When many farmers have an important decision to make, they draw a line down the middle of a piece of paper. To the left of the line, they write the word “Pro.” On the other side, they write “Con.”

As farmers go through this exercise, the choice they have to make becomes clear. If the benefits on the “Pro” side far outweigh the drawbacks on the “Con” side, that will dictate the decision.

Conversely, if the drawbacks outnumber the benefits, a different decision emerges.

It would be instructive to undertake such an exercise considering the proposal to demolish the four dams on the lower Snake River.

We were naive enough to think the White House-led mediation last year would weigh the benefits and drawbacks of tearing out the dams. Instead, the parties involved drew up a document that rains money on some parties and dodges the important issues.

The failure of the mediation was that some parties were left out and others were allowed to dominate the discussion. The agreement that emerged — described as a “roadmap” to demolishing the dams — failed to settle anything.

In last week’s Capital Press, we weighed the benefits and drawbacks of tearing down the dams. While the list of considerations was not all-inclusive, it was an eye-opener.

On the “Pro” side was a single item: fish. While there are no guarantees, environmentalists and a spokesman for one of the tribes involved assume the populations of fish impacted by the dams will increase, though they are already surviving now.

Remember that no species of fish is at stake. Chinook and other salmon species are plentiful along much of the west coast of the continental U.S., Canada, Alaska, Japan and Russia, according to the National Wildlife Federation’s website. Only populations of natural-run fish that migrate above the dams would be impacted in any way. Hatchery-raised fish continue to be plentiful.

On the “Con” side of the ledger is a long — and expensive — list. Among the items on it:

• Building generators with the ability to produce more than 3,033 megawatts of peak electricity the dams produce would be daunting, both logistically and in cost. Solar and wind developments only operate when conditions are right and would need batteries or some other form of storage. Otherwise, nuclear generation could be able to do it. Either way, the costs would likely top $16 billion.

• Rebuilding the system that irrigates more than 40,000 acres of farmland. Cost: $1 billion-plus.

• Barges could only get as far upstream as Burbank, Wash., so the use of roads, trucks and railroads would have to be expanded. Cost: More than $1 billion, plus farmers would have to pay an estimated $70 million a year in higher transportation costs.

• The value of farmland could plummet, and property tax rates could skyrocket, the Franklin County, Wash., assessor predicted. The overall impact would be “literally billions of dollars,” he said.

• More truck and rail traffic would produce an additional 1.25 million tons of carbon dioxide and nitrogen dioxide a year, the Pacific Northwest Waterways Association estimated.

Proponents of tearing out the dams say weighing the fish against the cost of the demolishing the dams is a “false duality perpetrated by political rhetoric.”

We prefer to consider it a reality check. Every dollar of the cost of tearing out those dams and rebuilding the region’s economy would be paid with O.P.M. — Other People’s Money. Congress can’t afford it. Washington state can’t afford it. Farmers and ranchers can’t afford it. Taxpayers can’t afford it.

And environmentalists aren’t offering to pay for it.