Which american naturalist convinced president roosevelt to set aside yosemite vally as a national park

 John Muir

Explanation/details:

John Muir (1838-1914)  was an ardent environmentalist and an early leader in movements to protect the American wilderness.  He helped campaign to get Congress to make Yosemite a national park, which actually occurred in 1890, during the presidency of Benjamin Harrison.  Land in Yosemite had even been set aside already as protected for public use by Congress in 1864, signed into law by President Abraham Lincoln.  This was known as the Yosemite Grant.   When Teddy Roosevelt became president in 1901, he was a strong supporter of the National Park and National Forest systems.  Roosevelt added 230 million acres of land to those public lands systems, including an expansion of the lands reserved for Yosemite National Park.  

In 1903, President Roosevelt personally took a 3-day camping trip in Yosemite with John Muir, and said of Muir:  "Of course of all the people in the world, he was the one with whom it was best worth while thus to see the Yosemite."

John Muir is well known also as the co-founder of the Sierra Club, along with Henry Senger.  The Sierra Club was one of the world's first major environmental groups, and to this day is highly active in promoting responsible environmental policies.