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Riparian vegetation is known to exert a number of mechanical and hydrologic controls on bank stability. In particular, plant roots provide mechanical reinforcement to a soil matrix due to the different responses of soils and roots to stress. Root reinforcement is largely a function of the strength of the roots crossing potential shear planes, and the number and diameter of such roots. However, previous bank stability models have been constrained by limited field data pertaining to the spatial and temporal variability of root networks within stream banks. In this paper, a method is developed to use root-architecture data to derive parameters required for modeling temporal and spatial changes in root reinforcement. Changes in root numbers over time were assumed to follow a sigmoidal curve, which commonly represents the growth rates of organisms. Regressions for numbers of roots crossing potential shear planes over time showed small variations between species during the juvenile growth phase, but extrapolation led to large variations in root numbers by the time the senescent phase of the sigmoidal growth curve had been reached. In light of potential variability in the field data, the mean number of roots crossing a potential shear plane at each year of tree growth was also calculated using data from all species and an additional sigmoidal regression was run. After 30 years the mean number of roots predicted to cross a 1 m shear plane was 484, compared with species-specific curves whose values ranged from 240 roots for black willow trees to 890 roots for western cottonwood trees. In addition, the effect of spatial variations in rooting density with depth on streambank stability was modeled using the bank stability and toe erosion model (BSTEM). Three root distributions, all approximating the same average root reinforcement (5 kPa) over the top 1 m of the bank profile, were modeled, but with differing vertical distributions (concentrated near surface, non-linear decline with depth, uniform over top meter). It was found that stream-bank F S varied the most when the proportion of the failure plane length to the depth of the rooting zone was greatest. Published in Figure 5. Field data showing the number of roots crossing each wall profile, with extrapolated sigmoidal regressions, and 95% confidence intervals. Sections of the regressions extrapolated beyond the range of field data for individual species are shown with a dotted line instead of a solid line.Figure 6. Mean number of roots crossing the wall-profile for different ages of trees of all species. Error bars in (a) represent maximum and minimum values recorded and those in (b) represent 25th and 75th percentiles of data recorded.Figure 7. (a) Cohesion due to roots at different stages of growth, for riparian species included in the RipRoot model. (b) Variations in streambank F S over time for a 3 m high silt bank, under drawdown conditions, with root reinforcement from each species. F S with no vegetation under the same conditions = 0·99 (unstable).

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