Rivers in Mediterranean-climate and other semi-arid regions tend to be more heavily impounded and thus their hydrology more strongly affected than rivers in humid climates because demand for water is greater (to supply irrigated agriculture) and runoff is out-of-phase with demand. The impounded runoff index (ratio of reservoir capacity divided by mean annual runoff) is 0.8 on the Sacramento and 1.2 on the San Joaquin Rivers of California, much higher than rates: encountered in humid Atlantic climate regions. As a result of these high levels of impoundment, the overall magnitude and seasonal distribution of flow has changed substantially. Flood peaks tend to be reduced: the Q2 declined on average 53 and 81% in the Sacramento and San Joaquin River basins, respectively. On many rivers, summer baseflows have increased to supply irrigation diversions downstream, creating a flatter hydrograph that no longer supports dynamic channel processes and the aquatic ecosystem that depends upon such channel dynamics. Vegetation has encroached in the formerly active channels of many rivers in response to reduced flood scour and sediment supply.