Terrain Reading

Part of Emergency Shelter

Understanding drainage patterns, wind behavior, and natural hazards lets you read the landscape like a map — and pick shelter sites that won’t try to kill you overnight.

Reading Drainage

Water flows downhill. That sounds obvious, but in an emergency people routinely build shelters in places where water will collect, channel, or pool. Learning to read drainage patterns before the rain starts is a survival skill.

How Water Moves Through Terrain

• Ridgelines and hilltops shed water in all directions. Good for drainage, bad for wind exposure.
• Slopes channel water downhill. Steeper slopes move water faster. Look for rills, small gullies, or lines of eroded soil — these are the paths water already takes.
• Valley floors and depressions collect water from all surrounding slopes. These flood first and drain last.
• Saddles (low points between two high points) can channel wind and water. Sometimes useful as sheltered passes, but avoid camping in the lowest point.

Reading Drainage Indicators

Even in dry conditions, the landscape tells you where water goes:

Indicator	What It Means
Dark, rich soil in a line or depression	Water channels here regularly — organic material collects in drainage paths
Gravel or sand deposits in a pattern	Fast-moving water sorted and deposited material here during floods
Debris lines on tree trunks or rocks	High-water mark — water reached this level before and will again
Green vegetation in an otherwise dry area	Subsurface water or drainage concentration — wet ground below
Erosion cuts in soil or exposed roots	Active water movement during rain events
Smooth, water-worn rocks	Persistent water flow, even if the channel is currently dry
Moss or algae on one side of rocks	Indicates persistent moisture; that side stays wet longer

The Stick Test

If you are unsure about slope direction (subtle grades are hard to see by eye), place a round stick or water bottle on the ground and watch which way it rolls. Do this in several directions to map the slope. Water will follow the same path.

The Dig Test

Dig a small hole 15-20 cm deep at your potential site. Wait 10 minutes. If water seeps in, the water table is too high for a ground-level shelter. Move upslope or pick a different area entirely.

Reading Wind

Wind determines how fast your body loses heat. Understanding local wind patterns helps you position your shelter to block it or use it.

Prevailing Wind Direction

Most regions have a dominant wind direction. You may not know what it is, but the landscape records it:

• Tree lean — Trees grow leaning away from prevailing wind. One-sided canopies (flag trees) point away from the dominant wind direction.
• Snow or sand drifts — Accumulate on the leeward (sheltered) side. The wind blows from the opposite direction.
• Erosion patterns — Windward sides of hills and ridges show more weathering and less vegetation.
• Branch growth — Branches and foliage are denser on the leeward side of trees.

Local Wind Effects

Beyond prevailing wind, terrain creates predictable local wind patterns:

Valley Winds (Anabatic and Katabatic)

• During the day, sun heats valley slopes. Warm air rises, creating an upslope breeze (anabatic wind). Valley floors are relatively calm during the day.
• At night, slopes cool rapidly. Cold, dense air sinks downslope into valleys (katabatic wind). Valley floors and low areas become the coldest spots — often 5-10 degrees C colder than slopes above.

This is why you should avoid valley floors for overnight shelter. The ideal overnight position is one-third to halfway up a slope — above the cold air pool, below the windswept ridge.

Funnel Effects

Narrow gaps between hills, cliff walls, or rock formations squeeze wind through at higher speeds. A gentle breeze on open ground can become a strong gust through a gap. Avoid building in natural wind tunnels — notches, narrow valleys, or gaps between large boulders.

Eddies and Turbulence

On the leeward side of large obstacles (buildings, rock walls, dense tree stands), wind creates turbulent eddies. The area directly behind the obstacle may feel calm, but just beyond it the wind swirls unpredictably. For reliable wind protection, build close to the windbreak — within a distance roughly equal to the windbreak’s height.

