What is carryover storage (and why September 30 matters)

Carryover storage is what is left in a reservoir when the water year ends on September 30. It is the balance a system carries into the next year before the new winter has delivered anything. Think of it as the savings account at the turn of the fiscal year: a full one buys a cushion against a dry winter, a thin one means next year starts exposed.

Why September 30 is the line

The water year runs October 1 to September 30, so September 30 is the low point of the natural cycle, after a full summer of releases and before winter refill begins. Measuring storage on that date captures the true carryover, the water that survived the demand season. It is the cleanest annual checkpoint for whether a reservoir is entering the next year ahead or behind.

Why managers watch it so closely

Carryover is the first line of defense in a dry year. If winter underdelivers, carryover is what keeps deliveries flowing and cold water available for fish through the following summer. District managers plan summer releases with one eye on the carryover target, holding water back in a shaky year rather than spending down to a dangerous September balance. A reservoir that ends the year well below its normal carryover is starting the next year already rationing.

Worked example: reading the end-of-season posture

On the Shasta chart above, look at where the storage line sits as it approaches the end of the water year and compare it to the five-year median for the date. A line running above its normal band into late September is healthy carryover, a comfortable start on next year. A line well below the band is the setup water managers dread: a dry system handing an even drier one to the next winter. Carryover is the number that turns one bad year into the risk of two.

Carryover and the water year go together. For the calendar behind it, see the water year guide.