Phoenix Lake—Bon Tempe Connection Project

by Sandy Guldman

Two watersheds: Lagunitas and Bon Tempe reservoirs in the foreground, and Corte Madera Creek watershed bounded by White Hill and Loma Alta. Photo by Charles Kennard

“Marin Water proposes to create a connection between Phoenix Lake and Bon Tempe Reservoir to improve operational efficiency and allow for more frequent use of Phoenix Lake water without the existing required intensive system modifications.” This statement is found early in the environmental review document prepared for the project, the Initial Study/Mitigated Negative Declaration for the Phoenix Lake—Bon Tempe Connection Project.

On its face, this seems like a simple project to move water from one reservoir to another by building a new pipeline. This would avoid the need to repeatedly modify the existing water infrastructure to alternate between conveying treated and raw water directly to and from the Bon Tempe Treatment Plant. However, we are concerned about damaging effects on aquatic life in Ross Creek that the project will have unless measures are taken to avoid them.

However, the environmental document ignores some crucial aspects of the project: It would move water from the Corte Madera Creek Watershed into the Lagunitas Creek Watershed, with potential impacts on water quality, riparian habitat, and the steelhead population in Phoenix Lake and in Ross and Corte Madera creeks. It does not mention the significant existing impacts of the Phoenix Lake Dam on those same resources. It does not acknowledge the persistent presence of steelhead/rainbow trout (Oncorhynchus mykiss), a federally listed species, in Ross Creek. It ignores the need to consult with the SWRCB, which regulates inter-basin water transfers, and with NOAA Fisheries, with authority over O. mykiss.

Presence of Fish in Ross Creek

There is ample evidence that Ross Creek supports O. mykiss. A crew led by Eric Ettlinger electro-fished five pools in Ross Creek between the dam and the parking lot at Natalie Coffin Greene Park on August 3–4, 2005. They found 185 O. mykiss, 62% of all the fish captured.

Over the years before and after, numerous O. mykiss have been seen by people visiting Ross Creek. Although many of the O. mykiss may not practice anadromy, that could easily change when fish barriers downstream of Phoenix Lake are removed. This is especially true when a major project will be implemented to remove barriers to fish passage caused by the Ross fish ladder and the concrete channel in Ross and Kentfield. It is scheduled for fall 2025.

Temperature and Dissolved Oxygen

Phoenix Lake typically stratifies in the spring, meaning that water in the lake is divided into stable layers, with warm water on the top and increasingly cool water as depth increases. The stratification ceases when cooler weather and winter wind and rain lower the surface water temperature and the layers mix. Temperature and dissolved oxygen (DO) data collected in Ross Creek since 2008 show that when Phoenix Lake is discharging over the spillway in the spring after the lake has stratified, the water entering Ross Creek is well oxygenated, but too warm for O. mykiss. When the water level drops and discharge over the spillway stops, water in the upper portions of Ross Creek comes mostly from leakage from the low-level release valve or from groundwater seeping through the base of the dam. Its characteristics are very different: it is much cooler and has virtually no DO.

Iron and Manganese Levels

Oxidized iron and manganese color rocks orange and black, respectively, near the base of Phoenix Lake dam. Photo by Sandy Guldman

A serious water quality problem in Ross Creek is a direct result of the low DO and high iron and manganese content in water at the bottom of Phoenix Lake. A report prepared by Friends in 2011 presents the results of water tests that show harmful levels of iron and manganese in the water leaking through the low-level release valve into Ross Creek. The excess iron is obvious in orange deposits on anything the water contacts near the valve. Black deposits on the rocks further downstream of the dam are likely deposits of oxidized manganese. Independently of the proposed project, Marin Water should implement measures to aerate the water column in Phoenix Lake so that water entering Ross Creek has better water quality.


The analysis of Ross Creek hydrology in the environmental document was prepared to assess whether the proposed diversions would “significantly delay or reduce flows at times when steelhead may be using fall pulse flows in Ross Creek as a cue to trigger upstream migration and spawning activity.” The study does not recognize that O. mykiss typically spawn in January through March, not the fall. It also fails to account for the hydrologic impact on rearing and emigration. The project’s impacts on all life stages of O. mykiss should be evaluated. Protecting only part of a life cycle is not enough.

