Geologists Were Wrong About Silt Transport by Rivers

New study “muddies the waters”
about sediment transport by rivers,
with important implications

Back in 2007, we reported the beginning of a paradigm-shifting realization about mudstones, the world’s most abundant kind of sediment (2007, 2009, 2019, 2021). Today, another study uses the phrase “new paradigm” in its model of sediments carried by rivers. Silt particles, it turns out, dominate over smaller mud particles—not the other way around.

Background

The mudstone revolution began by observing mud particles (around 2 micrometers in diameter) forming clumps, called floccules or flocs. These flocs can be transported by moving water. This “flocculation” mechanism rapidly speeds up deposition of mudstone, implying that mudstones do not have to settle out slowly in placid seas, as long taught, but can form both in static and dynamic conditions.

Now, a research letter by the American Geophysical Union (AGU) finds flocculation in larger silt particles (<63 micrometers in diameter) that were thought to be too large to form flocs. Lacking the electrophysical properties of mud particles, silt particles were thought to be immune to flocculation. If they indeed flocculate, they could settle out more rapidly than expected.

Flocculated Silt, Not Clay, Dominates River Suspended Sediment Load (Nghiem et al., Geophysical Research Letters, 17 Jan 2025). Sediment transport by rivers is a highly important process. Rivers distribute nutrients, create fertile floodplains, and reinforce coastal shorelines against sea level rise.

Up till now, geophysicists had believed that mud particles were the primary components of suspended load in rivers, because they flocculate. Silt particles were thought to be minor players. Lead author Nghiem and four colleagues flip the script, saying that “flocculated silt, not clay, dominates river suspended sediment load.”

Rivers move large volumes of fine-grained mud across the land surface and build floodplains and deltas over time by the settling and deposition of mud. Flocculation controls these processes by aggregating sediment grains into larger, faster-settling particles. The cohesion of very fine clay mineral sediment has conventionally been thought to cause flocculation, but this idea clashes with observed widespread flocculation of silt, the coarser fraction of mud thought to lack clay minerals. We resolved this paradox using a grain size and mineralogy data compilation of river sediment transported in suspension. Results show that suspended sediment load is mostly flocculated silt with sparse clay minerals, implying that physical trapping of sediment and binding by organics, rather than clay mineral cohesion, cause mud flocculation in rivers. We thus propose a new baseline understanding in which flocculated silt dominates river suspended sediment load.

Meanders on the East River, Colorado. Photo by DFC.

If silt is bound by organics that cause particles to flocculate, this has potentially large implications. Organic material is being distributed by silt flocs, which can settle out more rapidly than smaller mud particles. The organic matter thus transported could revise theories of carbon sequestration by river sediment transport.

Organic-driven flocculation suggests an underappreciated mud-carbon feedback: Mud transport not only governs organic matter flux, but also is affected by organic matter. All else equal, rivers containing more organic matter and clay minerals might sequester more mud and carbon by flocculating more mud and increasing floc settling velocity. High organic matter coating can shield grains from aggregating (i.e., steric stabilization, but such high coverage is rare in natural flocs. The coupling between flocs and organics might also augment the building of fertile floodplains and coastal land offsetting sea level rise. However, the degree to which river flocs maintain their organic matter once they reach the ocean remains unclear.

The paper uses the phrase “new paradigm” four times, as in the following:

Our analysis reveals a new paradigm: Lowland rivers primarily transport flocculated silt. The sparsity of clay minerals suggests that organic matter binding and physical trapping of silt drive flocculation in rivers, not clay mineral charge and salinity as classically assumed.

There is no mention of this process might implicate rates of sedimentary rock formation, but any new paradigm lends itself to new thinking that overturns assumptions.

See also “Experts Were Wrong About Fossil Deposition,” 13 Jan 2026.

We publicize this news item for deep time skeptics to consider. Some potential questions come to mind:

• • Does it affect the rate of sedimentary rock formation?
  • Can flocculated silt particles be transported at higher flow rates?
  • Does flow rate predict fractionation of silt and mud deposition? Could this explain laminations in a mudstone stratum?
  • How does the new paradigm predict the organic content of mudstones?
  • How does the efficiency of carbon sequestration of flocculated silt affect climate models?
  • Does the new paradigm explain the fertility of flood plains and distribution of vital minerals?
  • How does the flocculation process operate in atypical catastrophic flood conditions?

We hope that the new thinking will cause geophysicists to question other assumptions and reconsider rates of deposition under a wide range of conditions. And we hope that textbook writers will take note and update scientific “understanding” of processes that have been “classically assumed” but were wrong.