A River of Change: The Rio Grande in the Big Bend Region

In the heart of the Chihuahuan Desert, the Rio Grande — the Río Bravo in Mexico — forms the border between the United States and Mexico. The Big Bend is named for the great northward bend the river makes before running southeast to the Gulf of Mexico. Both countries protect the region, yet the river itself has been substantially altered.

Changes in its hydrology — the seasonality, size, duration and variability of its flow — have physically transformed the river, costing habitat for native and endangered fish and raising flood risk. USGS scientists, with other agencies, universities and non-governmental organizations, are working to understand these changes and inform management.

The Rio Grande about 20 kilometers downstream from Castolon, Texas, and Santa Elena, Chihuahua, looking upstream. George Grant, National Park Service, 1936.

Two floods a year, once

In the late 1800s, flow through Big Bend was dominated by two flood pulses of roughly equal size:

Together they kept high water in the Big Bend reach for nearly six months a year.

What changed

• Diversions from the northern branch, upstream of Presidio, Texas / Ojinaga, Chihuahua, rose sharply in the late 1870s.
• Large dams went up on the Rio Grande and the Río Conchos in the early and mid-1900s.
• Dams, their management, and growing farm and consumptive water use cut both total flow and flood size — most sharply in the 1940s, as a regional drought began. That era of low flow and small floods is when the channel and floodplain began to change.

A narrower river

Where not hemmed in by canyons, the late-1800s river was wide, split into multiple channels, and apt to jump course in big floods.

• Why it narrowed: dry-wash tributaries dump large loads of sediment into the river in flash floods. Without big, long floods to carry it away, the sediment settles in the channel.
• Plants lock it in: vegetation grows on the unscoured deposits and traps more sediment, and over time they become floodplain.
• How much: in the wide valleys, channel width has shrunk by about 50 percent on average since the 1940s. The river went from wide, shifting and multi-channeled with sparse streamside plants to one stable channel lined with dense vegetation.

The river's channel near Castolon, Texas (Big Bend National Park) and Santa Elena, Chihuahua (Cañon de Santa Elena Protected Area), traced on historical aerial photographs from 1941 to 2019. Base: 2019 National Agriculture Imagery Program image. USGS.

Why it matters:

1. Fish habitat lost: the wide, slow-water parts of the channel filled with sediment. Those low-velocity areas are where native and endangered fish rear and grow.
2. More flooding: the narrow river can no longer carry floodwater without spilling over, so floods hit riverside communities and historic sites more often.

Channel-reset floods

Occasional huge floods — channel-reset floods — scour the channel and briefly undo the narrowing.

• Cause: heavy rain from tropical storms and hurricanes over the Río Conchos basin fills upstream reservoirs and forces big dam releases.
• 2008: a channel-reset flood enlarged the channel by 26–52 percent in the wide valleys, but narrowing soon resumed.
• Then and now: in the early 1900s, floods that size came about once every four years, keeping the channel much wider. Today they come once every decade or two.

Hot Springs Canyon, Big Bend National Park, from one spot: (A) 1901, H.C. Oberholser; (B) before the 2008 channel-reset flood, with stands of non-native saltcedar and giant cane; (C) after it. B and C by David J. Dean, USGS.

Managing sediment and plants

Managers in both countries are looking for ways to slow narrowing, habitat loss and flood risk between channel-reset floods.

• Surveys: USGS and National Park Service scientists, with partners, regularly re-measure channel and floodplain topography — how fast and how much they change, and whether wide valleys and canyons differ.
• Sediment gauging: USGS continuously measures sediment moving in the river to see how much is eroded or deposited in floods (data).
• What the data show: large dam releases lasting many weeks or more can erode sediment and create new aquatic habitat — but floods much larger than typical releases are needed to fully offset what builds up.
• Removing invaders: the NPS, the State of Texas and non-governmental organizations are clearing non-native giant cane (Arundo donax) and saltcedar (Tamarix spp.) to raise native plant diversity and let banks erode in floods.
• Biocontrol: the arundo wasp (Tetramesa romana), a parasite of giant cane, and the tamarisk beetle (Diorhabda spp.), which eats saltcedar leaves, have been released.

The long-term physical and ecological results are still being evaluated.

Looking upstream in Boquillas Canyon. David J. Dean, USGS.

Sources

Based on A River of Change—The Rio Grande in the Big Bend Region, by David J. Dean, U.S. Geological Survey Fact Sheet 2021–3036 (DOI; publication page), prepared with the National Park Service, Utah State University, Sul Ross State University, World Wildlife Fund, Alpine Test Services, Rio Grande Scientific Support Services and RiversEdge West. Photographs and map from the fact sheet's PDF; the 1901 photograph is by H.C. Oberholser. The fact sheet's location maps, drawn on a commercial basemap, are left out. A work of the United States government in the public domain.