|This figure depicts the entire Portneuf River watershed area. It is subdivided into three sub-watersheds:|
|1) The upper Portneuf-Gem Valley watershed.
2) The Marsh Valley watershed.
3) The lower Portneuf watershed. It is in the latter that our aquifer (shown in yellow) is situated.
The three blue triangles indicate locations of river gauging stations maintained by the U.S. Geological Survey.
The McCammon station reflects runoff only from the uppermost two-thirds of Marsh Valley watershed, although its annual average discharge (21.5 billion gallons) plus the estimated amount of additional water derived from the lower third of the valley (no more than about 4 billion gallons) is within the range of annual estimated recharge derived from precipitation and snow melt (primarily from the highest elevations) from the Marsh Valley watershed (24-30 billion gallons). In contrast, the Topaz gauging station's discharge is much larger than total recharge that can be accounted for from precipitation sources in the upper Portneuf-Gem Valley watershed. Therefore, additional water must enter from outside the basin, most likely as ground water along faults and fractures in the earth's crust.
The total river discharge of about 76 billion gallons annually through the Portneuf Gap (the gateway to the lower Portneuf Valley watershed) is in good agreement with that measured at Pocatello's gauging station, and together with other indirect evidence suggests that the Portneuf River does not contribute much recharge to the aquifer as it flows over it in the southern part of the lower Portneuf Valley watershed.
One of the most surprising facts associated with this figure, however, is that less than 2% of all the water originating in the upper parts of the Portneuf River and Marsh Creek watersheds exits through the Portneuf Gap as ground water (about 1 billion gallons annually). In other words, the amount of ground water recharge originating from the upper Portneuf and Marsh Valley watersheds is relatively small.