How long whales live in fresh water has become a matter of some debate with several visits of whales to fresh water environments. The interest in this matter first peaked when Humphrey the humpback whale, took a detour into the Sacramento River in 1985 during annual whale migration season. Humphrey managed several weeks in the brackish water (fresh water) of the Sacramento River before finally returning to the San Francisco Bay and then to the Pacific Ocean.

The matter of how long whales in fresh water can live was revisited in May of 2007, when a mother humpback and her calf entered the Sacramento River. In general, some experts say that it is uncommon to see whales in fresh water for more than a few weeks. There are a few species of dolphins that are specifically fresh water mammals, but most whales are built for salt water environments. Scientists are not exactly sure how long whales can live in fresh water since they are salt water mammals. One issue is that in fresh water, they don't have access to their regular food sources.

An additional concern for whales in fresh water is that salt water performs some natural functions for the whale. Small cuts or scrapes, for example, are easily healed in salt water. Whales in fresh water environments do not have access to the beneficial healing aspects of salt. It was noted in the May 2007 Sacramento incident that both whales appeared to be injured by the propeller of a boat. Fortunately, the two whales in this instance were herded back to the Pacific Ocean, roughly two weeks after making their appearance in the Sacramento Delta.

Fresh water environments are also not natural for a whale's swimming and diving activities. Most whales swim in water that is at least 20 feet (6.1 m) deep, if not considerably deeper. Whales in fresh water rivers may be swimming in water only a few feet deeper than the girth of their bodies. This makes them more likely to be beached.

The humpback whale and many others species of whales are migratory animals by habit. Diversions from typical migration patterns can create problems for whales in fresh water. Since they are unaccustomed to the route they are taking, whales can easily get confused. Instinct to migrate is innate, so as interesting as it may be to see whales in fresh water, it’s a matter of concern to whale lovers.

When possible, governments and environmental groups use their resources to help whales back to their natural environments as quickly as possible. Marine biologists are certain that the biology of the whale cannot survive indefinitely in brackish water, and that at most, few whales in fresh water will survive more than a month.