The Sun article refers to a somewhat common situation on the lower river during spring, when farmers in Imperial Valley and other places along the river must place orders for irrigation water 2-3 days in advance to give it time to come down the river to where their turnout is located. If it rains during those 2 o 3 days the farmer may decide to not take the water he ordered letting it flow down the river. From the article:
The almost .03 inches of rain was enough to cause some water users to not take the water they requested from the U.S. Bureau of Reclamation almost three days before. And that caused the river to rise about four to five feet, according to Bob Walsh, external affairs officer with the bureau.
Obviously it doesn't take much rain to alter irrigation schedules and have a big impact on river levels.
...with no place to store the excess water, it runs downstream to Yuma and into Mexico.
Las Vegas funded a project to eliminate this situation somewhat on orders from Imperial Valley. It's called the Drop 2 Reservoir, which would store water off the river, adjacent to the All-American Canal in SE California. When completed it will allow water ordered but not taken to be stored for the next call, which is supposed to save about 70,000 acre feet per year. I believe Vegas funded the project in exchange for any water saved.
I'm not sure whether water ordered but not taken is counted against a given farmers allowable water allocation in a given year - I suspect it isn't. But I'm sure there are a lot of water agencies that shudder to think that, at times, water is being released from Lake Mead that isn't used by anyone (unless farmers in Mexico are grabbing it). Do you suppose efforts will be made to tighten up management of what is currently occurring on the river if it can delay or lessen the impact of water shortages in the basin?