Streambank erosion is a significant environmental problem that degrades cultivable land and increases sediment pollution in waterways. Previous research has shown how various aspects of plant anatomy (i.e. aboveground parts and roots) influence soil erosion. However, few studies have considered how naturally diverse assemblages of plants and interspecific interactions among species influence soil erosion. Therefore, we performed a greenhouse experiment in streambank mesocosms designed to quantify how plant species richness (monoculture - six species polycultures of common herbaceous species), interspecific interactions (facilitation and competition), and root traits (root length, biomass, and volume) control soil erosion rate (mm/min) and bank strength (Kg/cm2). Preliminary results suggest that both root length and biomass decreased as plant species richness in mesocosms increased (P < 0.10). In turn, decreasing root length and biomass led to significant declines in bank strength on the order of 0.07 Kg/cm² per additional 1 mm of root length (P = 0.01). These results suggest that multi-species assemblages had higher erosion and lower bank strength, perhaps because the growth of belowground root structures was inhibited by interspecific competition in the multi-species assemblages. To better understand the nature of these interactions, we are in the process of comparing relative yields of each species and relating those to erosion rate and bank strength. Our study highlights that soil erosion and bank stability differ between single vs. multi-species communities, which serve to highlight the need to study more realistic plant assemblages when considering streambank erosion and sediment loads to streams.