At Every Stone’s Throw: Erosion Control Strategies for New Trails

Sept. 1, 2000

An upslope breeze chases over McClure Meadow, just shy of 10,000 ft. in the High Sierra of California on an ordinary August afternoon. The glossy surface of Evolution Creek catches the purple sky, the bright granite ridges, and the soft cumulus and plays them against the deep green shadows of lodgepole pine where we are plodding along on the third day of a two-week odyssey—beasts of burden in a landscape of indescribable and beckoning beauty. Our shoulders scream, the sunlit meadow beckons, trout rise, but we are compelled to move on to a higher camp from where we hope to make it over a 12,000-ft. pass the next morning.

Everyone we meet in this section of the backcountry is hurrying under the strain of two constant worries: rations and weather. Some are walking toward a rendezvous with a packer and supplies that will enable them to prolong their trek in the high country another few weeks. Others are hurrying to or from high passes where sudden afternoon thunderstorms can ruin chances of getting over a pass. It seems that the success or failure of entire backcountry vacations comes down to the availability of calories and the behavior of capricious air masses.

One foot after the other, we climb out of the lush valley into pockets of sparse timber on a canyon wall scoured smooth by glacial ice. The wind picks up, the sky glowers, thunder rattles, huge drops spatter, and within minutes the slickrock that had been sunlit and magical has become dark and glistening. Thunder clatters as if the gods are bowling with giant granite boulders directly overhead. We scuttle downslope to a place we hope will not attract a lightening strike and hunker down to watch as the valley sides begin to stream with runoff.

An hour later, we resume our upward plod along the sodden trail and get a textbook tour of the relations among the factors of the Universal Soil Loss Equation: slope length and steepness, soil conditions and cover, rainfall intensity and duration. It looks as if a packer with a string of mules was hightailing it down from the pass when the weather hit. Where sheetwash from upslope has flowed onto the trail and concentrated on it, the loose earth has been whisked away, leaving lengthwise rills on the pitches and newly laid sediment deposits on the flats. Here the sharp hooves of the heavy animals have churned the wet soil to a depth of several inches.

Despite this seeming destruction, it is easy to see that our trail has been located and designed with worst-case conditions in mind: heavy wear, high-intensity storms, and erosion. These are the facts of life for trails. The best chance for their longevity comes from good initial planning, thorough field investigation, and thoughtful drainage design. The process starts in the office with an overview that encompasses soils, climate, vegetation, groundwater, geology, and slope.

Get a Hawk’s View of the Landscape

To shake out large-scale considerations about where to locate trails and how to drain them, it is essential to get an overview of the general landscape using aerial photos and topographic maps. A good place to start is by noticing where the trees are. About 30% of a year’s precipitation falling on short-needled conifers is intercepted by the needle surfaces, never even hitting the ground. This percentage goes down for long-needled conifers (pines) and drops to about 11% for broad-leafed trees. It is easy to see how the canopies of trees, particularly conifers, can be the first line of defense from erosion of trails by direct precipitation.

The tremendous absorptive capacity of leaf litter on the forest floor is an added bonus. The natural mulch of litter under a hardwood forest has been shown to absorb as much as an inch of rainfall per storm before precipitation even begins to filter through it to the soil. The litter safeguards the porous structure of the soil beneath, enabling water to infiltrate quickly during heavy or prolonged storms. This accounts for the fact that little if any surface runoff occurs in forested areas where streams are fed primarily by base flow.

Understand How Sheet Flow and Groundwater Will Behave

To avoid drainage surprises in unforested locations such as meadows and the delicate zones above the timberline, it is important to know the nature of the vegetative cover, the slopes, and the soils. Is the vegetation continuous or spotty? How long are the slopes above the proposed trail? How deep is the soil? These factors need to be considered together. At first glance, it could be inviting to locate a section of trail through a dense hanging prairie. But seasonally perched groundwater over shallow bedrock in such a setting can be daylighted by a trail cut, and the trail itself can be at risk of becoming a diversion ditch that inadvertently captures groundwater moving downslope. Likewise, a trail running perpendicular to a slope with spotty vegetation in a matrix of exposed soils can intercept sheetwash and overland flows. Drainage considerations such as these often result in the need for special alignments (such as stacked switchbacks at the edge of a hanging prairie) to avoid setting up a situation in which the trail becomes an eroding ditch. It is better to go into a trail project knowing this and including it in the budget than paying for it by default through years of expensive maintenance and repairs.