Board Policies

Finally, the proposed connection does not comply with Marin Water’s stated policies. Board Policy No. 7, adopted September 10, 2010, PART 2−Biological Diversity, begins with this goal:

Protecting the integrity of the watershed is best achieved through maintaining natural conditions on watershed lands consistent with District policies and federal and state laws. The District is committed to restoring and sustaining native biological diversity on District lands, in particular the variety of living organisms, the genetic differences among them, and the natural communities and ecosystems providing their habitat.

Applicable policies in PART 2 include:

A. Species and Habitats – The District will protect and restore species richness and complexity of habitats on District lands, and seek to preserve or restore natural habitats to the fullest extent possible.

B. Rare Species – The District will identify and promote conservation of all special status plant and animal species especially those listed under federal and state Endangered Species Acts.

C. Adverse Impacts – The District will minimize adverse impacts to spatial and temporal patterns of native species for reproduction, feeding, migration, and dispersal….

J. Streams − The District will take actions to protect native fishery resources, in streams within the District’s sphere of influence, consistent with California public trust doctrine and Fish and Game Code. The District will be an active partner in stream protection and enhancement efforts that other agencies and groups are pursuing in streams within the District’s sphere of influence. The District’s sphere of influence includes those streams that are directly affected by the District’s land or water management activities.

Friends has requested that the Initial Study/Mitigated Negative Declaration be revised to more accurately reflect the presence of fishery resources in Ross Creek and the impacts of this project and Phoenix Lake Dam have and would continue to have on those resources and to acknowledge the regulatory requirements. The District has an opportunity to improve conditions for aquatic life in the creek fed by Phoenix Lake, consistent with its policies, and should not sidestep this opportunity and responsibility.

We urge concerned citizens to let Marin Water know that a more thorough analysis is called for. In addition, it should identify the mitigation measures necessary to improve habitat in Ross Creek

All of the reports mentioned in this condensed article are available. If you would like more details, email us at

Docks and Ducks

by Nicholas Salcedo

An interesting conversation developed the other day, when I was talking about lower Corte Madera Creek. It came out of a misheard word.

“I thought you said dock.”

“No, I said duck.”

Turns out there are quite a few of both in this part of the creek, the tidal portion below Bon Air Bridge down to the mouth of the creek, where it enters San Francisco Bay by the Larkspur Ferry Terminal. The beginning of the conversation revolved around what’s a dock. Then it turned to what actually is a duck, and eventually finished with thanks for some new-found knowledge and appreciation of just a couple more things that are Corte Madera Creek.

First up came docks, specifically for boats, including sculls, canoes, kayaks and the like. A boat dock is basically a place where a boat parks on the water. Docks can be fixed or floating, or a combination of the two, and can be made of concrete, metal, rubber/foam/plastic, or wood. They are often treated with chemicals to retard deterioration, a necessity in salty water. Fortunately, one of the worst offenders, creosote, is no longer permitted, but many creosote docks or their remnants persist.

Greenbrae Boardwalk’s docks extend into Corte Madera Creek, opposite Wood Island and the Ferry Terminal. Photo © Scott Hess

Not all docks function in the same way. The small one at the end of Piper Park, currently closed while the City of Larkspur evaluates whether it can be repaired or must be replaced, is seldom used. On the other hand, the larger US Coast Guard-inspected Larkspur–San Francisco ferry docks see regularly scheduled activity every day. Maintenance, including dredging, is a necessity. Private docks, like those at Larkspur Marina and Greenbrae Boardwalk, fall somewhere in between. But it’s safe to say, the bigger the dock, the more intensive the activities and maintenance. Boat and dock use also disturbs wildlife, particularly those waterfowl (and other bird species) resting and refueling during their migration. Responsible boating and dock use helps minimize this impact.

Dock use also varies by season. While some docks are used year-round, like the ferry docks or Marin Rowing Association dock, their use picks up during summer. Not only is it the longer days and warmer weather, it’s also the increase in events.

In this classic, shallow estuarine environment, to reach navigable water at all tides, docks need to extend far from the shoreline or necessitate regular dredging, or both, which increase the open water, mud flat and marsh area impact. A Google Earth view in this two-mile section revealed 175–180 docks. If the average size of a dock and gangway is 750 square feet, for 175 docks that’s over three acres of aquatic habitat directly impacted. The Ferry Terminal, including its associated gangways, landings and buildings, adds at least another acre to this impact, not including dredging.

