Smallmouth Bass Below Glen Canyon Dam: Racing to Keep a Predator Out of the Grand Canyon

Decades of climate-driven drought, and a Colorado River asked to supply more water than it carries, have left Lake Powell and Lake Mead low. Beyond the shortages for cities and farms, the threat to hydropower and the loss of almost all Lake Powell's boat ramps, there is a quieter danger: invasive fish slipping through Glen Canyon Dam into the Grand Canyon.

The lower slough at Lees Ferry, Glen Canyon National Recreation Area. Image from the National Park Service

The problem

• Smallmouth bass (Micropterus dolomieu), a popular game fish in Lake Powell, live mostly in its top 46 feet (14 m). As the lake drops, that warm surface water — and its fish — comes close to the power intakes, and fish can be carried through the turbines and survive.
• Below the dam they are unwanted predators. Smallmouth bass are rated the greatest predatory threat in the Colorado River Basin, have helped drive native fish declines across the West, and were implicated in the loss of a humpback chub population in Dinosaur National Monument.
• The Grand Canyon holds the largest remaining populations of humpback chub, recently down-listed from endangered to threatened partly because of how well they are doing there. The river's fish community is now largely native, and nonnative fish, smallmouth bass especially, have been rare.

Figure 1: as the lake falls, the warm surface layer lines up first with the power penstocks, then with the lower bypass tubes. Image from the National Park Service

A smallmouth bass swallowing a juvenile flannelmouth sucker, Yampa River, Colorado. Image from the National Park Service

Why they had not arrived before: releases from the dam rarely rose above 14°C (57°F), below the bass's 16°C minimum for spawning, and while the lake stood above 3,525 feet few fish were drawn through. Pearce Ferry rapid, formed in 2007 where the river meets Lake Mead, seems to block most bass from moving upstream.

What changed: in spring 2022, with the lake at about 3,525 feet, fish began coming through — gizzard and threadfin shad, alive and dead, and unusual numbers of green sunfish and bluegill. The threat was no longer theoretical.

Water temperature at Lees Ferry, 2011–2022. U.S. Geological Survey

The task force

Partners in the Glen Canyon Dam Adaptive Management Program set up a Smallmouth Bass Task Force, led by the U.S. Fish and Wildlife Service, in February 2022, with biologists, hydrologists and ecologists from the National Park Service, the U.S. Geological Survey, the Bureau of Reclamation, the Western Area Power Administration and the Arizona Game and Fish Department. Thanks largely to fast modeling by USGS scientists Charles Yackulic and Drew Eppehimer, it reported by April 2022. Its findings:

• Reversibility: stopping bass before they breed is feasible; once they breed at Lees Ferry, reversal might take many years; established downriver, it would take extreme dam changes over decades; established in tributaries, beyond the dam's reach, it is likely irreversible. So prevention comes first.
• Numbers: somewhere between 9 and 46 adults per kilometer could found a population. The chance of reaching that rises as the lake falls and as fish accumulate over years.
• Conditions: in 2022, water of 16°C or more from June to September was expected from the dam to Lake Mead, with enough slow water for nests, and suitable conditions in tributaries from the Little Colorado River to Kanab Creek and beyond. Around Lees Ferry, 49–124 acres of spawning habitat existed across normal flows.

Smallmouth bass captures below the dam, 2003–2021. U.S. Geological Survey

Figure 6: conceptual probability of establishment at Lees Ferry. Image from the National Park Service

The levers: limiting fish passage and controlling release temperature were the only large-scale tools. Passage risk climbs steeply once the surface is within 50 feet of the penstocks. Options included keeping the lake higher — a 2019 Drought Response Operations Agreement moved an extra half-million acre-feet into Lake Powell in 2022, mostly from Flaming Gorge, and held back another half-million — releasing cold water through the bypass tubes, or building screens or bubble barriers. Modeling pointed to cooler releases, below 16°C, as the most effective single step.

Projected river temperatures, 2022–2023; humpback chub and smallmouth bass share the same optimal temperatures. U.S. Bureau of Reclamation

What happened next

2022. Monitoring expanded, and a citizen-science flyer asked river runners to look for bass. Lees Ferry stayed above 16°C from mid-June through October. Adults turned up below the dam, and 20 fingerlings in a backwater about 3 miles downstream showed local breeding. The fish were removed and the backwater treated with piscicide in September; 37 more fingerlings were then caught outside it. Six electrofishing trips in November–December removed 320 bass — nearly all young of the year — but capture rates of 1% to 60% meant many remained.

2023. Lake Powell bottomed at 3,519.5 feet on April 15, then a big snowpack raised it to 3,582 feet by early July. Yet releases stayed warm into fall, and bass bred in the mainstem and in the slough, despite a block net; the slough was treated again in August. Captures that summer and fall were the highest yet.

Lake Powell surface elevation, 2021–2024. U.S. Geological Survey

2024: "cool-mix." After an Environmental Impact Statement, Reclamation began mixing power-plant releases with carefully measured volumes of cold water (about 10°C) from the bypass tubes on July 9, 2024, aiming for 15.5°C or less at River Mile 61, near the Little Colorado River — a key humpback chub spawning site. It ran until early November. No mainstem spawning was seen in the first 76 miles below the dam. An improved, weekly-checked block net kept bass out of the slough, so no piscicide was needed. The cost: less hydropower, which future operations will try to reduce.

Water temperature at Lees Ferry, 2021–2024. Image from the National Park Service

Still to do

• Keep removing bass before earlier year-classes mature, in case future summers run warm.
• Reshape the slough: the National Park Service is assessing channelizing it to speed and cool the water, restoring the flowing side channel it was when the dam was built.
• Watch other invaders: green sunfish outnumbered bass in removals, and walleye, rated very high-risk and already breeding below the dam, like cool-mix temperatures — likely the next target.

The task force's quick assessment proved accurate: the risk was high and rose as the reservoir fell. Its work, and the rapid-response effort since, show what agencies pooling data and skills can achieve. If temperatures keep spawning in check, the bass should decline to where far less removal is needed.

Sources

Based on "Smallmouth Bass: Not a Small Threat to the Grand Canyon," by Melissa Trammell (National Park Service), Brian Healy (U.S. Geological Survey) and Kirk Young (U.S. Fish and Wildlife Service, retired), National Park Service; a work of the United States government in the public domain. The source puts the bypass tubes both "100 feet" and "120 feet" below the power intakes, so neither is given; misspelled scientific names ("dolomeiu," "lattipinnis," "Onchorhynchus") are corrected.