How water moves through the Ross Valley
Rain falling on the ridges above Fairfax, Sleepy Hollow, and San Anselmo runs off the steep slopes and soaks into the clay and colluvium on top of the bedrock. That water feeds Fairfax Creek, Sleepy Hollow Creek, and San Anselmo Creek, which join and become Corte Madera Creek on their way through Ross and Kentfield to the bay.
The valley floor is where it all collects. During and after atmospheric-river storms, the creeks rise, and so does the groundwater in the alluvial soils alongside them. Ross Valley residents know the flood history well, especially the New Year's Eve 2005 flood that put downtown San Anselmo and Fairfax under water. It drives the ongoing countywide flood-protection program for the watershed. Most years there is no flood. But high groundwater along the creek corridors returns every wet winter, and that is what fills crawlspaces.
Creek-corridor homes: water from below
On the flats near the creeks in San Anselmo, Fairfax, Ross, and Kentfield, crawlspace water usually doesn't come through the walls. It comes up through the ground. The groundwater rises until it reaches the crawlspace soil surface, and the crawlspace becomes a shallow pond that can linger for weeks. Homeowners often describe the crawlspace as "wet from the bottom" even on dry days between storms.
Uphill interceptor drains do little here, because the water is not coming from uphill. The fix is crawlspace dewatering:
- Shallow interior collection drains across the crawlspace floor, in fabric-wrapped drain rock, sloped to a sump
- A sealed sump basin with a cast-iron pump sized for long run times, since groundwater-fed sumps can cycle for weeks
- A battery backup pump and high-water alarm. Fallen trees on PG&E lines are common across the valley's heavy canopy in winter windstorms.
- A 12- to 20-mil reinforced vapor barrier, sealed at seams and piers, once the soil is kept free of standing water
Many of these homes also have low-lying downspouts and patios that send surface water toward the crawlspace vents. Solid tightlines and a few well-placed French drains stop that extra water from making a groundwater problem worse.
Sleepy Hollow
Sleepy Hollow is a long, unincorporated valley north of San Anselmo, with ranch homes on the valley floor and larger homes climbing the hillsides on either side. Valley-floor homes near Sleepy Hollow Creek deal with seasonal high groundwater. Hillside homes deal with seepage moving downslope toward their uphill foundation walls. Many properties also have long driveways and large lawns that collect runoff from the open space above. Outlets and right-of-way work are permitted through Marin County.
Brookside & Morningside hillsides
The hillside streets of Brookside and Morningside climb steeply off the valley floor, with homes on narrow, stepped lots and older wood and concrete retaining walls holding up driveways and gardens. Here the problem flips back to hillside seepage. Water travels along the clay contact and surfaces against the uphill side of the house or behind the walls.
A deep interceptor curtain drain cut to the clay or bedrock, combined with retaining wall back drains and weep hole relief, protects both the foundation and the walls. Narrow streets and limited parking often mean hand excavation and careful material staging.
Fairfax hillsides & Ross estates
Fairfax has some of the steepest residential streets in the valley. Homes are cut into wooded hillsides with seasonal springs, on lanes with no curb and gutter, which limits outlet options and often calls for an approved dispersion design or piped connection. Ross is the opposite: larger, flatter lots and historic homes, where runoff from neighboring slopes and long-buried, failed drain lines can leave lawns and basements saturated. A camera inspection of existing lines is often the first step on older Ross properties.
Discharge rules: keep it out of the sewer
In the Ross Valley, getting the water somewhere legal is half the job. Groundwater and stormwater must never be routed into the sanitary sewer, because the Ross Valley Sanitary District actively works to remove storm inflow from its system. Curb cores and street connections need encroachment permits from San Anselmo, Fairfax, or Ross, or from Marin County in Sleepy Hollow and Kentfield. Anything that discharges toward a creek bank should be designed with energy dissipation and may need additional county and state review.
Water rising in your crawlspace? Call (415) 555-0100, or request an estimate from a Ross Valley drainage specialist.
Drainage services in San Anselmo, Fairfax & Ross
- Hillside French & Curtain Drains → Deep interceptor curtain drains, French drains, and downspout tightlines designed for Marin clay and serpentine slopes.
- Crawlspace Waterproofing & Sump Pumps → Interior perimeter drains, cast-iron sump pumps with battery backup, and reinforced vapor barriers for wet Marin crawlspaces.
- Retaining Wall Drainage → Back drains, weep hole relief, and pressure relief for leaning or weeping hillside retaining walls.
Frequently asked questions
Can a drainage system protect my house from a creek flood?
No private drainage system stops a creek from overtopping its banks. What it does is control the groundwater that rises under your house for weeks around those events, and the runoff that reaches you from uphill. That covers most of the crawlspace water Ross Valley homes see in a normal wet winter.
Why does my San Anselmo crawlspace get wet from below when it is not raining?
Homes on the valley floor sit close to the creek corridors, where groundwater rises through the winter and stays high for days or weeks after storms. The water comes up through the crawlspace soil rather than through the walls. A collection drain and sump pump, followed by a reinforced vapor barrier, is the standard fix.
Can I connect my sump pump to the sewer cleanout?
No. Groundwater and stormwater must stay out of the sanitary sewer. Extra water in the sewer during storms is exactly what the Ross Valley Sanitary District works to eliminate, and illegal connections can bring fines. Sump discharge should go to the street, a storm connection, or an approved dispersion system.
Who permits drainage outlets in Sleepy Hollow and Kentfield?
Sleepy Hollow and Kentfield are unincorporated, so street and right-of-way work goes through Marin County. San Anselmo, Fairfax, and Ross each issue their own encroachment permits for work inside town limits. Anything that touches a creek bank can bring additional county and state review.