 Much of the basin of the Merced River is characterized by an alpine climate . The upper basin of the river receives heavy snowfall in the winter , which is usually enough to feed the river and its tributaries year round . However , most of the snowpack melts by the late autumn , reducing flows in the main stem and its tributaries significantly , and causing some smaller tributaries to dry up altogether . Snowmelt constitutes a majority of the river 's springtime and early summer flow , and provides up to 85 % of the flow above Happy Isles . The dry season depends more on groundwater to supply the river 's flow . The middle and lower basin are dominated by an arid , Mediterranean or even semi @-@ desert climate . 
