Past, Present, and Future Channel Possibilities North End Bolinas Lagoon

March 18, 2016

Presentation to North End Restoration Project Workshop A Special Meeting of the Bolinas Lagoon Advisory Council by Laurel Collins Watershed Sciences

Streams draining into the north end of Bolinas Lagoon Cretaceous Franciscan Complex

San Andreas Rift Zone

Early Pleistocene or Pliocene Sedimentary bedrock

Pleistocene Marine Terrace Deposits

USGS Geologic Map of Marin County From Scientific Investigations Map 2918 2015 Google Earth Image with 2006 USGS Geology map. Heavy black lines show faults. Red lines offshore of the Golden Gate show faults mapped by Bruns et al. 2002. Yellow and orange lines are possible fault lineations interpreted from LiDAR imagery Holocene graben shown in yellow mapped by Bruns et al. 2002.

Online: http://soun A’ dwaves.usg s.gov/2007 /03/

San San Golden Potato Gregario Andreas Gate Fault Patch Fault -East Fault Fault A

Holocene graben shown in yellow Cretaceous Franciscan Complex mapped Early by Bruns Pleistocene et al. Alluvium Holocene Alluvium 2002. (older) (younger)

Stippled

yellow

pattern is

sketch of

possible Pleistocene Marine extension Terrace Deposits Early Pleistocene or Pliocene Sedimentary bedrock of

Holocene

Graben

north of

Stinson

Beach

2015 Google Earth Image with 2006 USGS. 2015 Google Earth Image with 2006 USGS Geology map and pink highlights showing fault lines PostPink 1906 lines = earthquakehighlighted G. activeK. Gilbert faults is assessingfrom 2006 earthquakeUSGS Geology damagemap. downstreamRed lines = of northsketch end of of Bolinaspossible Lagoon. upper extent of the Heactive observed aboutHolocene a foot of downgraben. dropping in the lagoon on Yellow lines = the east side of possible fault thelineations 1906 San Andreasinterpreted fault in rupture.LiDAR.

DenudedRed outlined hillsidespolygons = northeastslides active of ~ Wilkins’last 30 barn years) probably reflectingPink polygons the = influenceslides of fires and(currently logging. active) 2015 Pink lines = LiDAR of highlighted active faults North End from 2006 Bolinas USGS Geology Lagoon map. Red lines = sketch of possible upper extent of the active Holocene graben.

Yellow lines = possible fault lineations interpreted in LiDAR.

Red outlined polygons = slides active ~ last 30 years)

Pink polygons = slides (currently active) 2015 Google Earth image and stream channels of the North End Bolinas Lagoon

Wilkins Ranch SR1

SR1

Warf Creek 1854 Detail of Coast and Geodetic Survey T- Sheet of North End Bolinas Lagoon 1854 Detail of Coast and Geodetic Survey T- Sheet of North End Bolinas Lagoon

Main channels may have have had intermittent to continuous flow depending on seasonal rainfall.

Note many trees on the lower alluvial fan. Tidal 1854 marsh boundary is depicted in green.

Note possible berm or gravel bar at the lagoon inlet at Salt Creek drainage. berm It is the current location of SR1. Lighter Wharf Orange 1854 polygon shows the depositional lobe of the Wilkins Gulch alluvial fan where coarse bedload was distributed across the fan. The area between the marsh and fan is a transition zone between fluvial and tidal processes that would have received finer-sized sediment.

At the apex of 1854 the fan, at the exit of the confined canyon, it is common and therefore likely that two main distributaries would alternately or simultaneously be active, depending on flow conditions or obstructions.

1854 1854 Overflow 1854 and additional tributary and distributary channels would also have conveyed water and sediment depending on seasonal rainfall and soil saturation.

Channels were probably fairly shallow and the fan surfaces were actively aggrading.

1854 Similar conditions likely existed on the Wilkins Gulch alluvial fan. It may have 1854 1854 had two mainstem distributary channels. North 1854 Tributary created its own fan. Many of the channels in the middle portions of the Wilkins and Lewis Gulch fans probably lose surface flow to ground water subsurface storage.

Yet near the 1854 toes of the alluvial fans, groundwater emerges and creates zones of saturation and increases streamflow.

The tidal waters of the lagoon establish a base level control on groundwater elevation and backwater flooding from streamflow.

1854 The alluvial fans of the Salt Creek tributaries probably looked something like this before the paved Fairfax Bolinas road increased runoff and sediment supply. The tidal marsh embayment was eventually blocked from the lagoon.

A thin pink 1854 line highlights a trail that skirted the tidal and seasonal wetlands.

A thick pink line shows dirt roads used for hauling timber by oxen to the lighter wharf.

Unfavorable saturated conditions probably led to early Lighter Wharf stream alterations and the need for structural stream crossings.

This sketch 2015 depicts the possible historical conditions on a modern image.

Note the narrowing and extension of tidal marshlands into the Bolinas Lagoon.

Legacy land use practices greatly increased the production, supply, and rate of sediment deposition to the Lagoon.

2015 The old LiDAR of northern route North End highlighted in Bolinas in pink. A Lagoon route that appears on 1892 maps is shown in yellow and is aligned west of Lewis Gulch Creek. Prior to this, Horseshoe Hill Hoershoe Hill Road Road was the necessary route into Bolinas. Bolinas Fairfax Road shows up on maps from 1910. LiDAR enhances the Denny’s 1910 Hiking map modern stream from Dewey Livingston. courses.

2015 The modern LiDAR of course of North End streams at the Bolinas north end of Bolinas Lagoon Lagoon are highlighted in blue. Major stream crossing structures are shown are red and yellow circles. Many segments of the channels have been purposely diverted and ditched (orange lines). Some require continual maintenance dredging.

Modern channels have become 2010-2011 deeply incised topographic into the upper map with 2 and middle foot contour portions of intervals their former alluvial fans. As a result, zones of coarse sediment storage is now limited to the polygons represented in yellow. The former fans are functionally disconnected from spreading and distributing water and sediment.

Incised channels are 2015 usually unstable. They accommodate more runoff. They create larger floods that arrive downstream more quickly. They erode their streambed and banks, producing and supplying more sand and finer-sized sediment that is deposited in their lower gradient downstream reaches at their tidal transition zones and the lagoon. During high tides backwater flooding upstream of their culvert inlets causes the channels to loose capacity to contain flows within their banks. Hence more flooding and more dredging is required.

Sketch of Future ? projected mid century sea level rise for highest tides Planning for the Future ? future. 3 potential alternatives should consider objectives: • make way for sea level rise

• minimize both fluvial and tidal flooding

• increase transportation safety

• restore ecological functions

• Minimize maintenance and construction costs

• Enhance aesthetics What amount of stream and ecological restoration is possible to fully restore natural processes? Theoretically, what amount of stream and ecological restoration is possible?

Toolbox of some options to consider: • Move roads • Elevate roads • Reestablish older road corridors • Move streams • Increase available space for lagoon to migrate without abrupt or armored shoreline • Allow natural channel migration (remove unnatural constrictions) • Reduce channel incision • Reduce sediment supply to lagoon

What is the least amount of ecological restoration possible that will still satisfy most of the objectives, especially minimizing future construction and maintenance costs?

How can we best address all the challenges of safety and potential earthquake and landslide hazards?