. Screens located in-channel require a sweeping velocity at least twice the design approach velocity. If located in the bypass channel, sweeping velocities should be greater than the approach velocity criterion for the screen. If no screen cleaning system is provided, higher sweeping velocities are recommended to move debris across the screen.

Screens placed in a stream environment characterized by large streambed material and substantial debris loads, such as Easkoot Creek, are typically constructed of stainless steel profile bar. This material is more durable and easier to clean than other types of screening materials, such as perforated plates or woven wire. To protect juvenile steelhead, the opening in a profile bar screen should not exceed 1.75 mm (0.0689 inches). A trash rack can be placed in front of the screen to catch larger debris.

Sedimentation Issues A screen would pass water and fine suspended sediment, not coarser sediment, to the high-flow bypass channel. Based on visual observations of sediment size in Easkoot Creek, the material on the surface of the channel between the Park Entrance and Highway 1 is predominately large gravels and cobbles. Therefore, nearly all sediment conveyed within Easkoot would be retained within Easkoot Creek. The remaining flows in Easkoot Creek will not be able to transport the volume of sediment and sediment deposition will occur in front of, and immediately downstream of, the screen. Therefore, management of sediment upstream of the diversion will be necessary. Refer to the “Potential Sediment Maintenance Issues with all Alternatives” section for more discussion of management of in-channel sediment.

Feasibility of a Screened High-Flow Bypass To provide only a 10-year level of flood protection to properties downstream of a high-flow bypass on Easkoot Creek may require diverting 600 cfs, or more, during the 10-year flood. If an adequate

Review of Background Information and Flood Control Alternatives for Easkoot Creek, Stinson Beach, California. Michael Love & Associates July 17, 2009 Page 9 screen cleaning system could be developed, the screen area would need to be at least 1,800 ft2. Given the debris load likely encountered during this type of flow event, an acceptable cleaning system may not be practical, requiring the screen to be four times larger (7,200 ft2). For a standard vertical flat plate screen, the topography of the channel and banks likely limits the submerged screen height to three feet or less. Based on this, the screen length with and without a cleaning system would need to be 600 feet and 2,400 feet, respectively.

The minimum necessary screen length of 600 feet to provide a 10-year level of protection is impractical from many perspectives, including cost, operations, maintenance, and environmental disturbance. Additionally, high debris and sediment loading rates associated with the site likely require a much larger screen. Therefore, if “take” of juvenile steelhead is not acceptable, then this alternative should not be further pursued.

An inquiry into the use of fish screens on flood bypasses found that resource agencies generally avoid using them due in part to the inability of cleaning systems to keep up with the debris load during flood flows, resulting in clogging. One example of a screened flood bypass channel was found in Vancouver, Washington. The main channel only carries low flows, while the bypass channel is intended to convey all higher flows. The screen does not have a cleaning system and commonly clogs with debris, making it ineffective (Kozmo Bates, former Chief Engineer, Washington State Department of Fish and Wildlife, personal communications). When clogged, flows overtop the screen, negating any fish protection.

Instead of screening, most contemporary flood bypasses attempt to provide suitable conditions for fish within the bypass. These often divert high flows into seasonal wetlands or side channels, which can provide foraging and rearing habitat for fish. These types of areas usually return the floodwaters back to the channel at some point, allowing the fish to return to the main channel as flows recede. Alternatives 2 and 3 present two different methods to divert high flows into large off-channel areas that could be suited for juvenile steelhead.

Alternative 2: Spillway Diverts Flow to Off-Channel Flow Detention Basin

Alternative 2, as described by the District, involves creating an off-channel flow detention basin (Figure 10). A lateral weir or spillway will divert high flows into a flow detention basin. The flow detention basin would include a high-flow bypass that spills on the wave slope. This structure would require a design that considers the wave climate of the area. If site constraints allow, the flow detention basin may be large enough to store high flows and attenuate peak flows in Easkoot Creek during frequently occurring high-flow events without spilling. As high flows recede, the off-channel flow detention basin could drain back into the channel through a flap-gate or other device.

