Spring Thaw and Mud Season

The transition from winter to spring is underway and our trails will be experiencing freeze-thaw cycles until the frost is out of the ground. This means checking trail conditions will be more important during the transition between frozen and completely thawed. In the big whiskey, we call it mud season and it usually starts in March and lasts 4-8 weeks. During that time when the ground is frozen solid, you can ethically ride open trails. To determine if the ground is frozen solid, it pays to look beyond the air temperature and look at the dew point. A dew point below 25 F will ensure that the ground will be nice and firm. The next thing to consider is cloud cover. Direct sunlight will begin to thaw the ground by 8:00 am, especially if there is no snow cover to reflect that sunlight. The best times to find frozen solid trail conditions this time of year are at night or very early in the morning (dawn patrol).

The inspiration for this article came from The Decorah, Iowa Human Powered Trail organization. I’ve read quite a few manifestos written about spring thaw that sound a little like sermons. I liked the way that this one sounded, so I copied it and added some of my own observations on how we might best navigate spring thaw. I do have one tip about determining when the frost is out of the ground. If you see earthworms after a spring rain, the layer of frozen soil that prevents normal drainage of surface water has thawed. It’s truly something to celebrate! #huzzah

I love the smell of earthworms after an early spring rain.

~gomez~

What’s the big deal with freeze-thaw cycles?

Dirt trails are extremely vulnerable to rut damage during the transition to and from winter because colder temps prevent the soil from drying. There is also a sub-layer of soil that remains frozen and prevents surface water from draining. the surface layer of soil and meltwater create mud pudding. When topsoil freezes the growth of ice crystals push soil particles apart leaving large gaps that can fill with water when the ice melts. In a thawed state this dirt is much like a sponge and will absorb large amounts of water. It is also hypersensitive to disturbance by foot/bike traffic and flowing water and will form ruts with little effort. Direct sunlight and above freezing daytime temperatures can thaw the top layer of frozen dirt and create an easily rutted, greasy, muddy mess on the surface. Overnight, lower temperatures refreeze the top surface of the trails, ruts included, and the process repeats when conditions allow (hence the name freeze-thaw cycle).

Why are ruts bad for trails?

When ruts develop along a trail they channel water, causing erosion of the trail surface, and slow the drying process. Rain is highly erosive to trails when the sub-soil is frozen and ruts greatly increase the chances of erosion by flowing water. When the sub-soil is frozen, water can’t soak in like normal and flows along the top in large quantities. Ruts on a trail can intercept these flows and divert them along the length of a trail. As the water flows down the trail and picks up speed it also picks up soil particles and washes them away from the trail. This can leave behind deep ruts tens or hundreds of feet long in extreme circumstances (top photo). Come springtime, ruts can also hold pools of water on the trail surface and prevent it from drying as quickly. If these puddles are disturbed by bike tires they can quickly turn into large mud pits. Fixing these areas eats up trail volunteer time that could be better spent on other projects. Wet areas like this also prolong trail closures.

Tips for riding during the winter/spring months where temps fluctuate between freezing/above

• Pay attention to the trail status – Don’t ride frozen trails when the temperature is above freezing.
• Ride frozen trails early in the morning before they have a chance to warm. or at night when conditions are frozen solid.
• Don’t ride if the temps are hovering around freezing and it is sunny. Wait until temps are below 28F and the dew point is below 25F even then there may be thawed areas if there is direct exposure to sunlight.
• If after following all the above guidelines and the trails are still too soft, turn around and come back another time. A single rider can damage many thousands of feet of trail under these conditions.
• Take advantage of paved bike trails and secondary roads during the short time of year that the trails are this fragile.
• Early spring is a great time of year to do trail maintenance. Almost every trail has a volunteer group that would welcome the help. You’ll meet other like-minded folks and reap the benefits when the trails open up for the season.
• Please respect the trails during the transition months in the winter/spring so everyone can maximize their enjoyment come springtime!