We used a computer model to estimate how these seasonal streams add water to the outlet of a watershed in Kentucky, USA. We placed sensors in the streams to track when and where they flow, and we compared the model’s predictions to these measurements to make sure it was accurate. We found that the smallest streams delivered about 30% of the water leaving the watershed, and overall the headwater streams contributed roughly half of the flow downstream. We also saw their contributions change with the weather: during rainstorms and wetter periods, these streams provided even more water. This shows that even streams that don’t flow year-round are crucial for downstream rivers. Including these streams in watershed models and management plans can lead to better water protection policies.