To describe a fictional creek for a story, consider its role in the story. If it's a place of peace, you can say that the air around the creek is always calm and the water is so still it reflects the surrounding trees like a mirror. The trees around it could be ancient and their branches reach out over the creek, creating dappled shadows on the water. There might be a hidden path leading to the creek, and only those who truly seek tranquility can find it. This creek could be a place where characters go to think, to find inspiration, or to heal from their wounds.
You can start by picturing its appearance. Maybe it has crystal - clear water that sparkles in the sunlight. The banks could be lined with moss - covered stones and wildflowers of all colors. There might be a small wooden bridge crossing over it, with vines hanging down from the sides.
Well, think about the sounds. A fictional creek for a story could have a gentle babbling sound, like a soft conversation. You could mention how the water gurgles as it flows over pebbles. Also, describe the wildlife around it. For example, there could be colorful fish swimming in the deeper parts and little frogs hopping on the banks.