Tidal marshes serve as the base of the food web, filter and process nutrients and contaminants, protect against erosion, and provide important habitat for fish and shellfish. A dock blocks the amount of light that reaches the underlying area, which can lead to a reduction in plant density. A reduction in the number of plants under docks can also have secondary effects. Fewer plants can lead to increased soil erosion. Reduced plant density also decreases the amount of food available to the animals in the marsh.

Holiday-makers at a dock in the vicinity of today’s Bon Air Bridge. Photo courtesy of the Ross Historical Society.

Docks can change the flow of water around them, with increased erosion around pilings. Floating docks that rest on the mud at low tide disrupt the algae that live on the surface of the mud as well as the animals living in the sediment that eat the algae. Floating docks made of Styrofoam can break apart and release bits of foam to the environment; these are notoriously indestructible and contribute to the microplastic load in the environment. Anyone who has participated in a cleanup along the tidal portion of the creek has encountered chunks of crumbling foam in the wrack.

Docks can have other impacts on water quality: pressure-treated lumber, impregnated with chromated copper arsenate (CCA), is a very common construction material for both the submerged pilings and the decking of piers and elevated walkways. Leaching of these three metals (chromium, copper, and arsenic) is fairly rapid in sea water; however, the metals tend to accumulate more in fine sediments, like the silt and clay common in the tidal reaches of Corte Madera Creek, than in sand. The metals then can accumulate in vegetation growing in the fine sediment and in the organisms eating the vegetation. High tidal flushing reduces these impacts.

Fuel for boats with motors poses a particular risk to water quality. The polyaromatic hydrocarbons in fuel are acutely and chronically toxic to plants and animals. Because many of the boats used in Corte Madera Creek do not have motors, fuel spills are probably rare in Corte Madera Creek. However, they cannot be entirely prevented. Additionally, if boats are cleaned or repaired near the creek, detergents, antifouling paints, and debris can enter the water, degrading water quality. The California Coastal Commission publishes several documents with recommendations for boat maintenance, as does the Environmental Protection Agency and San Francisco Department of Public Health.

It would be remiss if we didn’t equally discuss ducks. They are a type of waterfowl, part of the family Anatidae, that also includes swans and geese. Generally, ducks, with the exception of those classified as whistling ducks, are in the subfamily Anatinae, and range from perching ducks like the beautiful wood duck, to dabbling or puddle ducks, best represented by the ubiquitous mallard, to diving ducks, often referred to as bay ducks, like the stately canvasback, or sea ducks, like the cute little bufflehead or the elegant merganser.

Female merganser, photographed by Gary Leo

Ducks’ morphology and behaviors are what help set them apart. For example, perching ducks have sharp, strong claws with legs set more forward. A dabbling duck’s legs are more centrally placed, while a diving duck’s legs are set far back on the body to facilitate diving. Distinct behaviors include the wood duck’s preference for wetlands with adjacent woodlands, for they nest in tree cavities. And there are different feeding behaviors. Dabbling ducks customarily feed by tipping up, so their tails show above water, while diving ducks go completely underwater in search of food. Some ducks are known to dive over 100 feet deep!

Ducks are most numerous in Corte Madera Creek during the winter. There are also significant numbers during the fall and spring as they stop over to refuel during migration, as Corte Madera Creek is part of the San Francisco Bay estuary hot spot in the Pacific flyway. Cornell Lab’s eBird listed over 20 different waterfowl species observed last year in Corte Madera Creek. Audubon’s 2023 Marin Christmas Count recorded 16 different waterfowl species in our estuary. Those recorded at that time in the highest numbers (over 100 for each) were wigeon, northern shoveler, green-winged teal, scaup, and bufflehead.

Lower Corte Madera Creek has habitats for all of these ducks. Probably most important is the fact that this reach of creek is tidal, so the mud flats and marshes are accessible to different ducks at different stages of the tide. Also, the College of Marin’s Ecology Study Area, at the upper end of tidal action (and preserving the historic alignment of Corte Madera Creek), is a wetland with adjacent woodlands, one of the few remaining in the watershed.

Where did this conversation all lead? Problems were the first topic—habitat loss, disturbance of feeding birds, and pollution from the docks. But then we switched to joy. I mean, who doesn’t love ducks and ducklings? Watching a duck glide in and make a waterski landing on a glassy water surface is always exhilarating.

