A tensioning mechanism for a movable, tarp enclosure system including first, second, and third arms, and first and second support brackets which are all connected together in a substantially triangular, adjustable configuration in which the first arm is rotatable relative to support brackets and when the first arm is rotated, the first bracket moves linearly along the first arm toward and away from the second bracket. The second arm has one end thereof pivotally connected to the first bracket such that the first end of the second arm moves together with the first bracket, the third arm has an engagement member provided on one end thereof which is configured to operatively engage a vertically intermediate portion of an end one of the bows, and the third arm has a length adjusting mechanism for adjusting a length thereof.