The association between the El Niño/La Niña and seasonal streamflow for the Iowa River is investigated. The seasonal Southern Oscillation index (SOI) was ranked and the extreme quartiles for each season were selected to condition the composite analysis of streamflow. The either concurrent or lagged association between anomalous SOI index and streamflow was obtained with a composite analysis that windowed a 3-yr period. The existence of statistically significant streamflow responses to El Niño and La Niña has been demonstrated for lags ranging from zero to five seasons. The long lag of streamflow-SOI association is attributed to 1) the time to establish global and regional circulation conducive to excess or deficit rainfall in the Midwest and 2) the inertia of anomalous high (low) soil water. Streamflow responses to El Niño and La Niña are out of phase. Above normal streamflow is associated with El Niño, whereas dry conditions are associated with La Niña. Sensitivity analysis of the streamflow-SOI... Abstract The association between the El Niño/La Niña and seasonal streamflow for the Iowa River is investigated. The seasonal Southern Oscillation index (SOI) was ranked and the extreme quartiles for each season were selected to condition the composite analysis of streamflow. The either concurrent or lagged association between anomalous SOI index and streamflow was obtained with a composite analysis that windowed a 3-yr period. The existence of statistically significant streamflow responses to El Niño and La Niña has been demonstrated for lags ranging from zero to five seasons. The long lag of streamflow-SOI association is attributed to 1) the time to establish global and regional circulation conducive to excess or deficit rainfall in the Midwest and 2) the inertia of anomalous high (low) soil water. Streamflow responses to El Niño and La Niña are out of phase. Above normal streamflow is associated with El Niño, whereas dry conditions are associated with La Niña. Sensitivity analysis of the streamflow-SOI...