Wind Speed Estimation

You will not have instruments. Use this rough guide:

Observation	Estimated Wind Speed
Smoke rises vertically	Calm (< 2 km/h)
Smoke drifts, leaves barely move	Light (2-10 km/h)
Leaves and small twigs move constantly	Moderate (10-25 km/h)
Small branches sway, dust and loose paper lift	Fresh (25-40 km/h)
Large branches sway, whistling in wires/gaps	Strong (40-60 km/h)
Whole trees sway, resistance walking into wind	Gale (60+ km/h) — seek immediate solid shelter

Reading Hazards

Overhead Hazards

The number one overlooked shelter hazard. In a forest, dead branches fall regularly — wind, rain, ice, and simple decay bring them down. This is not rare; it is constant.

How to check:

1. Stand at your proposed site and look directly up. Scan every tree within falling distance (roughly 1.5x the tree height).
2. Look for dead branches — no leaves (in deciduous trees), bark peeling off, lighter color than living wood, broken ends hanging.
3. Look for leaning trees — if a tree leans significantly and has a shallow root plate or soil heaving at the base, it may fall.
4. Check for hung-up trees — one tree caught in another’s branches. These are unstable and can drop without warning.
5. After storms or high winds, hazards increase. Re-check even established sites.

Warning

“Widow maker” is not a colorful nickname. Falling branches kill people in forests regularly. Never sleep under dead wood. Period.

Ground-Level Hazards

• Ant mounds and wasp nests — Fire ants, wasps, and hornets nest in the ground, in logs, and in trees. Disturbing them at night in an enclosed shelter is dangerous. Scout for insect activity before building.
• Snake habitat — Rock piles, fallen logs with gaps underneath, and thick brush are common snake shelter. If you are building near these features, check thoroughly and make noise as you work. In snake territory, build a raised sleeping platform if possible.
• Poison plants — Know the dangerous plants in your region. Poison ivy, poison oak, and stinging nettles as shelter-building material will make you miserable. Learn to identify them: “leaves of three, let it be” covers many (but not all) of the worst offenders.
• Standing dead trees (snags) — Even if a dead tree has no branches overhead, it can fall in its entirety during wind. Give dead standing trees a wide berth — at least 1.5x their height.

Terrain Instability

• Undercut banks — River and stream banks that are undercut can collapse, especially when wet. Never build on or under an undercut bank.
• Loose scree slopes — Talus and scree (loose rock on steep slopes) can slide. Do not build at the base of a scree field.
• Saturated hillsides — After heavy rain, steep hillsides with clay soil can fail as mudslides. Look for cracked soil, tilted trees, or fresh exposed earth — signs of recent or imminent movement.
• Coastal zones — Tide lines, storm surge zones, and unstable dunes all change rapidly. Build well above the highest visible tide line and watermark.

Putting It All Together

When you arrive at a potential shelter area, spend 15 minutes doing a terrain read:

1. Identify the high ground — Where are the ridges, rises, and elevated platforms? These are your starting candidates.
2. Map the drainage — Where does water go? Follow the low lines, gullies, and dark soil. Eliminate any site in or near a drainage path.
3. Feel the wind — Which direction? How strong? Find the leeward side of a natural windbreak.
4. Scan for hazards — Look up (dead wood), look around (animal signs, unstable ground), look down (insect nests, wet soil).
5. Choose the best compromise — The perfect site rarely exists. Pick the spot that scores best across all factors, prioritizing safety over comfort.

Key Takeaways

• Water always follows the same paths — read the indicators (dark soil, gravel deposits, debris lines, green strips) to know where those paths are before rain arrives.
• Cold air sinks at night into valleys and low areas, making them 5-10 degrees C colder than mid-slope positions. Build one-third of the way up, not at the bottom.
• Wind is predictable: prevailing direction shows in tree lean and erosion patterns; local effects (katabatic flow, funneling) follow terrain features.
• Always look up. Dead branches, leaning trees, and hung-up trees are the most common and most overlooked shelter-site hazard.
• Spend 15 minutes reading terrain before committing to a site. That investment prevents overnight disasters.