Light can be a key driver of diatom assemblages, influencing biomass and diatom assemblage structure. Turbid rivers in aridland regions, like the southwestern US and Australia, often carry high sediment loads associated with tributary inputs that result in light-limited reaches. Algal communities grow in compressed habitats along shallow sandbars and bank edges (“bathtub ring”) with sufficient light availability to support photosynthesis. The goal of this ongoing research is to quantify the bathtub ring in terms of diatom diversity and structure in river systems that have in-stream light limitation co-limited by additional environmental drivers. To map this bathtub ring, we have sampled algal communities laterally along with environmental parameters (e.g., velocity, depth, turbidity, and hydraulics) in the Rio Grande, central New Mexico, USA. Initial work in shallow, sandy bottom reaches with moderate turbidity indicated that there were thresholds to growth at 18 cm water depth and 0.15 m/s flow velocity. Our additional research has shown that these thresholds are more dynamic in reaches with more complex geomorphology and heterogeneous substrates. Efforts to understand bathtub ring dynamics has led to modeling relationships among turbidity, depth, and light attenuation to predict the bathtub ring spatially and temporally. In recent summers with less frequent monsoonal events and more water withdrawals, lower flows of less turbid water have resulted in a much wider bathtub ring, sometimes spanning the entire width of the river. Additionally, this wider bathtub ring was composed of a greater proportion of green algae versus low-light adapted diatoms. We predict that wide bathtub rings related to increased water withdrawals and a warming climate will become more common with unknown consequences to the food web structure in the Rio Grande.