Identifying how temporal variation in the environment affects reproductive success of invasive alien species will aid in predicting future establishment and tracking dynamics of established populations. Asian carp Hypophthalmichthys spp. have become a nuisance in recent years in the Mississippi River basin. Their populations are apparently expanding, indicating favorable conditions for reproduction. During 2004 and 2005, we quantified mean density of Asian carp larvae, mean monthly gonadosomatic index (GSI) of adult males and females, and number of eggs within mature females in the lower Illinois River, a major tributary of the Mississippi River. A flood (water velocity ≥ 0.7 m/s) and drought (<0.2 m/s) occurred during apparent spawning in 2004 and 2005, respectively. During 2004, Asian carp larvae were found during 32% of sampling weeks; mean GSI and fecundity were relatively low for adults, probably reflecting partially spawned individuals and perhaps low reproductive investment. During the drought of 2005, larval stages were present during only one (5%) of the sampling weeks, whereas mean GSI and fecundity of adults were high through summer. Females resorbed their eggs instead of spawning during this year. Spawning conditions during low water periods appear to be unsuitable for Asian carps, inhibiting adult spawning and yielding few larvae. Spawning conditions during 2004 were better but still yielded low densities of larvae relative to native fishes. Reproduction in the lower Illinois River appears to be linked to river flow and its impact on adult spawning decisions, but conditions for strong year‐class production (i.e., high larval densities) may be rarer than previously expected.

Exchange of larval fish between a river's main channel and its floodplain backwaters is compromised by sedimentation. Restoration projects to reduce sediment loading are being implemented in large rivers of the midwestern United States to curb backwater habitat loss and restore backwater‐to‐river connectivity. During 2004 and 2005, drift nets were set bidirectionally (with and against the flow) within a constructed channel between the Illinois River and an adjacent, 1,100‐ha restored backwater, Swan Lake, to investigate the interplay between life history strategies and lateral drift on a diel and seasonal basis. Ambient larval density and species composition within the river and backwater also were quantified. Drift was positively correlated with water velocity in the main stem during 2004, and an estimated 32.3 × 106 larvae drifted at the surface of the channel into Swan Lake. In the absence of a flood in 2005, the density and composition of the larval fish assemblage in Swan Lake and the Illinois River appeared to drive larval drift timing, magnitude, and composition. Swan Lake's restoration has maintained some river connectivity and lateral drift functionality for resident fish, but its functionality compared with natural, connected river‐backwater systems remains unknown.