Estuaries provide critical habitat for species that are valued commercially, recreationally, and culturally. An estuary habitat displays variation, especially in salinity, tides, dissolved oxygen (do), and geomorphology (fig. However, tropical estuaries are usually more stable in time than temperate estuaries. The lower portion of the estuary has a free connection with the open sea and the salinity is usually around 35. The middle part of the estuary is the most dynamic in terms of variability and is where the most mixing occurs. The upper part of the estuary is primarily freshwater and has a salinity around 3. In addition, the tides are still affecting the water levels in the river at this point. Sedimentation plays a large role in the water level of an estuary affecting not only the biota that live there, but the civilizations that depend on the estuary for food. Estuaries are considered ephemeral and young due to being influenced by glaciation within the last 12k-17k years.