Robert Mac Nish is an adjunct professor in the University of Arizona's department of hydrology and water resources.

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A recent article described the formation of an advisory group to recommend measures to keep the San Pedro River flowing. The creation of this group is the latest in a series of ineffective steps to preserve the flows of the San Pedro in the Sierra Vista subbasin.

Hydrologists from the U.S. Geological Survey, the Arizona Department of Water Resources, the private sector and the University of Arizona have studied the basin extensively from the mid-1970s to the present day.

Computer simulations of groundwater flow in the basin have been made by all using mathematical models, all of which have demonstrated the impact of groundwater pumping on the river's flow. The river's flow is supplied by groundwater during periods when there are no runoff-producing storms, and this flow has dwindled to a third of what it was in the 1940s, before significant groundwater pumping began.

While the obvious cause of this flow diminishment is the groundwater pumping, many folks in the basin have sought, and continue to seek, alternative explanations, and this has led to delays in implementing an effective mitigation plan to protect the river.

In 1995, the U.S. Bureau of Reclamation drilled an array of wells for the U.S. Bureau of Land Management at Lewis Springs. The array was designed to detect the encroachment in the riparian corridor of the effects of ground-water pumping in the Sierra Vista/ Fort Huachuca area.

By 1997, the wells were operational and the USGS has been monitoring water levels in these wells since. Data from these wells demonstrate that the effects of the distant pumping reached the river by or before 1997.

Even if all groundwater pumping in the basin is stopped immediately, the effects of the historic pumping will continue to expand from the pumping centers, and the river will dry up, flowing only during storms.

If pumping were never to resume, eventually the river would return to something approximating its condition in the 1940s. That recovery could take more than a century.

In the absence of aggressive pro- active measures being taken immediately to protect the river from the expanding influence of the groundwater pumping, the river is toast. Given the historic responses of the folks in the basin to repeated warnings of the rivers potential demise, it is too late.