And who doesn’t love going out on the water in a boat? A ferry ride to San Francisco is always a treat and often spectacular. It’s been said that to fully understand the environment, one needs to experience it by land, air and sea. Or maybe it’s just by sitting on a dock by the bay, watching the tide roll away? Surely ducks love to sit on docks, too.

The Scoop on Mosquitoes

by Alycia Matz

What reactions do you have when you hear the word “mosquito?” Does it conjure the sound of insistent buzzing in your ears? Does it make your skin crawl with the thought of itching bites? Does it fill you with concerns for human health?
I’d hazard a guess that for most of us, the answers are yes, yes, and yes. These sentiments are not without good reason. Not only are their bites itchy and irritating, but there’s the risk of transmission of vector-borne diseases, such as West Nile virus and dog heartworm. In short, we all know mosquitoes are a nuisance—but how much do we really know about them?
According to the Marin/Sonoma Mosquito and Vector Control District (MSMVCD), there are over 20 different types of mosquitoes across the two counties. While their prevalence changes over the year given various environmental factors such as temperature, the five most common species documented in the 2023 MSMVCD surveillance report were Culex erythrothorax, Culex tarsalis, Culiseta inornata, Aedes sierrensis, and Aedes dorsalis. So far, no invasive mosquitoes have been detected in Marin or Sonoma counties, but MSMVCD is on high alert for Aedes aegypti and Aedes albopictus, both of which have become established elsewhere in California and have the potential to transmit viruses such as dengue and Zika.

Good mosquito habitat: a seasonal pond on the flank of White Hill. Photo by Charles Kennard

Mosquitoes’ life cycles depend on aquatic habitats, both fresh and brackish. Eggs are laid in standing water, where they develop into larvae, then pupae, and then leave the water as adults, all over 7−10 days. MSMVCD notes mosquitoes need as little as a half inch of water to complete their life cycle. Therefore, efforts around our backyards, such as clearing gutters, draining plant saucers, and regularly changing water in bird baths can go a long way in curtailing their breeding grounds.
Nevertheless, the aim to reduce larger areas of standing water to protect public health can become counterproductive. There are claims across the country that draining wetlands is a laudable method for controlling mosquitoes, as it reduces their habitat. However, mosquito eggs can remain dormant in dry habitats for over a year. That means even if an area is drained, it may hold enough water after a rainfall event to allow mosquitoes to hatch, especially since their life cycle is so short. Meanwhile, if wetlands remain intact, they can provide habitat for natural predators that help keep a mosquito population in check, such as birds, frogs, fish, and other insects.
But what if a mosquito population does get out of hand around us? MSMVCD mentions biological control is one effective method, often taking the form of mosquitofish. They are available from MSMVCD for individuals to release in ornamental ponds or other humanmade water sources, but it is essential that these water sources are not connected to waterways. Larvicides also reduce adult emergence and consequent disease risk. Microbial larvicides, such as Bacillus thuringiensis var. israelensis (Bti) and Bacillus sphaericus (Bs) have become preferred over chemical treatments in recent decades, as these bacteria selectively target mosquito larvae with minimal effects to nontarget organisms. However, other reports show they still prove toxic to closely related flies such as non-biting midges, which are an important food source for many wetland species.
Furthermore, the National Wildlife Foundation (NWF) points out we should think twice before hiring a mosquito control company to treat our yards with an insecticide. Many of these services use pyrethrin and pyrethroid-based pesticides to treat adult mosquito populations. While these chemicals are regulated and approved for use by the Environmental Protection Agency, it is not without a cost. These broad-spectrum insecticides kill other insects they contact. That means goodbye to beloved pollinators such as bees and butterflies, not to mention other beneficial insects such as ladybugs and dragonflies. When insects are eliminated in such a fashion, this has ripple effects across the food web. The NWF references research concluding as much as 96 percent of all North American birds feed their young exclusively on insects. For MSMVCD’s part, they note that the use of adulticides is considered a last-resort method.
Only female mosquitoes seek out blood, while mosquitoes of both sexes use a proboscis to feed on flower nectar and fruit juices. The historical perception was that mosquitoes were mere “nectar thieves,” that is, consuming nectar without providing any pollination services. However, recent research with plants such as orchids, tansies, goldenrod, and yarrow has shown evidence that mosquitoes do function as pollinators. These latest developments open up further avenues for exploration, including assessing the extent of mosquitoes’ contribution to reproductive biology and plant conservation, or determining if a nectar or sugar-based bait could be developed as an ecologically-sound control method.
In sum, there’s no argument that mosquitoes are an annoyance, and large populations are a reasonable cause for concern that necessitates control. There are simple actions we can take around our homes to reduce creating incidental habitat. However, it may be worth considering viewing mosquitoes and their management with nuance, now that we know more about them.
For further reading, see the Xerces Society’s 2018 report, “Ecologically Sound Mosquito Management in Wetlands.”

