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Soil types and where to ride in Winter, Part 1

Posted by Patty Fri, 19 Dec 2008 00:58:00 GMT

We get a lot of questions about where it is good to ride when the weather turns wet. We could just tell you where it’s good today, but that’s sort of like telling you which way to turn your rebound knob in the middle of the Braille Trail. Recommendations like these are transient, valid one-time only, and depend on current conditions.

Short of going out and actually sampling the trails ourselves, or perhaps having a cadre of trail samplers who would reliably report conditions to us, we instead use local knowledge of geology, topography, microclimates, and past experience to make predictions on local trail conditions.

Soquel Forest rain ride after effects: not so good for your drive train

For a detailed discussion of our weather, soil types, and winter trail conditions, click below to read on.

If you are new to California, you may not be familiar with our weather patterns. Our seasons are uneven. We have a 5-month long dry summer, short spring and fall, and a compact 3-month wet winter. Winter is the one time of year we are challenged by weather curtailing our riding. Also, Autum feels like Spring sometimes because that is when our new grass starts growing, even as the leaves start dropping from the trees.

Historical rainfall levels for the Los Gatos mountains

Seasons
Spring: March and April. Usually the rains taper off and storm fronts come less frequently with longer clear spells including a heat wave or two. We get about 22% of our rain in the spring. By May, our grasses are usually starting to turn brown as summer arrives.

Summer: May through September. Generally, it is either sunny or foggy in the summer and it doesn’t usually rain at all. Historically, on average we get only 5% of our annual rainfall during these five months.

Autumn: October and November. Occasional rain fronts pass through, spitting light rain. Temperatures can be quite warm between fronts. We will get about 23% of our annual rainfall in the Autumn.

Winter: December through February. This is when it rains a lot. Some people call it our “monsoon season”. We get a mix of light rain, windy storms with heavy rainfall, clouds and sunshine as storms develop over the Pacific Ocean and flow east in waves of wet weather typically lasting 3 to 5 days per front, with periods of clear skies lasting a few days or several weeks between storms. Fronts will drop anywhere from 1/2 to 2 or as much as 6 inches of rain. Sometimes fronts come back to back and we can get weeks of continuously rainy weather with flooding, mudslides and other hazards with only periodic calms between storms. Usually we get about 50% of our annual precipitation in these three months.

Ok, having gotten the weather discussion out of the way, here are some of the things we consider before guessing on trail conditions:

Slope, aspect and elevation
Slope is how steep the hillside is. Aspect is what direction it faces on the compass.

Water drains off of steep slopes quickly, but tends to slow down and pool on flatter ground. So trails on hillsides tend to try faster than the flatlands.

Trails on south facing slopes will also usually dry faster than ones on north facing slopes if the underlying soil is the same, because the south facing slope gets more sun whereas the north facing slope spends more time in the shade. East facing slopes are usually slightly wetter than west facing slopes during dry periods, because the morning sun is cooler than the afternoon sun and doesn’t evaporate as much water.

Russian Ridge viewed from Mendigo Hill to the south shows wooded north facing canyons and warm south facing grassy hilltops. Trees appreciate the runoff, but trails under the trees can be wet much longer after a rain

High elevation slopes tend to feature quick draining, coarse rocky soils because the fine sediments wash off, whereas lowlands tend to feature fine, heavy soils that hold water. Our mountains have interesting elevation-related microclimates. As storm fronts come in off the ocean they are pushed up by the ridgeline. The summit gets way more rain than either the coast or the bayside cities. The storms usually drop more rain on the slopes that face the ocean, but the rainfall also spills over the top and gets the high slopes just east of the summit. These high elevation areas above 1500 feet tend to really get dumped on and be much wetter during storms. They also can be moist in the summer when fog sits on the ridgeline.

Underlying geology.
In the Bay Area we have several distinctive geologic blocks and soil types so if you know a little bit about geology that can help you understand why the dirt is the texture it is. In the lowlands, most of the soil has been deposited there after washing off the hills above and is high in clay. Clay is made up of microscopic flakes. The space between each flake acts like a sponge, sucking water up. The tension water creates between those flakes makes clay sticky. The particles don’t want to separate from each other. This is not good. Once you get a fine layer of clay on your tires, if you keep riding the clay will build up, making your tires fatter and fatter until they are rolling donuts that are too big to fit through your fork stanchions or chainstays. Clay can also be found up in the hills where erosion has deposited it in silty flats.

