The diurnal cycle is a significant part of the variability of over fifty rivers in the western United States and can be used to understand the dominant forcing from land and atmospheric inputs in a given river basin. The clearest differences in diurnal cycles of streamflow among rivers are those between water diurnally added to the river, like snowmelt, and water diurnally removed from the river, like evapotranspiration and infiltration into groundwater. Snowmelt-dominated rivers have their highest sustained flows and largest diurnal fluctuations during the spring melt season. These fluctuations are characterized by sharp rises and gradual declines. At the end of the melt season, in large basins, the hour of maximum discharge shifts to later in the day as the snowline retreats to higher elevations. Evapotranspiration/infiltration-dominated rivers in the western states have their highest sustained flows during the winter rainy season but exhibit their strongest diurnal cycle during the summer months of minimum total discharge, when the diurnal fluctuations comprise a large percentage of the total flow. In contrast with snowmelt-dominated rivers, the hour of maximum discharge in evapotranspiration/infiltration-dominated rivers occurs in the morning and is consistent between rivers and over time as the summer progresses. In these rivers, diurnal changes are characterized by a gradual rise and sharp decline each day.