Unvegetated rivers no longer exist: Plant-landscape interactions in the Mojave and Amargosa rivers, SW USA

Vegetation is a key influence on fluvial processes, but isolating its impacts is challenging when it is one amongst many potential controls. Accordingly, there is geomorphological and sedimentological interest in understanding unvegetated rivers, which allow for the investigation of fluvial systems in isolation from the biogeomorphic controls imposed by plants. Previous studies have assessed unvegetated rivers in two ways: 1) by interpreting alluvial signatures in the ancient sedimentary rock record that pre-date the evolution of land plants; and 2) through geomorphic observations of “unvegetated” modern rivers, where plant coverage is limited or absent at the reach scale. Archetypal “unvegetated” reaches have previously been reported from the catchments of the Amargosa and Mojave rivers in the Great Basin, USA. However, the present study demonstrates that such reaches cannot be considered truly unvegetated, through the combined application of original geomorphic observations and published historical data that reveal a fundamental influence of vegetation at multiple time and length scales. This study catalogs in situ plants, wood matter, and biogeomorphic structures in reaches previously described as “unvegetated”, both before and after flood events, and finds that the abundance of organic matter jams and biogeomorphic structures are comparable to those recorded in other dryland systems. Further, some of the biogeomorphic structures are shown to influence reach-scale morphology by affecting parameters such as floodplain strength and aggradation. These observations are supplemented with those of upstream biogeomorphic structures to account for the role of vegetation in controlling downstream sediment transmission. Combining these observations with evidence of inherited vegetation influences from the recent and deep time, we conclude that neither river adequately reflects unvegetated conditions and should not be considered as such. This conclusion can be generalized to other purported modern “unvegetated” rivers, which are likely subject to the biogeomorphic influences of vegetation 1) at the reach scale, 2) imported from upstream catchment areas, and 3) inherited as landscape memory.