In the hills, the nature of topsoil is usually dominated by the underlying rock. Granite or other old metamorphic rock, and younger “Graywacke” or Sandstone are the most common rocks around here, although we also have many areas of Serpentine, which is usually bluish in color. Volcanic rock is uncommon in our nearby area. Old hard granitic rock makes for good winter trails, because the crystal structure of the rock tends to have square edges which create a rough surface that provides good traction. Also coarse sandstone is good because it drains well and is tacky when moist, although finer grades of graywacke tend towards mudstone or siltstone and the soils produced can have a lot of clay and be not so tire-friendly.

Serpentine is a strange stone that is formed in deep fault zones. It fractures in curves, like glass, and consequently the small rock fragments tend to slip and slide around against each other instead of bind. It is almost impossible to build a stable rock wall out of serpentine. Road cuts that go through serpentine are prone to wavy slumping of the road, and mudslides. If you drive on Edgewood Road near 280, or 280 north of there, you can see the blue rock and mudslides in the area. Trails that go through serpentine to have stability problems too, and the soils derived from serpentine tend to be covered by grass, and thus are usually gooey when wet.

Serpentine rocks are formed in the sliding zone of faults

Vegetation.
Grass is pretty in the sunny springtime, but in the winter it should raise a great big red flag warning you of black clay. Grass decays slowly and the organic matter tends to be greasy. Clay dries very slowly and even after several sunny days in the wintertime it can still be a gooey saturated mess. If grass is growing from brown colored wet dirt instead of black, the clay content is probably lower and it may make for good trail conditions.

Chaparral is usually found on south or west facing slopes where the sun bakes the earth in the summer. Usually there is not much topsoil under chaparral, so the earth you roll across is usually very close to bedrock with little to no organic matter. Suitability for riding depends on whether that bedrock is sandstone, granite, or serpentine.

Oak and mixed broadleaf woodland soils tend to be wetter than chaparral. Usually these woodlands are on shady north-facing slopes. Oaks and other hardwoods produce a lot of leafy litter, which decays into a greasy black topsoil. So the heavy topsoil and short period of winter sunshine under Oaks and Bays can make for wet and gooey trails.

Redwood and Douglas Fir forests feature amazing towers of timber that hold thick canopies way up in the sky. These canopies shade the dirt underneath, but the root systems of these huge trees are incredible. When the sun comes out, these trees become giant water-sucking devices that will pull water out of the ground rapidly. The organic litter under redwoods tends to be thick, airy, and twiggy, and usually is less greasy than under oaks, and in our area, these trees tend to grow on sandstone, which drains well.

Hours of daylight.
December 21 has 9.5 hours of daylight on the solstice, January 21 has 10, February 21 has 11 and March 21 has 12 hours of daylight on the equinox. The soil has a tendency to stay cold during winter because it spends more time chillin’ in the dark than it does warming in the sun. However, as the days get longer, the sun also rises higher above the horizon so the warmth provided by the sun in February is considerably greater than in December. So December to February the cold dark dirt tends to stay wet unless we have a whole lot of sunny days. The lengthening days and heightening sun combine magically in February in California to bring forth amazing wildflowers and periods of joyous spring-time feelings. Be ready for them.

Weather patterns and the days of the week.
If it’s been raining for a week, one day of sunshine isn’t going to do much to dry out the trails. Likewise, if it rains for a week straight in January, and then we have a week of cold cloudy weather, the sun won’t have had much of a chance to do its work. If we have several weeks of rain, the ground is going to get saturated and soft. If it’s sunny for 10 days, chances are good there is good riding to be found. So what does that have to do with the days of the week you ask? Well, if it rains on the weekend, few people are going to go out and ride while the wet stuff is falling and the trails are really wet, and if the sun comes out during the week, few people are going to be able to split from work and enjoy the trails when they firm up. Lack of use on wet trails keeps them in better shape. The hard compacted surface stays firm if you don’t squish it around.

But if it rains during the week and the sun comes out on Saturday, people’s desire to get out doesn’t coincide with the timing needed for the sun to dry things out. If you can’t resist and you ride on wet trails leaving tracks behind, the movement of the wet dirt softens it, and knobby tire tracks can collect and channel water in the next downpour. Squishing dirt and turning it into mud drives water into the microscopic cracks and pops the air pockets, and leaves a layer of silt floating on the top which is the slowest-drying component of the soil. When EVERYONE does this the trail surface will turn into a slurry, which will take a lot longer to dry and be a nuisance to all while its wet. Look for multiple days of sunshine to provide a firm surface to ride on.

I will post more on specific parks and trails and why they make for good or bad winter riding in the next article. —Patty