On this morning, Morgan’s team was employing passive seismic recorders and GPS devices as part of their field campaign. The Horizontal-to-Vertical Spectral Ratio (HVSR) method, a sophisticated but portable geophysical technique, allowed them to measure vibrations in the ground. These subtle signals would help determine how deep beneath their boots the bedrock lay. It was a puzzle piece that most traditional hydrological studies left out, but Morgan hoped it would help explain why some stream reaches vanished in the summer while others continued to trickle steadily through droughts and storms.
A few hundred meters downstream, fellow researcher Logan Flanagan marked a point on his handheld GPS device. He had just passed a section of channel that was dry as bone. Yesterday, following a night of steady rain, that same spot had been flowing energetically. He recalled how, in previous studies, such ephemeral behavior had been largely attributed to topography—water collection in low points, shallow flow paths along steep gradients. But here, among post-glacial soils varying in thickness and composition, topography alone did not always predict the stream’s presence.
Their team’s study focused on small watersheds—10 to 30 hectares in size—a fraction of the sprawling Hubbard Brook valley. They repeatedly mapped these delicate, headwater channels under varying flow conditions throughout the seasons. On some days they would find streams at only 4% of their maximum extent; on others, they could be as high as 70%. The variability was astonishing, and through careful analysis they found clues that the nature of the soils themselves, sculpted by ancient glaciers, played a huge role.
Where the hillslopes had thinner soils and shallower depth-to-bedrock, water percolated quickly, reappearing downstream after a good rain shower. In contrast, deeper soils and thicker layers of glacial till served like a sponge—more stable but also more insulating—leading to less dramatic expansions and contractions in network length. If the bedrock lay nearer to the surface, it often forced groundwater to emerge as a spring-fed seep after rain events, suddenly pushing the stream’s boundaries outward. When drier periods set in, that water would recede as quickly as it had come.
Flanagan and Morgan’s observations held broader implications than simply satisfying scientific curiosity. Headwater streams are ecological lifelines. They host unique communities of insects and microorganisms, supply essential nutrients to downstream ecosystems, and provide habitat for amphibians that rely on intermittent flows. Their patterns of appearance and disappearance shape biogeochemical cycles, influencing how carbon and nitrogen move through the environment. If scientists can better predict which sections of stream will flow when, they can better anticipate shifts in habitat availability, nutrient transport, and overall watershed health as climate conditions change.
The team also recognized the complexity in relating depth-to-bedrock to stream behavior. It wasn’t a straightforward, one-size-fits-all story. In some sections, the correlation was strong, while in others, the patchwork quilt of post-glacial soil and rock told a more nuanced tale. It became clear that while topography provides a broad framework, a deeper understanding requires peering beneath the forest floor into the hidden contours of the Earth itself.
As midday approached, Morgan’s and Flanagan’s team gathered near the forest road, comparing notes. Their boots were caked in mud, and the morning’s dew had soaked through their pant legs. But there was excitement in the air. They were uncovering the secret choreography of water, soil, and stone—an intricate interplay that determines when and where a stream reveals itself. In time, these insights would help refine hydrological models, improve predictions for stream intermittency, and ultimately inform efforts to protect these quiet, yet vital, forest watersheds.
Standing under the tall trees, the scientists felt a rare satisfaction. They were illuminating an unseen world, a realm beneath the surface that holds the key to understanding how ephemeral streams waltz across the landscape. Here, in New Hampshire’s headwaters, the forest whispered of mysteries yet to be fully grasped, and Morgan and his colleagues were listening more closely than ever before.
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The research that inspired this story is part of the American Geophysical Union 2024 Annual Meeting.
Does depth-to-bedrock explain stream network expansion and retraction patterns?
John Cole Morgan (Virginia Polytechnic Institute and State University)
Kevin J. McGuire (Virginia Tech)
John P. Gannon (Virginia Tech)
