Fluvial geomorphology, root distribution, and tensile strength of the invasive giant reed, Arundo donax, and its role on stream bank stability in the Santa Clara River, southern California

Authors: STOVER, JIANA - Lyon College; KELLER, EDWARD - University Of Southern California; DUDLEY, TOM - University Of Southern California; Langendoen, Eddy

Submitted to: Geosciences
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 8/7/2018
Publication Date: 8/14/2018
Publication URL: https://handle.nal.usda.gov/10113/6473319
Citation: Stover, J., Keller, E.A., Dudley, T.L., Langendoen, E.J. 2018. Fluvial geomorphology, root distribution, and tensile strength of the invasive giant reed, Arundo donax, and its role on stream bank stability in the Santa Clara River, southern California. Geosciences. 8(8):304. https://doi.org/10.3390/geosciences8080304.
DOI: https://doi.org/10.3390/geosciences8080304

Interpretive Summary: Invasive vegetation may out-compete native vegetation along channel margins, thereby significantly changing the morphologic dynamics of stream systems, and as a result adversely impact in-stream and terrestrial habitats. Researchers from USDA, ARS, Oxford, MS, in collaboration with scientists from the University of California, Santa Barbara, compared the effects of the invasive giant reed (Arundo donax) on the stability of river banks along the Santa Clara River, CA, with those of native red willow (Salix laevigata) between 1981 and 2009. Over the study period the amount of A. donax increased about 1 to 2 ha per year. The roots of S. laevigata were more numerous and exhibited greater strength. The study results indicate that lateral stability of the Santa Clara River will be reduced when A. donax becomes dominant. Study outcomes provide State of California floodplain managers insight of expected changes in channel morphology of southern California streams at areas with increasing populations of A. donax.

Technical Abstract: Arundo donax (giant reed) is a large, perennial grass that invades semi-arid riparian systems where it competes with native vegetation and modifies channel geomorphology. For the Santa Clara River, CA, active channel width and intensity of braiding varied over several decades with width and braiding generally increasing during high flow events that removes A. donax. Nevertheless, over several years from 1981-2009, the amount of A. donax at the study sites increased at ~1 - 2 ha/yr. Effects of A. donax on bank stability are compared to those of a common native riparian tree, Salix laevigata (red willow) at two sites on the banks and floodplain of the Santa Clara River. There is a significant difference of root density of A. donax compared to S. laevigata, and the latter has a higher number of roots per unit area at nearly all depths of the soil profile. Salix laevigata, for similar sizes, has a greater number of roots distributed throughout the bank. Tensile root strength for S. laevigata (for roots of 1- 6 mm in diameter) is about five times stronger than for A. donax and adds twice the apparent cohesion to weakly cohesive bank materials than does A. donax (8.6 kPa compared to 3.3 kPa, respectively). Modeling of bank stability for banks of variable height suggests that S. laevigata, as compared to A. donax, increases the factor of safety (FS) by ~ 60% for banks 1 m high, ~ 55% for banks 2 m high, and ~ 40% for banks 3 m high. For 3 m high banks, the FS for banks with A. donax is < 1. This has geomorphic significance because, in the case of A. donax growing near the water line of alluvial banks, the upper 10-20 cm has a hard, resistant near-surface layer overlying more erodible banks just below the near-surface rhizomal layer. Such banks may be easily undercut during high flow events, resulting in overhanging blocks of soil and A. donax that slump and collapse into the active channel, facilitating lateral bank erosion. Therefore, there is a decrease in the lateral stability of channels if the mixed riparian forest is converted to dominance by A. donax.