Passage of San Anselmo’s Measure F: Gain or Loss?

by Laura Lovett

In 2007, voters in the Ross Valley authorized a flood control fee to be assessed to each household. The fee was approved by a majority of voters and will be collected through 2027. The fee revenue is used to fund the Ross Valley Flood Protection and Watershed Program, the goal of which is to manage stormwater and reduce the damage from flooding during significant storms.
To date, approximately $35 million has been collected and spent on items as diverse as the detention basin above Fairfax, purchase of Building Bridge #2 in San Anselmo, various bridge and culvert projects, and development of a detailed computer model of how creeks in the Ross Valley function. Additional grants amounting to $20.2 million have been received to help fund further projects. $18.7 million has been spent in the town of San Anselmo.
While a great deal has already been done, much of it may not be visible to residents: it is essential to complete costly engineering and feasibility studies first; permits are expensive and time-consuming to apply for; staff spent time applying for the $20 million in grants received. These happen long before any construction takes place. In recent years, however, many San Anselmo residents have been feeling that a lot of money has been spent and that there is nothing to show for it.
Last year, town residents gathered signatures and placed Measure F on the 2024 March 5th election ballot. The Measure asked, “Shall an initiative measure be adopted to withdraw the Town of San Anselmo, including all parcels of land in Town, from the Marin County Flood Control and Water Conservation District Zone 9?” It passed with a majority vote.
The measure’s proponents told voters that this would get them out of paying the fee going forward. However, fees must still be paid for work already committed and underway, which will take us to 2027. The town residents, along with the rest of the Ross Valley, will be paying fees until then.
One of the major items affecting the flow of water through San Anselmo has been the historical structures built over the creek as it passes through town, some of which have foundations within the creek flow channel. These, and the buildings’ low walls, cause the water to back up, and they catch debris which further blocks water flow. Removal of these obstructions has been a major objective of the Flood Control District.
The county purchased 632−636 San Anselmo Avenue, which is known as Building Bridge #2, and removed the buildings in preparation for removing the concrete platform they sat on, and their supports in the creek bed. Then Covid changed everyone’s lives, the work came to a halt, and the town started using the platform as an outdoor gathering spot. The county has now put the work back on schedule and this platform and its supports are due to be demolished next year. The passage of Measure F hasn’t affected that timeline. Engineering studies show that removal of this obstruction will reduce the overall depth of future floods by as much as a foot and improve stream flow for fish migration. Many residents were angered by the closing of their gathering spot when the structure was deemed unsafe, and this anger spurred the voter rebellion.
In the end, the one clear thing achieved by Measure F was to remove San Anselmo from a seat on the Flood Zone 9 Advisory Board. San Anselmo councilmember Brian Colbert remarked that, “It’s problematic that we lost our voice on the board and I regret that much of the information voters received was misleading.”
What is most troubling about San Anselmo’s choice to operate as a separate agent is that the Corte Madera Creek watershed flooding issues can only be successfully addressed if all towns along the waterway work together. The political actions of one town can jeopardize towns above and below them in the watershed. Corte Madera Creek is a single, interconnected stream drainage that a great many smaller creeks feed into. Decisions need to be made with the entire watershed in mind and in conjunction with all other affected parties, including those that can’t vote on a local measure.
Supervisor Katie Rice said, “While we must respect voters’ wishes to remove San Anselmo from Zone 9, the fact is that the only way to address flood risk in Ross Valley is to work together. The District is trying to address potential impacts to the greater community. Progress has indeed been made and ultimately the work will provide real benefit to the communities of San Anselmo, Ross, and downstream.”
Colbert expressed a hope shared by many when he said, “The end of the flood fee provides a way to rethink how the individual communities engage with each other around solving the flooding issues.” We certainly hope to see this happen.