Since 2000, most changes to federal regulatory oversight of environmental protection have been due to agency rulemaking and executive action.
Agency rulemaking is a process in which agencies create or promote regulations. First, they have to notice that they want to issue a rule. Then, they go through a comment stage of rulemaking in which they have to accept written comments from the public. Finally, a new rule can be published with a statement, specifying the rule's purpose and basis.
On the contrary, executive orders are official actions or directives from the president to the agencies. These orders have the same force of a law.