Tinajas: Water Pockets in Glen Canyon's Sandstone

Hiking in Glen Canyon and find a pool of water in the rock? It's probably a tinaja ("tihn-AH-hah") — also called a waterpocket, tank, ephemeral pool or pothole.

• What: a depression in sandstone bedrock that holds rain and snowmelt, carved — and still being carved — by water, wind and chemical weathering.
• How long: small ones may dry out; larger ones can hold water year-round, and some even support riparian plants.
• Who uses them: amphibians, aquatic invertebrates and mammals; historically, Indigenous communities; and backcountry campers today, who may drink tinaja water once purified.

A tinaja and its plants, in the shade at lower right. NPS photo.

Tinajas in Glen Canyon

Tinajas are the most numerous and widespread surface water in Glen Canyon National Recreation Area away from Lake Powell, especially in the Waterpocket Fold along the border with Capitol Reef National Park. Few studies have looked at them.

Weathering pits are related but different: shallow depressions formed with a larger role for wind, and not in drainages as many tinajas are. The giant pits at Cookie Jar Butte (Lower Lake Powell watershed) may be among the deepest in the world — up to 125 feet wide and 56 feet deep.

Threats

• Climate change: warmer, drier conditions will change how much water tinajas hold and for how long.
• People and cattle: visitors and livestock can foul the water, bring in non-native species, disturb aquatic life and trample plants. Visitors add sunscreen, bug spray and E. coli; livestock add E. coli too.

The assessment

A Natural Resource Condition Assessment (NRCA) under the NPS NRCA Program studied water-related resources away from Lake Powell and the main-stem Colorado River; tinajas were one of five focal resources. With so little known, Utah State University ecologists:

• summarized what is known about tinajas here and at Capitol Reef;
• mapped likely tinaja areas in the Waterpocket Fold from geology, vegetation data and aerial images, marking single tinajas and clusters (more than 10);
• proposed indicators for future monitoring.

What it found

• Most lay on the east side of the fold, in large clusters that looked shallow, with some plants. Many more are probably invisible to the methods used.
• Suggested indicators: water quantity, water quality, community composition and disturbance, plus surveys of vertebrates (such as amphibians) and invertebrates (fairy shrimp, snails, insects).

Clues from Capitol Reef

Studies next door may apply here:

• More than half of 463 tinajas surveyed held plants — 91 species in all.
• Vertebrates: five native amphibians and one non-native fish.
• More than 50 macroinvertebrate species, including fairy shrimp (Streptocephalus texanus), pond snails (Physella spp.) and many insects.

What managers can do

• Map the rest of the park for tinajas.
• Prioritize larger tinajas for monitoring and management: they hold more water longer and matter most in drought.
• Work across the fold: the Waterpocket Fold joins Glen Canyon and Capitol Reef, a chance to manage tinajas together.

How you can help

• Don't trample plants in or around tinajas.
• Keep sunscreen, bug spray, soap and other chemicals out — don't bathe or swim in them.
• Don't splash water out or stir up the sediment: even the microscopic life counts.
• Don't walk through dry tinajas; organisms there may be waiting for water to "come back to life." A Canyonlands video shows what lives in dry and wet potholes.

More: ephemeral pools at Arches, Canyonlands, Hovenweep and Natural Bridges; waterpockets at Capitol Reef.

Sources

Based on "Water in the Rock: Tinajas in Glen Canyon," part of the series NRCA 2022: Condition of Glen Canyon's Tributary Rivers and Associated Resources, National Park Service, summarizing Albright J and others, 2022, Natural resource conditions at Glen Canyon National Recreation Area: Findings & management considerations for selected resources, Natural Resource Report NPS/SCPN/NRR—2022/2374, National Park Service, Fort Collins, Colorado, DOI; a work of the United States government in the public domain.