|Note the principal directions of ground water flow - through the
Portneuf Gap and from the southern Bannock Range - and across the bedrock lip which
separates the southern and northern valley.
One of the most important features of this aquifer system is the presence of coarse gravels at and near the surface (the portion shown in yellow). These gravels were deposited by a catastrophic flood some 14,000 years ago, when glacial Lake Bonneville (located in the Great Salt Lake basin) broke through its natural dam at Red Rock Pass and emptied in the space of a few months.
Because the deepening of the water table, most of the Bonneville gravel is not part of the aquifer in the northern valley. However, it is the single-most important feature in the southern valley.
Visit the Hydrology section of the Digital Atlas of Idaho to learn more about the Lake Bonneville Flood.