Droughts are increasingly affecting water quality in streams, thereby adding to water shortages. However, we still do not fully understand the processes driving these changes. Here, we looked at how a stream in a forested area in Switzerland responded to drought, using data on ten different dissolved substances in the water. During the dry summer of 2018, stream flow and groundwater levels kept dropping, but the stream never completely dried up. Less rain turned into stream flow than usual, showing that the area's water storages were getting used up. The relationship between stream flow and dissolved substances changed from normal conditions. Most substances showed less variation in concentration, except for sulfate. This change can be explained by the stream being fed only by old groundwater, while shallower water sources, like soil water, were cut off from the stream. Other biological and chemical processes also affected the availability and mobilization of different substances, such as changes in oxygen levels and nutrient use. Overall, we showed that understanding how drought affects water quality needs more than just looking at stream flow. We must also consider both water dynamics as well as biological and chemical processes to find the root causes.