Mobile Bay Has More Branching Brooks Than Shown on Federal Maps

Headwater streams are usually the narrowest parts of river networks, but they make up the majority of those networks’ length. Land managers strive to protect these waters to keep the river’s entire drainage area pristine.

A large national database of watersheds created by the U.S. Geological Survey (USGS) is an essential reference to understanding these upstream waters. Some hydrologists, however, have wondered how well those maps capture the narrow channels of headwater streams.

Researchers have now figured out how much is overlooked in the flat, swampy Mobile Bay watershed, revealing that the stream network is 2.4 times longer than shown in the USGS database. That finding could transform efforts to protect Alabama’s waterways throughout their length. The team presented their results on 9 December at AGU’s Annual Meeting 2024 in Washington, D.C.

From Trickles to Torrents

“There’s the field approach, and then there’s the modeling approach,” said Ph.D. student Delaney Peterson of the University of Alabama, who led the study. “And that’s not to say you can’t marry the two. It’s really effective to use both.” Peterson and her colleagues used foresters’ field maps of streams that drain to Mobile Bay to model the starting points of creeks throughout the watershed.

As part of normal logging and land management operations, foresters trek into the field to document where creeks flow. They also take notes on how much and how often water flows through the stream. A forestry company shared 5,800 of these maps from the Mobile Bay watershed with the researchers, who used them to predict where streams start throughout the drainage area.

This is a “pretty unprecedented field-mapped component,” Peterson said. “And we’re trying to see if we can use that to get at larger-scale patterns.”

From the field data, the researchers looked at each point where a creek began and used the slope of the surrounding hillsides to calculate the total area draining to that point. They found that on average, the head of a channel in the Mobile Bay watershed forms when it collects water from 12 acres (5 hectares) of land, the size of nine U.S. football fields.

The USGS map, which covers watersheds in the contiguous United States, does not capture as many fine details as the field maps and thus misses streams higher in the watershed. With its lower resolution, it shows the heads of creeks beginning where water collects from 91 acres (37 hectares) of land on average. That’s an area more than 5 times larger than the grounds surrounding the White House.

But the field maps did not cover the entire area that drains to Mobile Bay, researchers found. They scaled up their findings by modeling where stream heads would be located if headwater streams began everywhere that had an upstream drainage area of 12 acres (5 hectares), rather than the roughly 91 acres (37 hectares) observed on the national map.

The finer-scale mapping and modeling showed the watershed contains 2.4 times more kilometers of creeks than what appears on the USGS reference map.

Protecting Drainages

That more detailed look at the upper reaches of rivers could help foresters protect them from erosion, Peterson said.

Careless clear-cutting can trigger landslides because logging machinery and felled trees loosen topsoil. Just one afternoon thunderstorm could send sheets of sediment sliding into the nearest stream. The U.S. Forest Service tries to prevent this erosion by creating areas around streams that they suggest loggers leave untouched. Peterson’s research will help land managers find the tiny creeks that cross commercial forests.

“The way that we manage our forests is directly centered around our maps of stream networks,” said research hydrologist Adam Price of the U.S. Forest Service in La Grande, Ore., who was not involved in the research. “If we extend our stream networks into these headwater areas, we’re extending the amount of buffers,” Price said.

Peterson and her colleagues are now applying their methodology across a bigger region in the southeastern United States. The team has 60,000 maps of waterways in the area, capturing terrain that ranges from flat, sandy islands to the rugged Ouachita Mountains in Arkansas.

They are also extending their analysis beyond simply looking at drainage area to explore how curved valleys, steep slopes, and other variables dictate the origin of streams.

—Mark DeGraff, Science Writer

Citation: DeGraff, M. (2024), Mobile Bay has more branching brooks than shown on federal maps, Eos, 105, https://doi.org/10.1029/2024EO240575. Published on 16 December 2024.

Text © 2024. The authors. CC BY-NC-ND 3.0
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