The Silurian-age shift in fluvial stratigraphic architecture is associated with the predominance of shallow pancontinental braidplains in the vegetation-free, pre-Silurian period. Recognition of deep, single-threaded channels in pre-Silurian strata challenge this paradigm; however, it is unclear how these rivers sustained stable banks. Here, we combine measurements of fluvial cross-strata from the 1.2 Ga Stoer Group with quantitative paleohydraulic tools to reconstruct river geometry and planform. We show that the deposits are consistent with deep (4-7 m), low-sloping rivers (5.6x10-4 to 4.5x10-5), similar in morphometry to modern single-threaded rivers. We show that mud in floodplain sediments provides sufficient resistance to erosion to withstand the reconstructed bank shear stress, indicating that sediment cohesion from mud alone can foster deep pre-Silurian rivers. We suggest that the apparent stratigraphic shift coeval with the greening of continents marks a kinematic change in channel migration rates rather than a geometric shift in river planform.

Primary data (data sets 1 and 2) was collected by authors at Stoer Group outcrops across NW Scotland. Additional data (data set 3) used in comparative analysis is from Galeazzi et al. (2021). Hydraulic reconstructions were conducted using general purpose calculating functions in Microsoft Excel and MATLAB. All formulas are defined and all calculations are described in the main text and supporting information, therefore no unique/novel code or scripts are required to reproduce these results.

All data is held in Microsoft Excel files, and can be opened in any spreadsheet software.