Dynamics of a vegetated mixing layer around a patch of submerged plants was examined theoretically and experimentally in a lowland river. Theoretical analysis explored analogy with hydrodynamic mixing layers and introduced equations that describe expansion of the layers and alterations in mean flow. Field experiments provided empirical data for validation of the theory and examination of the effects of population density on the mixing layer dynamics. Detailed measurements assessed the structure of turbulent flow, plant morphology, biomechanical properties, bending, and streamlining due to interaction with the flow. The theory was compared with the field experiments and an agreement was concluded. It was found that near the upstream edge of the patch, the vegetated mixing layer expands similar to the canonical mixing layers, although its statistics scale on the velocity differential characteristic for the stabilized part. Downstream mean momentum flux reduces in magnitude and reverses in direction, while turbulence became dominant. This was observed to stabilize the velocity differential and increase the mean velocity of the layer that result in its limited growth described by a logarithmic function. The population density of vegetation was found to control the flow. In dense vegetation, the analogy with mixing layers resembled best, while in sparse vegetation, the flow behaved similar to the boundary layers. This theory can be used for quantitative assessment of seasonal effects in natural vegetated rivers because the population density of vegetation varies during the vegetation season. The companion paper focuses on the structural properties of turbulence in the vegetated shear layers.