The solution to seriously polluted rivers was to stop the additions of pollutants and dilution. Very little else was done. The Corps of Engineers or Department of Interior could have done the job of EPA and then gotten back to their core mission.
Politicians and high profile CEOs don't do things in a small, quiet, practical and efficient way. They don't like to use the tools at hand, just not spectacular enough and are a closet admission of their inattention to problems. They have to make a headline and so they create monsters and monuments to their stupidity and arrogance. That is how superfluous irritations and trouble makers like EPA get started.
Tricky Dick also gave us OSHA and the NTSB as part of his legacy. Nixon was as much a big gooberment wonk as many now.
Yes, he was “big government “ but that was also the right & left political culture of the era. “Big Government “ solved the Depression and won WWII at least that’s what everyone believed. So “Big Government” can solve all problems! Even Republicans believed that; FDR’s socialism was ok just needed to be managed better. Remember Social Security is a retirement system also it’s your money and other pet myths of the New Deal. My father called those GOPers that thought that way “Me Too” Republicans. Nixon to a great extent represented that era & political thinking though not completely. For example, he had to be brow beaten into accepting Keynesian economics.