For most shark species, spending a day in fresh water would be similar to placing us humans on the moon without a spacesuit. They most likely would not be able to survive due to the inhospitable surrounding environment. One of the main problems that would pose a serious threat to sharks in this case is the process known as osmosis. Osmosis refers to when a fluid moves through a semi-permeable membrane from a solution with a low solute concentration to a solution with a higher solute concentration until there is an equal concentration of liquid on both sides of the membrane. In this case, the dissolved substances involve sodium and chloride.
Because sharks evolved in salt water, they are equipped with salty bodies. Even sharks in fresh water contain more than twice the amount of salt and chloride as other fish that are more common among freshwater. In theory, sharks that are placed here should burst like a balloon when it is overfilled with air, given the osmosis effect; however since they urinate a lot they are able to avoid this problem.
The sharks take in a lot of extra water however they excrete much of it as urine that is diluted and has a rate of over 20 times that of typical saltwater sharks. What this means is that their kidneys are required to work harder than normal, thus utilizing additional energy. Much like humans that have become accustomed to life in low oxygen regions, sharks in fresh water appear to adapt to what would seem to be formidable conditions.
Although there have been several studies over the years that have determined that there are in fact some species of sharks residing in freshwater environments, relatively few sharks spend a substantial amount of time here. In fact, river shark populations are now at dangerous lows. Bull shark’s population numbers are higher since they often move between fresh water and salt water environments. Other species of sharks however, that are more adapted to life in lakes and rivers are faced with having to withstand both natural and human induced problems within their habitats. Problems that these sharks face include changes in temperature, oxygen, mineral content, and other climate changes. Human activities such as damn building, water modifications such as irrigation and the introduction of pollutants to the water all pose a serious threat to these particular species of shark.