A fast-growing aquatic weed that wraps around boat propeller shafts, clogs drainage canals and stalls irrigation pumps has appeared again in Northern California's Sacramento Delta waterways, causing extensive damage to the area's recreational environment and creating even more worries for lowland farmers. According Lars Anderson, a research coordinator at the UC Davis Aquatic Weed Research Laboratory, the problem weed is Egeria densa, a member of the hydrilla weed family. A native of Brazil, Egeria densa was accidentally introduced to the area more than 40 years ago. It lay dormant until about three years ago, when a change in the Delta's environment caused by the drought allowed the weed to flourish.