Floodplain water quality is naturally regulated by river floods, but is increasingly affected by anthropogenic activities. Accordingly, we evaluated the combined influence of river flooding (pre- and post-flood periods) and rotational grazing (cattle presence or absence) on water quality of floodplain lakes. We hypothesised that river floods decrease the concentrations of most water constituents and dilute grazing effects on water quality, and that differences in cattle pressure (i.e. grazing duration) promote differences in water quality among lakes. Nutrient concentration decreased after the flood through washing-out and dilution, but other water constituents showed different responses. Contrary to expectations, water-quality changes in relation to cattle presence or absence occurred only after the flood, with higher turbidity and phosphorus during periods with than without cattle. Finally, water-quality variability among lakes was attributed to their different sizes (and therefore dilution capacities), but not to differences in cattle pressure. Our results suggest that river–floodplain connectivity and grazing location in areas with large lakes are important for preserving water quality. Pre-flood changes in water quality in relation to cattle presence or absence could have been obscured by wind-driven sediment resuspension, which has important effects during shallow-water stages, and by high phosphorus concentration, which can prevent removal during periods without cattle.

洪泛区的水质自然受到河流洪水的调节，但越来越多地受到人为活动的影响。因此，我们评估了河流洪水（洪水前后）和轮牧（牛有无）对洪泛区湖泊水质的综合影响。我们假设河流洪水降低了大多数水成分的浓度，并减轻了放牧对水质的影响，牛压的差异（即放牧时间）促进了湖泊之间水质的差异。洪水后，营养素的浓度通过冲洗和稀释而降低，但其他水成分表现出不同的响应。与预期相反，与水牛存在与否有关的水质变化仅在洪水发生后才发生，与没有养牛的时期相比，其浊度和磷含量更高。最后，湖泊之间水质的变化归因于它们的大小（以及稀释能力）的不同，而不是牛压力的差异。我们的结果表明，大湖泊地区的河水平原连通性和放牧位置对于保持水质很重要。风驱沉积物的再悬浮（在浅水阶段具有重要作用）和高磷浓度（可以防止无牛时期的清除）可以消除与牛存在或不存在有关的洪水前水质变化。但不是为了牛的压力差异。我们的结果表明，大湖泊地区的河水平原连通性和放牧位置对于保持水质很重要。风驱沉积物的再悬浮（在浅水阶段具有重要作用）和高磷浓度（可以防止无牛时期的清除）可以消除与牛存在或不存在有关的洪水前水质变化。但不是为了牛的压力差异。我们的结果表明，大湖泊地区河流与平原之间的连通性和放牧位置对于保持水质很重要。风驱沉积物的再悬浮（在浅水阶段具有重要作用）和高磷浓度（可以防止无牛时期的清除）可以消除与牛存在或不存在有关的洪水前水质变化。