Under the rule, farmers using land near streams and wetlands were restricted from doing certain kinds of plowing and from planting certain crops, and would have been required to obtain E.P.A. permits in order to use chemical pesticides and fertilizers that could have run off into those bodies of water. Those restrictions will now be lifted.

A wide range of environmental protections are similarly being eliminated. The administration is rushing to clear the way for oil exploration in the Arctic National Wildlife Refuge, for instance, and weakening protections for endangered species in an effort to help drillers. And, in December 2017, Mr. Trump embarked on the biggest land protection rollback in United States history when he reduced two national monuments in Utah by some two million acres.

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The Environmental Protection Agency and the Army Corps of Engineers, which worked together to write the original Obama water rule, are expected to issue a new, looser replacement regulation by the end of this year. It is expected that the new measure, still being developed, will retain federal protections for larger bodies of water, the rivers that drain into them and wetlands that are directly adjacent to those bodies of water.

But it will quite likely strip away protections for streams that run only during or after rainfalls, and for wetlands that are not adjacent to major bodies of water or connected to them by a surface channel. Those changes would represent a victory for farmers and rural landowners who aggressively lobbied the Trump administration.

Lawyers said the interim period between the repeal of the Obama rule and the implementation of the new Trump rule this year could lead to regulatory confusion for farmers and landowners.

“The Obama clean water rule had very clear lines defining which waters are protected by the Clean Water Act, versus which waters are not, while repealing the rule means replacing those lines with case-by-case calls,” said Blan Holman, an expert on water regulations with the Southern Environmental Law Center.

Mr. Holman said the administration was replacing “clear, bright-line rules” with a case-by-case system. “This will be very unpredictable,” he said.