Identify Rock Outcrops and Geologic Contact Zones

Rock outcrops can require extensive blasting, which increasingly is being prohibited in wildland settings and, in any case, is expensive. A trail may need to take a big detour to avoid the costs of constructing through rock. The shortest route is not always the cheapest. In fact, once several alternative routes have been flagged on the ground, surveyed, and undergone preliminary design, construction costs usually play a major role in route selection.

The hawkeyed assessment should also pick up the location and nature of geologic contact zones in the area. These can often be zones where groundwater daylights in the form of midslope seeps and springs where valley incision has exposed the contact of a pervious stratum overlying a denser one. Sometimes locally moist conditions in these zones show up on topo maps and aerial photos as dense lines of trees or other vegetation. This moisture may intensify both chemical and physical weathering processes, creating a local zone with potential for large and small mass movements of earth materials. These materials may be saturated by both groundwater and surface water for a greater period of time than areas farther away from the contact zone. All of these conditions can result in the need for ongoing maintenance and for construction methods and materials to provide support, stability, and drainage for a trail traversing such a zone. Sometimes it is better to avoid these places.

Avoid Geomorphically Active Zones

Areas of active mass wastage or accelerated erosional or depositional processes should be identified from the get-go. Examples include avalanche and debris chutes, alluvial fans, talus and scree slopes, glacial outwash zones, floodplains, and slopes subject to slumping or dry ravel. Classic “signature” landforms tend to develop in discrete climatic zones, geologic materials, and slope locations. They are clues to weathering and energy-transfer processes that tend to operate in these discrete conditions. An earth scientist can interpret such features from aerials and topos during the overview phase of project planning.

Avoid Areas of High Wind Exposure

Exposures subject to unusual wind conditions should be identified because of the potential hazards to livestock, recreational users, and maintenance crews. Of equal concern is whether wind will cause difficulties in keeping the trail or trail improvements in place. Unusual wind conditions can occur where air masses squeeze through saddles, rise and fall during convective storms, roil on the lee sides of high ridges, or slam into slopes as a result of topographic funneling or orographic lifting. In certain ashy or low-cohesion soils, wind erosion might obliterate the trail or deposition of wind-shifted material might cover it. Wind can move snow from drifts on one side of a ridge to another, creating a lingering snowbank that delays use of the trail until late in the season. Snowmelt might keep the trail soft, which can lead people to pioneer other routes when the trail becomes an eroding conduit for meltwater.

Avoid Flat Ground

Flat ground should be identified and avoided at the earliest planning stage. A rule of thumb for siting trails is that a minimum cross-slope of about 5% is needed to route water off the trail surface and keep it out of the subgrade. A trail on flat ground is a trail that sooner or later will have drainage problems. If the soils are deep, the problems will probably be worse, and the longer and harder the trail is used, the worse the problems will become.

With this in mind, the terrain overview should carefully assess the opportunities to get an alignment up on the slope where good drainage can be achieved. Instead of crossing a flat, the preliminary trail alignment should be just above or below the break in slope leading into or out of the flat area. This might require a longer trail, but most of the time, higher construction costs for a longer trail will be justified by lower long-range maintenance needs.

Finally, the overview stage should identify wet meadows, perched groundwater, seasonal sheet flow, and crossings of both year-round and intermittent drainageways to the extent that the engineering implications and construction costs of working with these conditions are evident.

Plot Candidate Routes

After identifying the opportunities and constraints posed by all of these landscape factors, candidate routes can be plotted on maps and photos. A desired beginning point, intermediate points, and an ending point for each alternative are then marked on maps and photos. An intermediate point might offer a territorial view or be a likely camping or watering spot. It could be a point where the trail needs to intersect another trail, or a bench where the trail must switch back to gain elevation or avoid an unsatisfactory route ahead. To derive an approximate gradient for the trail between two points, the elevation difference between them (in feet) is divided by the distance (in feet) between them. But here’s the rub: Even an 800-lb. gorilla in excellent physical condition will tire if it has to spend the entire day plodding up a trail as steep as 10%. Sometimes a trail must make a short pitch that is steeper, but such pitches are almost always an exception, and they are avoided where possible.