An initial evaluation of limited topographic information and the location of existing infrastructure within the GGNRA suggests that the area in and around the northwest parking lot would be most suited for a detention basin. (Figure 11). It may be feasible to design the detention basin to be compatible with parking, minimizing loss of parking spaces associated with construction of a detention basin. Flood containment berms may be necessary to prevent an increase in flooding on adjacent private property. Determining the actual location of a flow detention basin with a high- flow bypass would require additional information and analysis and be dependent on, among other things, size of basin, local topography, height of the adjacent dunes, risk of impacting exiting private and public infrastructure, and ability to perform routine maintenance.

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Determining frequency and rate of flow spilling from Easkoot Creek into the flow detention basin, and from the detention basin into the bypass involves numerous considerations, including hydrology, local topography, downstream channel capacity, height of the adjacent dunes, risk of impacting existing private and public infrastructure, and ability to perform routine maintenance.

Potential Impacts to Steelhead Trout Alternative 2, as described above, would have a risk of entraining juvenile steelhead into the flow bypass. Juvenile salmonids, including steelhead, often utilize off-channel ponds and wetlands, as well as moving into wetted side-channels and onto inundated during large flood events (Henning et al., 2006). These environments provide refuge from the high velocities in the main channel. Therefore, during large flows juvenile steelhead would undoubtedly travel into the flow detention basin, utilizing it as off-channel refugia. If the flow event is large enough to spill out of the basin into the high-flow bypass there is a chance that juvenile steelhead could be swept into the bypassed flow and to the ocean. Screening of the bypass is impractical for the reasons previously described in Alternative 1. Additionally, a screen located in a detention basin would have no sweeping velocities, increasing the risk of impinging fish on the screen and collecting debris and sediment.

The environment that would be created in Alternative 2 is not unlike that found in coastal lagoons that breach near-shore dunes and drain to the ocean during high flow events, similar to what may have been the historical configuration of Easkoot Creek/Willow Camp/Poison Lake. To mimic these hydraulic conditions, when high flows overflow through the dunes and into the ocean, the spillway leading to the high-flow bypass channel could be made sufficiently wide that water velocities approaching the spillway remain relatively low (within the swimming abilities of the fish) and water depths would be shallow and behaviorally undesirable to the fish. This would reduce the risk of entraining fish into the bypassed flow.

In an off-channel flow detention basin, a flow return must be provided to allow detention basin water to drain back into the channel to reduce the risk of stranding fish in the drying detention basin after flows recede. A remedy may be to include a small-restricted opening or conduit between the basin and the channel that would allow the detention basin to remain wetted and connected to the stream during low-flow periods. This could reduce the potential for stranding fish in the basin as high-flows recede, as well as provide flow exchange between the stream and detention basin that could allow fish to use the basin as off channel winter rearing habitat. Attention should be given to avoid sedimentation of the return connection, which would isolate the detention basin.

Sedimentation Issues Like Alternative 1, the lateral weir or spillway leading to an off-line flow detention basin would only divert water and fine sediment, not coarse sediment, to the flow detention basin. All coarse sediment conveyed within Easkoot would be retained within Easkoot Creek. The remaining flows in Easkoot Creek will not be able to transport the volume of sediment and sediment deposition will occur. Therefore, management of sediment upstream of the diversion will be necessary. Refer to the “Potential Sediment Maintenance Issues with all Alternatives” section for more discussion of management of in-channel sediment.

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Feasibility of Creating an Off-Channel Flow Detention Basin Depending on the size of the flow detention basin, and if a bypass can be designed to reduce the risk of entraining juvenile steelhead, this alternative may be feasible. However, creating a structural off-channel flow detention basin with a high-flow overflow may not be aesthetically appealing, may have maintenance and water stagnation issues, and would not provide the ecological benefits that a more natural system would provide. Alternative 3 expands on Alternative 2 with the creation of a contained floodplain/wetland system that provides the same functions as Alternative 3, while recreating some of the physical processes that were in this location historically.

Alternative 3: Floodplain / Seasonal Wetland Restoration

Alternative 3 considers limited restoration of historical floodplain/wetland processes, as interpreted from the geomorphic context of the site and depictions of the area in historical maps and anecdotal accounts. As previously described, portions of the Calles and nearly the entire GGNRA at Stinson Beach was comprised of a seasonal floodplain/wetland basin (approximately 20.5 acres in size) contained by dunes. Evidence suggests that waters from the wetland would occasionally breach the dunes and overflow to the ocean during high flows. Restoring the entire floodplain/wetland is likely infeasible given current residential areas and recreational use of the land. Even without full restoration, restoring a portion of this seasonal floodplain/wetland basin could have multiple benefits, including:

- Attenuation of peak flows through both floodplain/wetland floodwater storage and through breaching of low dune areas during large

- Reduction in downstream channel sedimentation, depending on the configuration of the restored wetland and its ability to accumulate both fine and coarse bedload, and

- Creation of productive off-channel riparian and wetland habitat suitable for use by aquatic, avian, and terrestrial organisms.

