USING THE TOOLS OF CONSERVATION PALEOBIOLOGY TO PROVIDE A BASELINE FOR ESTUARINE RESTORATION RESULTING FROM THE 2014 ENGINEERED SPRING-FLOOD OF THE COLORADO RIVER

The over-allocation of the Colorado River’s water has led to the ecological devastation of its delta. In the past, the Colorado River annually delivered over fifteen million acre-feet (~1.85x10¹⁰ m³) of water to its delta but in recent years that number has been zero. Recently, environmental pulse flows equal to approximately 1% of the natural annual river discharge were negotiated by the United States and Mexico to be released over a five-year trial period.

In the week prior to the first pulse, which occurred on March 23, 2014, we traveled to the Colorado River Delta (CRD) to collect a pre-pulse flow baseline for the benthic molluscan community. We expected low live-dead agreement in species composition given the consensus that the damming of the Colorado River significantly changed the molluscan community. Preliminary analysis of our data, from 91 live-dead samples from two localities, using a plot of taxonomic similarity (Jaccard-Chao; JC) versus rank-order abundance (Spearman’s Rho; Rho), indicated, however, that there is high concordance between live and death assemblages; that is, all data points fall within the upper right quadrant of the JC-Rho plot. It is possible that the death assemblage has been altered in the decades since the Colorado River was dammed and that its pre-dam signal has been overprinted by recent inputs. However, we also collected 15 bulk samples from nearby beach-ridge accumulations (cheniers) known to be from the pre-dam era. In a follow-up analysis, comparing the chenier assemblage with the live-dead dataset, all points fell once again in the upper right quadrant of the JC-Rho plot.

Although live-dead fidelity in species composition was high, Hurlbert’s Probability of Interpecific Encounter (PIE) showed a large difference between live (PIE = 0.667), dead (PIE = 0.457), and chenier (PIE = 0.259) samples, suggesting that the abundance of species in the live assemblage is more even than the death assemblage, and that both are more even than the pre-dam chenier assemblage. These results are consistent with the consensus that the once dominant bivalve, Mulinia coloradoensis, has declined in recent years as a result of increasing salinities in the CRD. As a baseline metric for assessing the restoration effects of the pulse-flows, PIE is more sensitive than the JC-Rho plot in detecting ecological changes.