Emigrant Wilderness, August 2025

I had hiked 18 miles with a 50-pound backpack. Then, I cached 40 pounds of food and started a little overnight bushwhack loop to visit some new-to-me alpine basins.

It was sunny, hot, and dry.

And I was tired of carrying pack weight, so I looked at the map and poured out my water. I’d tank up at water sources.

An hour later, I reach the first creek. It was dry.

After another hour, the pond that was on the map turned out to be a putrid cesspool of mosquitoes, slime, and a rotting deer carcass. I skipped that and kept moving.

Finally, after three and a half hours and five miles of bushwhacking since I poured out my water, I heard the glorious sound of running water while sidehilling up a steep canyon. I made a beeline to the bottom, dropping 300 feet of elevation just for the chance at some cool liquid refreshment:

man filling up a water bottle in a creek

By the time I arrived at the creek, my heart was racing, my legs were shaking, and my nerves were on edge.

I drank two liters at the creek and packed another two for the 3-hour climb out of the canyon, over a pass, and into the drainage where I was aiming to camp.

This experience had me revising The Water Question we often ask as practitioners of ultralight:

Should I carry water, or tank up at water sources?

That decision comes down to how fast dehydration changes your physiology – and the math isn’t in your favor if you skip drinking for too long.

The Numbers

A 160-pound hiker carries about 44 liters of body water. Losing 1% of body mass as water (i.e., 0.73 liters, about 25 ounces) is the first threshold where performance starts to slip.

How long does it take to lose that much without drinking?

  • Cool/easy pace (~0.3 L/h loss): ~2.5 hours
  • Moderate/warm (~0.6 L/h loss): ~1.2 hours
  • Hot/hard (~1.0 L/h loss): ~45 minutes

Those numbers shorten if you’re smaller, heavier, or sweating more than average.

What Happens at Each Stage

1% loss (~0.7 L): Thirst kicks in. Mental sharpness dips – you may not notice (that’s part of it), but pay attention to your decisions here.

2% loss (~1.5 L): Endurance drops 5–10%. Climbs feel harder, heart rate climbs, heat feels oppressive.

3% loss (~2.2 L): Now you’re degraded – dizziness, slow reaction time, poor judgment.

5% loss (~3.6 L): Crisis. Nausea, confusion, high risk of heat exhaustion or worse.

The Lessons

Dehydration arrives fast. In warm conditions, you can cross the 1% line in under an hour.

Matching intake to loss (roughly 0.4–1.0 L/h, scaled to conditions) keeps you stable. It does pay to carry water if dehydration is a risk and water sources are scarce.

Cognitive decline affects navigation and safety as much as muscle fatigue does – so don’t forget that dehydration isn’t just a physical problem. The body and mind are intertwined and feed off each other. If one starts to fail, the other isn’t too far behind.

The Takeaway

When you weigh the choice between carrying water and tanking up at sources, the physiology leans one way: steady hydration keeps you sharper, safer, and more efficient. The ounces in your bottle may feel heavy, but the cost of skipping them compounds quickly.

So, should you carry water, or tank up at water sources? At least carry enough to keep the needle below 1%.