Flag Preliminary Trail Location in the Field

For a trail alignment to successfully connect two points, it must make grade, which means that it must get from point A to point B at a maximum gradient of about 10% or less. Although gradients between intermediate points can be estimated in the office by taking distances and elevations from topographic maps, it is essential to get out in the field and actually locate a preliminary trail alignment on the ground to learn if it will make grade.

Break Grade While Making Grade

Making grade is the supreme challenge to siting a trail, because a well-designed trail consistently breaks grade in order to get the water off the surface. Each section of trail alignment must allow grade breaks for drainage relief at intervals appropriate for the gradient, the soils, the slope above the trail and the cover on it, rainfall and runoff conditions, the kind of use, and the worst-case conditions expected. So a trail needs to break grade while making grade. What this means is that the trail will climb for 80 or 100 ft. at 8-10%, then ease off to 3% or so for 15 or 20 ft. The easing off is the grade break, and it provides an opportunity for the designer to install rock lead-off features, rolling dips, or other drainage-relief features that effectively break up concentrated flow, slow it down, and get it off the trail before it causes damage.

Grade breaks should be carefully located to take advantage of downed logs, large rocks, or little patches of flat ground on the downhill side of the trail, where flow velocity can be further dissipated, sediments can be deposited, and water can be level spread back out into the rough. When the surveyors come through later to gather topographic information for the designers, they will be on the lookout for these features.

Break Grade at Water Crossings

At any location in which the trail must cross a seasonal draw or a live water at grade, care must be taken to ensure that the trail itself does not capture the crossflow. This is commonly done by creating a camelback crossing in which the trail alignment descends to the water from higher ground on both sides of the crossing. To accomplish this, the alignment must give grade at every crossing, even though it might be climbing as steeply as possible in order to make an intermediate point.

Drop Back to Make Grade

A section of alignment that is forced to give grade too often can end up getting off course or not being able to make grade to the intermediate point. When this happens during preliminary work to site a trail, there is nothing for it to do but tear out the flag line and go back and move a previous section of the line so that it avoids the problem location. This might require the beginning point to be moved, a switchback to be added to a previous section of alignment, or an alternate route to be tested. Adjustments such as these are cheaper to make during preliminary trail location than after the trail has developed drainage problems.

Other Essentials

Construction details should specify that drainage relief features, such as rock or log lead-off bars, should be keyed deeply into the soil so that they will not be undercut by piping and erosion. Trails should be outsloped where feasible so that gravity can be used to get water off the trail as sheet flow, before concentrated flow develops. Trails should be placed as near to perpendicular to the cross-slope as possible, and grades should be rolled so that trails do not become streams during intense runoff events. Sections of trail that must cross soft or wet ground should have rock support so that the traveled way does not become a muddy walkaround. When properly done, such structures tend to keep people and livestock on the trail through wet areas because there is no reason to want to step off it. Construction details for rock support should specify that the upper soil horizon is to be overexcavated to create a key of large rock through which groundwater or seasonal runoff can easily pass. The large rock serves as a foundation for a fill of smaller rock for the trail surface above it.

Take a Hike in the Rain Before Designing

A good way to review basic techniques for minimizing trail erosion is to take a hike in a heavy rain. While the cosmic bowling alley crashes overhead and the mountains wring moisture from the clouds, you will surely see rivulets running down steep sections of the trail. But if it is a well-located and well-designed trail, you will notice negative-grade water crossings, grade breaks, rolled grades, rock armoring, lead-off features, and other means by which stormwater is slowed down and directed off the trail surface. These techniques, applied literally every stone’s throw along the trail, allow bare-earth surfaces to remain reasonably stable despite the vicissitudes of weather and heavy use. They are time-honored backwoods practices that achieve what EC practitioners can chant in their sleep: reduce slope length and steepness, increase cover and surface roughness, slow the velocity of moving water, dissipate concentrated flows, provide for sediment settling, design for worst-case rainfall intensity and duration, and by all means get out for a great hike as often as possible. And don’t forget your rain gear. 

About the Author

Martha S. Mitchell

Martha S. Mitchell, CPESC, is principal of ClearWater West Inc. (, consultants in erosion and natural resource planning in Portland, OR.