The floodplain/wetland restoration would likely consist of removing the fill placed in the historical wetland during construction of the northwest GGNRA parking lot. This alternative could include reconfiguring the channel and inclusion of side-channels and off-channel wetlands. Restoring the floodplain/wetland in this area may necessitate removal of parking areas. A flood containment berm would likely be required to avoid impacts to adjacent private property (Figure 12). Under normal flow conditions Easkoot Creek would be contained within its channel. During periods of high flow, streamflow would be out-of-bank in this reach, inundating the floodplain and wetlands. An overflow through a breach in the dunes could be engineered into the project to function as a high-flow bypass to discharges flood flows onto the wave slope. This structure would require a design that considers the wave climate of the area, among other factors. There would be opportunity to construct the overflow out of natural materials (large wood and rock) to ensure it remains stable and functions as intended during flood events. With this alternative, it would be important to ensure that the elevation of the outlet is stable. Native vegetation along the sides of the overflow could help locally stabilize the dunes. If it was allowed to down cut, the entire floodplain/wetland could drain rapidly, potentially stranding fish. Figure 12 provides a schematic description of this concept.

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The most suitable location for siting a floodplain/seasonal wetland restoration project appears to be within the approximately 3-acre hatched area shown on the Stinson Beach GGNRA property upstream of the Calles (Figure 11). Based on historical maps and current topography, this area is located within the historical boundary of the Willow Camp wetland area. The dunes are lower in this area than further to the south, making it more suitable for incorporating a high-flow overflow into the project. This area is currently a GGNRA beach parking lot, and nearest to the channel the lot is at an elevation of approximately 11 feet (NGVD29). For reference, mean higher high water (MHHW) at Stinson Beach is 3.21 feet (NGVD29).

Like Alternative 2, determining the most suitable location of a floodplain/wetland system along with the frequency and rate of flow overflowing into the bypass would require additional analysis and be dependent on, among other things, size of basin, local topography, height of the adjacent dunes, risk of impacting existing private and public infrastructure, and ability to perform routine maintenance.

Potential Impacts to Steelhead Trout Like Alterative 2, juvenile steelhead would be expected to move into an inundated floodplain or wetland during high flows. If the floodplain/seasonal wetland overflowed onto the wave slope there would be risk of some fish being entrained in the flow and washed downstream to the ocean. Screening of the bypass is impractical for the reasons previously described in Alternative 1. Additionally, a screen located in a detention basin would have no sweeping velocities, increasing the risk of impinging fish on the screen and collecting debris and sediment. The presence of micro- topography in the floodplain, downed trees and wetland vegetation will provide numerous low- velocity areas where juvenile steelhead can find refuge from high flow velocities. Because this alternative would be restoring the dynamics that historically occurred within lower Easkoot Creek, the potential for loss of a limited number of juvenile steelhead may be offset by the increased amount of potentially productive habitat made available by restoring a wetland environment.

Sedimentation Issues Like the Alternative 2, sediment settling will occur when the flow velocities drop in the slower water stored in the floodplain/wetland basin area contained by the berms. Sediment accumulation in a floodplain is a natural process. Dependent of the size of the floodplain/wetland basin, allowing sediment accumulation to occur may be feasible. However, given the sediment loads from the watershed, it is expected that the flow storage capacity of the basin will be quickly filled with sediment. Dredging the entire basin will be extremely costly and difficult to permit because the basin area will be considered a valuable wetland. If necessary, sediment could be managed upstream in an area that is contained, easily accessible, and permittable. Refer to the “Potential Sediment Maintenance Issues with all Alternatives” section for more discussion of management of in-channel sediment.

