"If someone is using Colorado River water without an entitlement, that harms the entitlement holders in Arizona, California and Nevada who do have one," said Lorri Gray, director of the bureau's Lower Colorado Region.

The crackdown comes at a time when the bureau and all seven states that rely on the Colorado River are trying to stretch water supplies to meet growing demand and avert drought-related shortages.

The states -- California, Arizona, Nevada, Wyoming, Utah, Colorado and New Mexico -- adopted a drought plan last year that outlines efficiency measures, such as lining earthen canals to stop seepage and removing invasive plants that use too much water.

Gray said bringing illegal wells into compliance would help ensure the river's long-term sustainability and protect users with a legal right to the resource.

Bureau officials have not identified any major water users in Nevada who may be drawing river water illegally.

Most of the well owners in question are in Arizona and California, and most are private citizens who have drilled their wells too close to the river.

Instead of pumping groundwater, to which landowners have a right, they are drawing water from the river's subsurface flow. Well owners must get approval to siphon water from the river's surface or subsurface.

To comply with the new procedures, well owners could seek an individual water right, join an existing water district or become a customer of a provider with rights to Colorado River water. They could continue to pump water from the well but only within the limits of the water right or provider.

Well owners who can't acquire water rights couldn't continue to use their wells.

The bureau is trying to determine how many wells have tapped the river or the exact amount of water pumped. Hydrologists estimate annual losses at 9,000 to 15,000 acre-feet, enough to serve Lake Havasu City for most of a year.

More than half of the wells identified are in Arizona.

Gray said the bureau will try to work with the well owners to keep a water supply.

"A key goal of this rule is to help those well owners who are using Colorado River water without a legal entitlement become lawful users of that water," she said.

The bureau is seeking comments on the proposed rule, which was published last week in the Federal Register.

The Review-Journal contributed to this report.