The San Marcos River, in Hays County, arises from the largest spring in Texas; three federally-endangered species are endemic in the upper few kilometers of the river's length. During the morning we will visit the San Marcos National Fish Hatchery and Technology Center and the River Systems Institute at Aquarena Springs. Following a box lunch on the shaded river bank at City Park, we will tour the upper San Marcos by rented canoe - a relatively easy, 2-mile float. Throughout the day, seven guest lecturers from U.S. Fish and Wildlife Service, Texas State University-San Marcos, and San Marcos River Foundation (among others) will present informal talks on the ecology, hydrology, threats, and recovery of the riparian ecosystem and its endangered species. The subtropical heat of the August afternoon may be ameliorated through periodic immersion in the crystalline spring-fed stream, at the participant's discretion and/or by accident. No children under 7 years of age will be allowed on this trip.