Feasibility of Creating a Floodplain/Wetland At a scoping level, Alternative 3 appears most feasible and beneficial to fisheries and the geomorphic function of the stream as a whole. This alternative would likely provide similar flood protection as Alternative 2, is also much more aesthetically appealing, provides GGNRA with an educational feature, and provides enhanced ecological and geomorphic function. Because Alternative 3 does not depend on a structure to divert flow out of the channel, like as Alternative 2, less maintenance is expected for Alternative 3.

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We recommend further exploring this alternative.

Potential Sediment Maintenance Issues with all Alternatives

In Alternative 1 and Alternative 2, the proposed high-flow bypass systems would divert only water and fine sediments, not coarse sediment, to the receiving high-flow bypass channel or flow detention basin. The most appropriate application of bypass channels is usually for streams with relatively low bed material loads (ACOE, 1994). Reducing flooding on Easkoot Creek with a floodwater bypass channel or off-channel flow detention basin has a substantial risk of disrupting channel sediment transport capacity downstream from the point of diversion. It is physically infeasible to divert both high-flows and sediment out of Easkoot Creek, but still maintain normal flows and protection of juvenile steelhead in Easkoot Creek downstream of the diversion point. All sediment conveyed within Easkoot Creek to the diversion point would be retained within Easkoot Creek and need to be transported by Easkoot Creek downstream to Bolinas Lagoon to prevent channel aggradation. The likely result of bypassing all but normal flows would be a lack of adequate stream power to transport bedload within the channel downstream of the diversion point. This would lead to localized aggradation immediately downstream of the diversion point, requiring regular maintenance. A sediment basin located upstream of the diversion point would likely be required to prevent sediment aggradation in the downstream channel.

In Alternative 3, flow is not diverted, but sediment settling will occur when the flow velocities drop in the slower water stored in the floodplain/wetland basin area upstream of the flood containment berms.

A sediment analysis prepared by Tetra Tech (2001b) found that the primary sediment source to Easkoot Creek is a result of mass wasting processes. The one to two feet of channel aggradation within a 2-year period also indicates that high amount of sediment loading possible from the watershed. Controlling mass wasting processes in a watershed is likely infeasible. Providing ways to manage sediment after mass wasting event is more feasible.

For all alternatives, sediment management upstream of the diversion or storage feature will likely be necessary. A sediment budget and transport analysis for Easkoot Creek would aid in determining the method and extent of sediment management necessary.

Use of Sedimentation Basins A means of addressing issues concerning sedimentation and resulting loss of channel capacity is to construct and maintain a sediment basin (Figure 13). Sedimentation basins are also used to manage downstream sediment loads and address channel aggradation resulting from large erratic sediment inputs, such as episodic mass wasting (Saldi-Caromile, 2004). Sediment basins are used to trap coarser channel materials such as coarse sands, gravels, and small boulders. Sediment basins typically do not provide enough flow detention time to settle out fine sands and silts, which typically wash downstream. Sediment basins are generally preferred over routine channel dredging because of reduced disturbance to the channel bed and riparian vegetation associated with maintenance. Additionally, they are constructed in areas that are easily accessible for maintenance, and can be designed to mitigate potential impacts to fisheries during dredging activities.

Depending on their sized and efficiency at removing sediment, sediment basins can cause degradation of the downstream channel bed through “sediment starvation”. They can also be used

Review of Background Information and Flood Control Alternatives for Easkoot Creek, Stinson Beach, California. Michael Love & Associates July 17, 2009 Page 14 to lower an aggraded channel bed immediately upstream of the basin. This is accomplished by setting grade control at the head of the basin below the existing channel bed, thus initiating a small head-cut and localized lowering of the upstream channel bed. These two properties of sediment basins can be used in Easkoot Creek to reduce in-channel aggradation both upstream and downstream of the sediment basin.

The channel of Easkoot Creek may need to be dredged concurrently with the initiation of a high- flow bypass system to achieve the appropriate stable bed elevation and established channel capacity. Note that less frequent channel dredging may still be necessary because of accumulation of fine sediment not trapped upstream. A sediment transport analysis could assist in determining if, and how often, channel dredging would be needed to maintain channel capacity.

Potential Location of Sediment Basins When used in conjunction with a flood bypass, the sediment basin is typically placed upstream of the point of diversion. Alternatively, the point of diversion can be located at the downstream end of the basin. For Easkoot Creek, a reasonable site for a sediment basin is where the stream enters the GGNRA near the Parkside Restaurant (Figure 11). This marks the location where the channel enters the historical wetland area and is upstream of the high-flow diversion points for all alternatives.

