We conducted a four-year study of the life history parameters of the endangered freshwater turtle Podocnemis lewyana (Testudines: Podocnemididae) in four channels connecting wetlands to the Magdalena River in northern Colombia. Using capture–mark–recapture techniques, we documented body size growth rates, sex ratios, and size class distributions, and estimated survival rates for juveniles and adults. We also used body size/clutch size and body size/track width correlations to estimate the body sizes of females that nested in beaches in the Magdalena River near the study sites. The body size at first nesting (30 cm straight-line carapace length) was comparable to the estimate obtained by inspecting the reproductive tracts of 70 females using a sonograph. Growth rate analyses showed adult males to be smaller than adult females with an estimated size at sexual maturity of 20 cm straight-line carapace length. Females begin nesting at 5–6 years of age and may lay up to four clutches annually with a mean clutch size of 22 eggs. The overall sex ratio was 1♀:0.72♂, but this parameter varied among study sites, as did size class distributions, with juveniles more abundant in the two shallower channels and adults more abundant in the two deeper channels. Survival estimates increased with increasing body sizes. We constructed a projection matrix using these estimates of the key life history parameters for this population and used an uncertainty analysis to create a distribution of possible asymptotic population growth rates (λ), to estimate the vectors of stable age distribution and reproductive values, and conduct an elasticity analysis. Results indicated that the population is declining 8.8% annually, probably as a result of illegal over-exploitation of adults that are harvested as by-catch by local fishermen, and especially of reproductive females that are captured while nesting. Elasticity analyses indicated that the most effective way to help this population recover is to increase adult survival rates, since this transition alone accounted for 62% of the contribution of all vital rates to the population growth rate. Traditional management methods that rely entirely on nest transfer and head-starting will likely be of limited impact.