Dinosaur fossils on a Colorado reservoir shoreline get buried, moved or lost within a year

Dinosaur fossils on the shore of Blue Mesa Reservoir, Colorado's largest body of water, can be exposed, buried, moved downhill or swallowed by the lake within a single year. Alison Mims, Jack Wood, Michael Bozek and Forest Frost of the National Park Service describe how Curecanti National Recreation Area staff measured that, and what they would change.

Typical rock block with multiple dinosaur fossil fragments. The bones were broken before or during deposition and are now eroding further.

Why the site matters

Most specimens are broken, disarticulated fragments, but the shoreline sites have produced 15 identifiable fossil vertebrate taxa, mostly dinosaurs. The more recognizable genera are Apatosaurus, Allosaurus, Camarasaurus, Ceratosaurus, Diplodocus and Stegosaurus. The specimens help fill a geographic gap in the known distribution of certain Late Jurassic taxa across the mid-continent of North America, and the potential for further important finds is high. But the reservoir's seasonal swings cover and uncover fossil locations in cycles.

An atypical find: an Allosaurus vertebra lying isolated on the beach sediments.

A harsh shoreline

Erosion can expose new fossils, or damage specimens and the site. Reservoir-controlled water levels at Blue Mesa can rise and fall around 92 feet. The sandy, muddy shore sees rain, snow, high winds and many freeze-thaw cycles. Storm waves reach around 3 feet, winter ice several feet thick scrapes the shore or piles up on beaches, and high-water cliffs shed blocks of sediment that carry clay, sand, gravel and rocks downhill. In a year, one fossil on the beach could face open air, an ice layer, flooding, waves, scraping ice and burial.

Staff faced practical questions. Fossils are heavy and hard to move, and using a rock saw on site takes time. If only a tip of fossil shows in a block, how long until erosion reveals whether it is identifiable? If it is left this year, could it be lost forever? One visit can turn up hundreds of fragments, so what to collect, what to leave, and when to return?

One shoreline locality at low water. The high-water mark is at the base of the cliff.

First steps

The team began logging water levels, days a site was flooded, wind, precipitation and temperature. Scuba trips checked whether specimens could be found underwater; that proved hard. A remote camera took daily shoreline pictures. With two NPS divisions, Geologic Resources and Inventory & Monitoring, a pilot monitoring project began in 2016, with two parts.

Small scale: tracking individual fossils

Numerous fossil and rock specimens were marked with metal pins and set along the slope between high water and the water's edge, with baseline GPS positions. Over several years they were relocated by eye or with a metal detector and re-surveyed, and a few were photographed to build 3D photogrammetry models that reveal even tiny losses of bone or matrix. Searches were annual, sometimes twice a year at the lowest water, at the end of April and beginning of November.

Specimens marked with metal pins, ready to be placed along the slope to track yearly movement.

What they found:

• Specimens get buried and some are lost within a year; some were underwater at the time of the visit and others deeply buried.
• The burial was unexpected, because the team had not predicted large sediment loads moving across the site by season. Some specimens lay under several inches of sediment and some could not be found.
• Recovered specimens had moved downslope, up to several feet between spring and fall.

Left: marked specimens placed on the shoreline sediments in April. Right: the same specimens in November, moved downslope and buried under several inches of sediment.

What they would improve: mark each specimen uniquely, for example with numbered pins, since fragments are hard to tell apart from photos and descriptions; try other metal detectors or Passive Integrated Transponder (PIT) tags for deeply buried pieces; weigh specimens, which may beat time-consuming 3D photogrammetry for spotting direct damage; and search at least yearly.

Large scale: tracking the site

Photogrammetry of two shoreline sites, covering roughly 20 feet by 20 feet, was done in 2016 and again in 2019. Software compared the ground-surface point clouds for the two years and colored the changes. Repeat photogrammetry detected sediment build-up and erosion to a centimeter scale, plus block movement and widening cracks.

Heat map of one locality from images taken in 2016 and 2019. Yellow to red areas gained material; green to blue areas eroded.

What they would improve: use trained staff with better GPS and photogrammetry tools instead of doing it themselves, monitor on a 5- to 10-year schedule, and consider terrestrial lidar for broader shoreline change.

What it means

Important specimens can become undiscoverable between April and November from direct damage, burial by slope collapse or loss into deep water. The staff say they will need to spend more time on the beach searching for and rescuing fossils.

Sources

• National Park Service, Curecanti National Recreation Area, "Survival is Still Tough for Dinosaurs at Curecanti National Recreation Area" by Alison Mims, Jack Wood, Michael Bozek and Forest Frost, Park Paleontology News, Spring 2021, https://www.nps.gov/articles/000/curecanti-dinosaur-fossil.htm; rewritten in hubnx's own words.