An ongoing disagreement in the fluvial geomorphology community deals with the impact forested riparian areas have on channel morphology. Despite the large amount of published studies on the benefits of riparian forests such as bank stability and reducing soil loss, some researchers suggest that reforesting floodplains increases the inputs of large wood (LW), which ultimately causes erosion, can decrease sediment storage, as well as promote channel widening. The majority of the research on the influence of riparian forests and LW on channel morphology has occurred on the coasts of the United States where streams are often steep and rough, the substrate is rock or coarse sediment, and the riparian areas are dominated by coniferous forests. The statistical power of the current research is also limited by small numbers of study reaches and watersheds and often focuses on smaller watersheds.

The purpose of this research was to explore the relationship between riparian vegetation and stream morphology in Iowa by amassing a large geospatial dataset characterizing channel width and riparian land cover across diverse physiographic regions of the state. To investigate whether streams running through forest were wider than streams running through riparian areas dominated by grass, pasture, or row-crops, a statistical relationship between riparian vegetation height and along-channel deviations in channel width was obtained using the Iowa LiDAR product for each. The results indicate that wider stream reaches are more likely to be surrounded by forest than narrow reaches, consistent with our hypothesis. Analysis of field observations from 10 forested stream reaches around the state was unable to clearly identify the cause of this relationship, but suggests that reaches with log jams are wider than those without. These results imply that conversion of riparian vegetation to forest could lead to channel widening, consistent with some previous studies. The field observations further highlight the need to better understand the role of recruitment and persistence of large wood and log jams in the hydraulic geometry of alluvial channels.