Generally, people in southern Idaho are accustomed to the need for close regulation of water rights. Water is limited; if there isn't someone to control how it's apportioned, trouble is going to ensue. In northern Idaho, where there's been somewhat more water than in the arid south, people are less accustomed to the water rights regime. There, attempts to adjudicate and close regulate water is just seen as, well, intrusive - and maybe threatening.
Those people may want get a load of this: A Washington state Department of Ecology study: “Spokane Valley-Rathdrum Prairie Aquifer Optimized Recharge for Summer Flow Augmentation of the Columbia River.”
The department notes, "The research was done as part of ongoing efforts to ensure adequate water supplies in the SVRP aquifer and in the Spokane River in the face of population growth, ever-increasing groundwater pumping and expected effects of climate change. Large amounts of aquifer pumping have already decreased summer low flows in the Spokane River, the report says."
So where would this water come from? "The study looked at three alternative sources for water to recharge the aquifer and the drier reaches of the Spokane River: the Spokane River during high flows; pumping aquifer water from a site in Washington up into Idaho; and using groundwater near the southern end of Lake Pend Oreille. Spring runoff water would be piped to Idaho and discharged to the aquifer, arriving in the Spokane River in the late summer. The technically preferred source is the latter."
As described here, Washington wouldn't necessarily be making a grab for Idaho water. But we're talking about working right along the state borders, with an aquifer that cuts across both states. If you're a Panhandle Idaho resident with water rights, this report may come as a cold slap.
Especially if you're one of those who've been opposed to state adjudication of water rights - which is another way of saying, protection of them.