Aquatic invertebrate assemblages are dynamic over space and time.  Spatial variability is well studied and appreciated, whereas temporal variability is poorly studied and underappreciated.  Here we report on temporal variability in aquatic invertebrate assemblage measures using a dataset of monthly samples collected at the same location using the same methods over 7 years.  Variation in mean assemblage measures was low for taxa richness and high for abundance measures.  Mean coefficients of variation within months and among months and years were 8, 14, and 13% for genera richness and 25, 68, and 74% for total assemblage densities.  Assemblage composition was highly variable with numerous genera and species coming and going at varying intervals.  Taxa accumulation curves showed that 75, 90, and 95% of the 50 total genera collected to date were collected within 18, 56, and 72 months.  Mean monthly genera richness was 25.8, half of the total genera collected to date.  These results have important implications when developing standardized sample collection periods for bioassessment programs and suggest that assessments based on abundances and the occurrence of individual genera or species should be used with caution, especially when these assessments are based on single sampling events.