What They Are and How They Were Formed

The word pluvial is Latin for the word rain; therefore, a pluvial lake is often thought of as a formerly large lake created by excessive rain paired with little evaporation. In geography though, the presence of an ancient pluvial lake or its remnants represents a period when the worlds climate was much different from present-day conditions. Historically, such shifts changed arid areas into places with extremely wet conditions. There are also present-day pluvial lakes that show the importance of various weather patterns to a location. In addition to being referred to as pluvial lakes, ancient lakes associated with former wet periods are sometimes put into the category of paleolakes. Formation of Pluvial Lakes The study of pluvial lakes today is mostly tied to that of ice ages and glaciation as the ancient lakes have left distinct landform features. The most prominent and well studied of these lakes are usually related to the last glacial period as this is when they are thought to have formed. Most of these lakes formed in arid locations where there was initially not enough rain and mountain snow to establish a drainage system with rivers and lakes. As the climate then cooled with the onset of climate change, these dry locations turned wet because of different air flows caused by the large continental ice sheets and their weather patterns. With more precipitation, stream runoff increased and began to fill the basins in the formerly dry areas. Over time, as more water became available with the increased moisture, the lakes enlarged and spread across places with lower elevations creating enormous pluvial lakes. Shrinking of Pluvial Lakes Just as pluvial lakes are created by climate fluctuations, they are also destroyed by them over time. For example, as the Holocene epoch began after the last glaciation temperatures around the world rose. As a result, the continental ice sheets melted, again causing a shift in world weather patterns and making the newly wet areas once again arid. This period of little precipitation caused the pluvial lakes to experience a drop in their water levels. Such lakes are usually endorheic, meaning they are a closed drainage basin that retains precipitation and its runoff but it does not have a drainage outlet. Therefore without a sophisticated drainage system and no incoming water, the lakes began to gradually evaporate in the dry, warm conditions usually found in their locations. Â   Some of Today’s Pluvial Lakes Though the most famous of todays pluvial lakes are significantly smaller than they used to be because of the lack of precipitation, their remnants are important aspects of many landscapes around the world. The United States Great Basin area is famous for having the remains of two large pluvial lakes -- Lakes Bonneville and Lahontan. Lake Bonneville (map of former Lake Bonneville) once covered nearly all of Utah as well as portions of Idaho and Nevada. It formed about 32,000 years ago and lasted until approximately 16,800 years ago. Lake Bonnevilles demise came with reduced precipitation and evaporation, but most of its water was lost as it overflowed through Red Rock Pass in Idaho after the Bear River was diverted to Lake Bonneville following lava flows in the area. However, as time passed and little rain fell into what remained of the lake, it continued to shrink. The Great Salt Lake and the Bonneville Salt Flats are the largest remaining portions of Lake Bonneville today. Lake Lahontan (map of former Lake Lahontan) is a pluvial lake that covered nearly all of northwestern Nevada as well as parts of northeastern California and southern Oregon. At its peak about 12,700 years ago, it covered approximately 8,500 square miles (22,000 square kilometers). Like Lake Bonneville, Lake Lahontans waters gradually began to evaporate resulting in a drop in lake level over time. Today, the only remaining lakes are Pyramid Lake and Walker Lake, both of which are located in Nevada. The rest of the lake’s remnants consist of dry playas and rock formations where the ancient shoreline was. In addition to these ancient pluvial lakes, several lakes still exist around the world today and are dependent on an areas precipitation patterns. Lake Eyre in South Australia is one. During the dry season portions of the Eyre Basin are dry playas but when the rainy season begins the nearby rivers flow to the basin, increasing the lake’s size and depth. This is dependent though on the seasonal fluctuations of the monsoon and some years the lake can be much larger and deeper than others. Todays pluvial lakes represent the importance of precipitation patterns and the availability of water for a locale; whereas the remains of ancient lakes show how a shift in such patterns can alter an area. Regardless of whether or not a pluvial lake is ancient or still existing today though, they are important components of an area’s landscape and will remain so as long as they continue to form and later disappear.