Sediment basins are commonly placed at the head of alluvial fans and other natural changes in channel slope and confinement. This section of channel is characterized by a reduction in channel confinement and an increase in width of the floodplain where construction of a sediment basin may be feasible. Based on an interpretation of the historical geomorphology, there was likely a noticeable break in channel grade near this location, as sediment transported from the steep channel upstream of Highway 1 transitioned as it entered into a low gradient seasonal wetland. If incorporated into the design, a sediment basin in this area could possibly be used to both starve the downstream channel of sediment, and lower the aggraded streambed a short distance upstream of the basin back to pre-aggradation conditions.

It may be feasible to construct sediment forebays upstream but adjacent to the high-flow diversion point of Alternative 2 and in the upstream portion of the Alternative 3 floodplain/wetland basin (Figure 11). If incorporated into the design, a sediment basin in this area could possibly be used to both starve the downstream channel of sediment, and lower the aggraded streambed upstream of the basin back to pre-aggradation conditions.

Sedimentation basins should be designed with features that allow fish to escape when draining for cleanout operations. A gated bypass pipe with should be included in the design to divert streamflow around the basin during cleanout operations.

Recommendations

Of the three alternatives explored in this memorandum, Alternative 3 (creation of a floodplain/seasonal wetland) appears to provide the most protection for native steelhead juveniles. However, all of the described alternatives are conceptual. Without conducting additional studies, the feasibility, effectiveness, and required maintenance and cost associated with any of the alternatives remains unknown. Sea level rise and seismic-induced land elevations should also be considered for each alternative. The following items discuss information gaps and potential studies

Review of Background Information and Flood Control Alternatives for Easkoot Creek, Stinson Beach, California. Michael Love & Associates July 17, 2009 Page 15 necessary to further evaluate the feasibility and functionality of alternatives. This list is not intended to be exhaustive, but provide a starting point for moving forward.

Detailed Topographic Survey

A detailed topographic survey of the project area is needed to further characterize the existing geomorphology and hydraulics of the channel and floodplain and for developing and evaluating feasibly of conceptual alternatives. Currently, there is only very limited survey data of Easkoot Creek and its floodplain. We recommend conducting a topographic survey that includes all lands within the existing floodway and floodplain, as mapped in the current FEMA FIS from Highway 1 to downstream of the Calles. Wave slope and sand dune topography should also be mapped.

Determine Design Flows

Design flows, including total flow volume, peak flow rate and flow frequency, are necessary to develop adequate facility designs including storage volume and weir overtopping elevations. Design flows area also used to prepare an accurate assessment of future flood risks and project benefits. Because there are no long-term flow records for Easkoot Creek, prior engineering reports estimated design flows using various methods. These methods resulted in a large range of flow rates for a given return period and few estimates of flows for frequent storm events, such as the 1.5, 2 and 5- year events.

We recommend that existing estimates of peak flow be re-evaluated and improved upon. A stream gaging station would assist in establishing the accuracy of the various methods of flow estimates. The calibrated flow estimation method can then be used to estimate other return period storm events and to develop “design storm” hydrographs. Storm hydrographs can be used for evaluating the effectiveness of a flow detention basin or floodplain/wetland restoration at storing and attenuating peak flows.

Identify Lower Easkoot Creek Channel Capacity and Desired Level of Protection

The elevation at which a high-flow bypass system becomes active and the rate of flow bypassing are dependent on the capacity of the downstream channel and the desired level of flood protection for downstream proprieties. The recommend topographic survey can be used to develop hydraulic models and establish the level of flood protection that various design alternatives could provide. Additionally, this information can be used to evaluate hydraulic capacity at existing road-stream crossings.

Identify Additional Areas for Flood Storage

Besides the three alternatives reviewed in the memorandum, there may be other opportunities to improve flood flow and sediment conveyance within lower Easkoot Creek while benefiting fisheries and the geomorphic function of the channel. There are a number of undeveloped riparian areas along Easkoot Creek within the Calles reach that are designated as floodplain by FEMA. Much of these areas are likely within jurisdictional wetland and un-developable. There may be opportunities to enhance the existing floodplain with these areas to provide additional storage of floodwaters, especially during high tides. There may also be possibilities to increase the tidal prism within areas closes to Calle del Arroyo, thus improving the ability of the stream to process sediment through tidal flushing.

