To avoid a loss of effectiveness in protecting streams, the stream buffer shall remain in natural undisturbed forest vegetation and no development or land-disturbing activities shall be undertaken therein, except as provided in section 17-3-5. Furthermore, there shall be a 20-foot transitional area immediately landward of the stream buffer which may be graded, landscaped and/or used for pedestrian or vehicular purposes so long as no impervious materials are utilized. Certain uses of land, as specified in section 17-3-5, below, may be permitted within the stream buffer so long as they meet the requirements of that section and are designed and constructed to minimize the amount of intrusion into the stream buffer and to minimize clearing, grading, erosion and water quality degradation. Nothing herein is intended to preclude the removal of downed vegetation from the stream bed in order to improve stream flow dynamics. If it is necessary to remove downed or dead vegetation, it should be cut and the root ball left in place to help maintain stability of the shoreline. Nothing herein is intended to [or shall] preclude any activity specifically authorized pursuant to Section 401 and/or Section 404 of the Clean Water Act.