High levels of pyrethroid pesticides in the Sacramento-San Joaquin River Delta, the number one river system on America’s Most Endangered Rivers List of 2009, has been linked to heavy urbanization in the region. Leading a study to understand the collapse of the delta’s ecosystem, University of California-Berkeley toxicologist Donald Weston, Ph.D. found that these pesticides most likely reached the river from urban storm drains, collecting household pesticide disposal and runoff from lawns of 1.4 million residents in the Sacramento region.