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Additionally, the vegetative and geomorphic characteristics of the existing riparian floodplains within the Calles reach should be characterized to identify opportunities to improve floodplain connectivity and function, if present.

Prepare a Sediment Budget

A sediment budget can be used to determine the necessity for, and size of, sediment basins associated with a high-flow diversion. The Tetra Tech (2001b) sediment budget study did not investigate Easkoot Creek in detail, but identified that the primary source of sediment in Easkoot Creek is a result of mass wasting processes. A sediment budget prepared for the Easkoot Creek watershed is necessary to estimate the likely frequency and volume of future sediment inputs. The composition of the streambed sediment should be characterized for use in future sediment transport analysis.

Conduct a Fisheries Habitat Study

Investigating the presence and condition of existing fisheries habitats, including identifying the extent of the channel containing viable habitat, would help establish baseline conditions and assist in characterizing potential affects to the fisheries associated with any selected alternative. Additionally, the District should open discussions with the NPS and the fisheries resource agencies (CDFG and NOAA Fisheries) to determine the potential acceptability of approaches described in Alternatives 2 and 3.

Assess Subsurface Conditions in the Northwest Larking lot

If investigation of restoring historic floodplain/wetland is pursued, we recommend that a study be conducted in the northwest parking lot that is similar to the study of historical conditions in Willow Camp/Poison Lake prepared by the National Park Service in the southernmost parking lot (National Park Service, 2004, circa 2006). The 2004 study in the south parking lot included the following items (summarized from NPS, 2004):

1. Establishment of a subsurface hydrologic monitoring network to determine hydrologic characteristics including water table fluctuations, water sources and gradients, and water chemistry.

2. Coring within the parking lot to determine depths of fill and to look for evidence of the former pond/wetland surface (e.g., buried wetland soils, wetland/riparian vegetation, or pond sediments).

3. Collect elevation and location data for creation an existing conditions topographic map, calculating fill depths and volumes, determining seasonal water table elevations, and mapping buried pond/wetland surfaces.

A similar study in the northwest parking lot could yield information on the extents of the historical wetland, wetland function (fresh or brackish), amount and characteristic size of sediment deposition, deposition rates, and seasonal water table elevations.

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References

ACOE. 1994. Channel Stability for Flood Control Projects. EM-1110-2-1418. Department of the Army, U.S. Army Corps of Engineers. Washington, D.C.

California Department of Parks. 1980. Mount Tamalpais State Park General Plan. Department of Parks and Recreation. www.parks.ca.gov/pages/21299/files/239.pdf

California Department of Parks and Recreation. (1982). Mt. Tamalpais State Park Slide Area Above Stinson Beach Memorandum. Provided by Marin County Department of Public Works.

FEMA. 1979 rev. 1997. Flood Insurance Study. Marin County California Unincorporated Area. Community Number – 060173.

Garcia and Associates. 2004. Channel Morphology of Easkoot Creek Following Lower Easkoot Creek Restoration Project Prepared for: National Park Service Golden Gate National Recreation Area.

Henning, J. A., R. E. Gresswell, and I. A. Fleming. 2006. Juvenile Salmonid Use of Freshwater Emergent Wetlands in the Floodplain and Its Implications for Conservation Management. North American Journal of Fisheries Management 26:367–376.

Lane, E. W. 1955. The Importance of Fluvial Morphology in Hydraulic Engineering. American Society of Civil Engineers 1955 Conference, Volume 81, Paper 745.

Maurer, B. circa 2006. Letter from Barbara Maurer/John Wooley in Stinson Beach community archive. www.stinson-beach.org/community-archive.html

Marin County Department of Public Works (1982). Letter regarding “Slide Area Above Stinson Beach.” Provided by Marin County Department of Pubic Works.

National Park Service. 2004. Memorandum: Trip report for travel to Golden Gate National Recreation Area (GOGA), Point Reyes National Seashore (PORE), John Muir National Historic Site (JOMU), and the Presidio of San Francisco, March 29-April 2, 2004. Prepared by Joel Wagner, Wetland Program Lead, WRD and Rick Inglis, Hydrologist, WRD. National Park Service Water Resources Division.

