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August 30, 2019

This year marks 150 years since John Wesley Powell and his nine-man crew conducted a scientific expedition of the Green and Colorado rivers. In addition to the goal of mapping the river basin, Powell’s group collected fossils and observed flora and fauna as they journeyed down the river. What did they observe, and what is the focus of USGS ecological studies in the Colorado River Basin now?

In the 19th Century

Imagine a vast wilderness, and the fascination that the people back East had for the exotic wildlife of the frontier.  Physical specimens, either taxidermied or otherwise preserved, were an exciting way to study these species.  

Powell had experience collecting specimens from a young age, when he would explore and learn about natural history through field exploration. As he traveled the Ohio and Mississippi rivers, he amassed a collection of fossils and mollusk shells. Later, when he was a professor, he became the head of the Illinois Natural History Society and curated its collections. One of the objectives of his early field surveys with students was to add to the museum’s collection, as well as to identify and characterize new taxa. Powell’s wife, Emma Dean Powell, served as an ornithologist during his academic field trips and described 175 species of birds.

At that time, it was understood that fossils reveal a history of faunal succession, by which species become extinct and are replaced by new species, and that assemblages of fossils can therefore help us to infer the relative ages of the rock layers where they are found.  During the river expedition of 1869, the crew would collect whatever small, transportable fossils they could find, such as those of trilobites, leaves and fish.  

Through the journals of Powell and crewmembers Jack Sumner and Billy Hawkins, we have descriptions of not only the geology and terrain, but also some of the riparian vegetation and wildlife that they observed during their three months traversing the Colorado River Basin. They referenced making camp near cottonwood groves and hackberry bushes, as well as mesquite in the arid portion of the basin.  While the men observed, with seeming fondness, kingfishers, songbirds and cliff swallows, the majority of their written references to wildlife relate to its potential as a meal for the hungry explorers. They hunted geese, mountain sheep, mule deer, ducks, grouse, beavers and rabbits, and they fished the productive waters of the Green River (and, with less success, the Colorado River) for mackerel, sucker, “Colorado salmon” and whitefish.  Powell noted, while in the upper basin on the Green River, “Mule deer and elk abound, grizzly bears, too, are abundant; wild cats, wolverines and mountain lions are here at home.” In Sumner’s report of their first week on the river, he noted, “Passing by the falls, we leisurely rowed down and camped in about the middle of Brown’s Hole, a fine camp, where we enjoyed ourselves to the limit, fishing, shooting ducks and geologizing.”

As shown by these descriptions, wildlife at that time was largely viewed as a resource to be exploited, with scientific efforts mostly focused on identification and classification of organisms, and it was not until the following century that the more integrated field of ecology developed, leading to such subdisciplines as population dynamics, restoration ecology and ecosystem services.


In the 21st Century

Like Powell’s expedition, this year’s Sesquicentennial Colorado River Exploring Expedition (SCREE), in which the USGS participated, collected data and gathered samples throughout the nearly 1,000-river-mile journey. However, this system has been fundamentally altered since Powell’s time, with three major dams on the portion that he traveled (and many more throughout the main stem and tributaries), hydropower generation, and irrigation infrastructure. Through monitoring, experimental research and modeling, USGS scientists study this ecosystem in order to inform decisions about how to manage natural resources, such as fisheries, riparian vegetation and streamflow.  

One such research topic relates to a fish that would have been abundant during Powell’s time, the humpback chub (Gila cypha).  Now an endangered species, the USGS studies its population dynamics in the Grand Canyon, which is downstream from the Glen Canyon Dam. Declines in this population during the 1990s are believed to be largely due to decreased water temperature (from deep impoundment dam releases), which impacts their reproduction, as well as competition and predation by nonnative salmonid species such as rainbow trout. The humpback chub population grew during the late 2000s, when drought conditions resulted in warmer (shallower) water being released at Glen Canyon Dam, and there were fewer salmonids. The population since then has been stable, and the USGS is using mark-and-recapture studies and population modeling in order to predict how the chub could respond to potential management strategies, such as control of invasive species or flow modification. 

Another area of study relating to altered flow regimes has to do with tamarisk (Tamarix ramosissima), also known as saltcedar. This nonnative, invasive riparian shrub succeeds in areas of low water availability, high salinity and streamflow altered by dams and irrigation infrastructure.  Unlike in Powell’s time, it now lines the Colorado River in many places. Due to its rapid spread and ability to alter its habitat, it displaces native woody plants such as cottonwoods, willows and western honey mesquite. Local, state and federal entities have tried a variety of control methods, including biological control by the northern tamarisk beetle (Diorhabda carinulata). The USGS uses remote sensing techniques to measure tamarisk biomass in areas where the beetle has been introduced and studies ecosystem changes associated with biological control of tamarisk, which will help to prioritize management actions and methods, facilitating restoration of native plant communities. 

River runners and fishermen are assisting the USGS in one of its ecosystems studies in the Grand Canyon. Through Citizen Science Light Trapping in Grand Canyon, USGS has collected far more data on insects that emerge from the river than would otherwise have been possible; these insects are a critical component of the aquatic and terrestrial food webs.  An important finding from this study is that the abundance and diversity of aquatic insects are constrained by hydropower operation at Glen Canyon Dam. The USGS is studying the effects of experimental flow releases on aquatic insects to better understand this relationship and inform decision-making. USGS scientists on the SCREE expedition used the same light trapping technique each evening at camp, so that the expedition could provide additional data to complement this important area of study.

After Powell completed his two scientific expeditions of the Colorado River, he published his findings, enlisted artists and photographers to portray the stunning landscapes, and engaged with the public and policy makers to inform them about western resources. The USGS hopes that the story of this year’s modern expedition, as told through artwork and photography by our partners and the results of the USGS’ sampling and data collection, will likewise inspire curiosity and stewardship in the unique resources of the Colorado River Basin.

scientists holding a test tube with aquatic insects.
USGS scientists collected the adult life stage of emerging aquatic insects each evening of the recent Sesquicentennial Colorado River Exploring Expedition. (Credit: Jaime Delano, USGS. Public domain.)
a view of a humpack chub in the Colorado River from underwater
A Humpback chub in the Colorado River. (Credit: Freshwaters Illustrated, USGS. Public domain.)
Lovely lavender color Tamarisk on the bank of the Colorado River
Tamarisk was introduced from Asia during the 1800s, due to its lovely lavender color, but it now threatens native ecosystems throughout the Colorado River Basin. (Credit: John Parks, USGS. Public domain.)
view of landscape of river with cottonwood galleries in background and tent in foreground
Both Powell’s crew and this year’s modern expedition camped along the river with views of cottonwood galleries. (Credit: Nick Hammond, USGS. Public domain.)



  1. The Great Unknown; The Journals of the HIstoric First Expedition Down the Colorado River, by John Cooley (1988)
  2. Population Dynamics of Endangered Humpback Chub in Grand Canyon, USGS Southwest Biological Science Center
  3. Southwestern Riparian Zones, the Tamarisk Plant, and the Tamarisk Beetle, USGS Southwest Biological Science Center
  4. Kennedy, T.A., Muehlbauer, J.D., Yackulic, C.B., Lytle, D.A., Miller, S.W., Dibble, K.L., Kortenhoeven, E.W., Metcalfe, A.N. and Baxter, C.V., 2016. Flow management for hydropower extirpates aquatic insects, undermining river food webs. BioScience, 66(7), pp.561-575.

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