A quick history lesson sheds light on why the 2015 passage of Minnesota’s new “buffer” law — which requires strips of vegetation along waterways to filter runoff pollution — garnered widespread praise as a dramatic step in water quality improvement.

Since 1972, the nation’s historic Clean Water Act has protected and rehabilitated rivers, streams and lakes polluted by decades of thoughtless industrial and municipal waste disposal. But the law signed by Republican President Richard Nixon carved out an exemption for agriculture.

The buffer law — championed by Gov. Mark Dayton — took an important step toward reining in sediment and fertilizer runoff. That’s why the recent surprise agreement to weaken the law should be greeted with dismay.

The drinking water in Minnesota farm country is already at risk from agricultural fertilizer. Lake Pepin, the Mississippi’s big river-lake, is choking on sediment carried there from the Minnesota River’s southern prairie basin. The water crisis in Flint, Mich., also underscores that this is a time to move forward not backward.

Now at issue in Minnesota is whether the law covers both public and private ditches. A meeting between Dayton and Republican legislative leaders this year unfortunately led to an agreement that private ditches won’t be mapped by the Minnesota Department of Natural Resources (DNR) when it develops a new “Buffer Protection Map.”

There is no official estimate yet of how many acres or ditches will go unprotected, since some private ditches still may be subject to local buffer requirements by Soil and Water Conservation Districts. But private ditches are common. The respected Friends of the Mississippi River advocacy group estimates “hundreds of miles” of private ditches may continue to carry pollution downstream.

In explaining the disappointing agreement, Dayton has said that Republican leaders threatened to withhold legislative support for the governor’s bonding bill unless the policy changed for private ditches. Republican House Speaker Kurt Daudt disputed that this week, saying that his party “doesn’t operate that way.”

Regardless of what was said, Dayton should not have yielded. His bonding bill would provide sorely needed financial assistance to upgrade many outstate water treatment systems. With the House and Senate up for election this fall, it would be foolish for a legislator of either party to oppose this. There was no need to give up anything in return for it.

Legislators should take up the private ditches policy in the coming session. In the meantime, Dayton should explore other ways to ensure that the buffer law provides the broadest protections possible.

One way to do that is to push the DNR to take an aggressive approach to determining which public waterways will require 50-foot buffers, a larger and more protective strip than the 16.5-foot mandate for public ditches. The DNR is utilizing an incomplete public water inventory list from the 1970s instead of updating the list. “One state analysis, covering 67 counties, includes 21,642 miles of streams — and omits 28,760,’’ according to a recent Star Tribune story.

Last week, Dayton reiterated his commitment to cleaning up the state’s waters, saying through a spokesman that the “water crisis is not going away and he is not going away.” He merits commendation for taking on this important issue. It’s good he understands how much work is left to do.