National Park Service. Circa 2006. Preliminary Evaluation of Hydrologic Data Collected at the South Parking Lot at Stinson Beach, Golden Gate National Recreational Area. National Park Service Water Resources Division.

National Park Service. Circa 2003. Environmental Assessment Easkoot Creek at Stinson Beach. Golden Gate National Recreation Area. Division of Natural Resource Management and Science.

Posada, J. circa 2006. The need to clear debris and build retaining walls at the Easkoot Creek landslide. Letter in Stinson Beach community archive. www.stinson-beach.org/community- archive.html

Review of Background Information and Flood Control Alternatives for Easkoot Creek, Stinson Beach, California. Michael Love & Associates July 17, 2009 Page 18

Saldi-Caromile, K., K. Bates, et al. 2004. Stream Habitat Restoration Guidelines: Final Draft., Co- published by the Washington Departments of Fish and Wildlife and Ecology and the U.S. Fish and Wildlife Service. http://wdfw.wa.gov/hab/ahg/shrg/21- shrg_stream_sediment_detention_basins.pdf

Sadjadi, M.M. 1971. Stinson Beach Drainage Study. photocopy of handwritten document provided by Marin County Flood Control and Water Conservation District.

Spangle, William & Associates. 1984. Alternative Mitigation Measures for Storm and Flood Hazards. Final Report Volume 1. Portola Valley, California. www.co.marin.ca.us/depts/PW/Main/floodcontrol/Z5/FcZ5VOL%201.pdf

Tetra Tech. 2001a. Historical Perspective of Bolinas Lagoon Watershed. San Francisco, California. http://www.spn.usace.army.mil/projects/bolinaslagoon/landuse.pdf

Tetra Tech. 2001b. Bolinas Lagoon Watershed Study Input Sediment Budget. Prepared for the U.S. Army Corps of Engineers, San Francisco, California. http://farallones.noaa.gov/ecosystemprotection/resources.html

Working Group. 2008. Bolinas Lagoon Ecosystem Restoration Project-Recommendations for Restoration and Management. Prepared for a Working Group of the Sanctuary Advisory Council, Gulf of the Farallones National Marine Sanctuary by the Marin County Open Space District and the U.S. Army Cops of Engineers, San Francisco, California. http://farallones.noaa.gov/ecosystemprotection/bolinas.html

Review of Background Information and Flood Control Alternatives for Easkoot Creek, Stinson Beach, California. Michael Love & Associates GGNRA

Figure 1. Portion of Stinson Beach showing Easkoot Creek and the Golden Gate National Recreation Area (GGNRA).

(provided by Marin County)

Figure 5. 1906 Plat of Stinson Beach showing multiple alignments of Easkoot Creek (black lines with arrowheads) and the location of Willow Camp. 42 40 38 HWY 1 36

34 Arenal Ave. Calle del Onda del Calle Calle del Pinos del Calle Calle del Sierra Calle

32 Calle del Arroyo Calle del PraderoCalle 30 Pedestrian Bridge

28 Park Entrance Bridge 26 24 22 20 18 16 14

Elevation in Feet (NGVD29) Elevation in 12 10 8 6 MHHW 4 FEMA 1979 3.21' 2 Marin County 2007 0 3,000 3,200 3,400 3,600 3,800 4,000 4,200 4,400 4,600 4,800 5,000 5,200 5,400 5,600 5,800 Stream Distance in Feet above Mouth

Figure 6. Easkoot Creek channel bottom elevation surveys conducted in 1979 by FEMA and in 2007 by Marin County. Mean higher high water (MHHW) indicated for reference. . .

25 . . . y 24 Park 23

22 Boundar 21 Park Boundary

20 Pedestrian Bridge

19 Park EntranceBridge 18 17 16 15 14 13 Elevation in Feet (NGVD29)Elevation 12 11 10 GGNRA 1999 9 GGNRA 2004 8 GGNRA 2006 7 3,900 4,000 4,100 4,200 4,300 4,400 4,500 4,600 4,700 4,800 4,900 5,000 Stream Distance in Feet above Mouth

Figure 7. Easkoot Creek channel bottom elevation surveys in the GGNRA conducted by NPS. GGNRA

(Figure adapted from Marin County Flood Control and Water Conservation district)

Figure 8. Alternative 1. Potential location of flood bypass channel.

Figure 13. Example of sediment detention basin.