Reservoirs hold more than rivers, yes.
But before providing facts re this I'[ll discuss an economic point to consider for person not locked into the misperceptions of water resource management that allowed cunning criminals in the public and private sector to steal many trillions of public dollars over the past century through complex waterscams:
More water is lost from the two largest reservoirs on the Colorado River by evaporation than people use from it. Reminiscent of the "Chinatown" movie theme, yes? The chronic "water shortage" of the Southwestern states that was created by this waste justified forming California's omnipotent Metropolitan Water District of Southern California and its massive State Water Project that continue to nourish a plethora of land/water/energy schemes for unseen directors in the private sector.
The Colorado River misplanning was promoted by Professional Engineers who pretended to be misguided by their fixation upon huge projects/structures into misinforming politicians and journalists about the need for these. (This gave them 'deniability' when thoughtful citizens finally began asking why they prescribed the most complex, least cost-effective flood control-water supply-electriticy generation mode.)
With considerable pressure brought to bear on dissenters by the private sector, plus studious avoidance of this issue by the major news media, those few thoughtful citizens were persuaded to drop the matter so that fifty huge dams could be built intstead of the thousands of small ones and onsite retention practices that would have done the job far better at much less cost. The cooperating civil engineers-bankers-lawyers-agency managers and other professional liars were rewarded generously for their 'cooperation' while the public got what it invariably gets when dissenting voices are stifled without due consideration of their information.
The politicians were talked-pushed-bribed into approving tremendously expensive structures instead of the many many tiny off-stream dams that would have turned the Colorado's huge (157 million acre) watershed into a sponge that kept its immense natural underground reservoir full through the worst of droughts.
An indication of the misinformation that still pervades public information sources may be found at http://www.waterencyclopedia.com/Ce-Cr/Col...-Basin.html>
. Careful analysis of this site will reveal the masterful misguidance that has kept honest politicians and journalists from recognizing the deceits of dishonest ones.
Example: We see a relatively meaningless figure for this drainage basin's runoff; "about 700 cubic meters (24,700 cubic feet -) per second". We do not see this translated to 185,250 gallons per second so that it can be readily compared to the only significant figure, total rainfall. (That amounts to more than 100 TRILLION gallons yearly.) Is it unreasonable to suspect that the total rainfall figure was withhelf to keep readers from wondering why planners-politicians-professional water managers have been unable to capture enough to serve all needs of people and wildlife?
Many of the legislators and local politicians-experts were wise enough to recognize that if the natural evaporation-proof underground reservoir were kept full then the springs and seeps that used to feed the Colorado River would come back to life, nurturing stable flows of much greater average total volume. But wise and honest persons were shouted down by professional liars so that the Big Dam Foolishness could go forward.
Competent professionals in the engineering field know quite well that a multitude of offstream reservoirs kept full by diversion of a portion of the much greater year-round flow that comes from sensible onsite retention of rainfall would have provided adequate flood-proofing and even more electricity than Hoover Dam, along with abundant drinking and irrigation water, without blocking the river flow. But capable PEs were wise enough to not jeopardize their careers by speaking these simple truths.
America's US Department of the Interior - Bureau of Reclamation has served the private sector well since corrupted US Congressmen invented it to give their mentors in the private sector direct control over the public's natural resource management. Deceiving and coercing politicians who cannot be bought has been its primary function and it has done a fine job of this. Its absurd management scheme for the Colorado River, deliberately wasting more water from it than is used, stands as an all-time classic in the art of water/power scamming.
Re river flow vs reservoirs:
If my memory serves me right it would take more than 7 years of average Colorado River flow to to fill its two largest reservoirs, ignoring the other 48 major dams on in this watershed. Assuming an average 15 mph flow rate for the main river indicates that its total volume wuld be emptied in just four days.
Using this yardstick, how do the world's other major rivers measure up in reservoir volume versus in-flow volume?
USA Daily Streamflow Conditions - http://waterdata.usgs.gov/usa/nwis/rt
has useful data. Surface reservoirs hold a lot more water than is generally supposed but we'd be a lot better off if our public servants were to focus upon guiding rainwater into subsurface reservoirs with due diligence.