The Debate about Unimpaired Flow The State Water Board “will try to determine what the flow needs are on balance” for every season of the year, with fall salinity control needs part of that discussion, said Howard. Possible changes in flows (including the ap- propriate range) will hinge on the percentage of unimpaired flow, which the State Water Board defines as “the total volume of water that would flow past a particular point of interest if no diversions were taking place in the watershed above that point.” “An unimpaired flow is not a
natural flow, those are two separate concepts,” said Howard. “Natural flow assumes no infrastructure at all. The vast riparian forest and wetlands that were native to this area would still be there.”
Discussion of unimpaired flow can become quite technical and not easily understood by the public. Opinions differ whether the amount of out- flow has changed or stayed the same. January to June outflow, as a percent of unimpaired flows “is about 60 percent and it has been about that level since about the 1960s,” said Johns, adding “there have been no real changes in Delta outflow as a percent of unim- paired flows for about 50 years.” Rosenfield with the Bay Insti- tute disagreed, saying the percentage of unimpaired runoff in the Central Valley that makes it out of the Delta as actual outflow “has dropped sub- stantially over time,” with the recent average “slightly less than 50 percent.” Furthermore, the percentage of available runoff that makes it out of the Delta “is always substantially lower in dry years than in wet years” because the amount of water that people take “has increased steadily over time and has not responded to changes in annual hydrology … so in dry years the percentage that gets diverted is higher than in wet years,” said Rosenfield.
The maintenance of artificial flow regimes to support water diversion schedules “is actually an additional
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problem” because there are “fewer and less dramatic peaks in flow,” said Rosenfield, noting it is those peaks that trigger “life history transitions” such as migration.
“By diverting or storing most of the water that would normally form an outflow pulse, we impair the migration and spawning life stages of numerous native fish species,” he said. Missing in the flows debate “is that the actual flows that the fish see in the Delta are much more affected by climate than by water project operations,” said Johns. “The fish have no idea about unimpaired flows. What they care about are actual flows.” Quinn and others believe unreal- istic targets could emerge if the state calls for a percentage of unimpaired flows to improve water quality. Basing Delta outflow projections on a per- centage of unimpaired flow “is grossly inefficient as a means of protecting fish,” and is not a true representation of any sort of legacy condition, said Quinn.
Unimpaired flow “is kind of a mythical concept [with] the theory behind it being ‘if it happened in nature and the fish were evolved under those conditions than we ought to reproduce something that happened in nature,’” said Johns.
“I kind of appreciate that to some
extent it’s almost kind of religious – ‘it’s what God provided and we ought to reproduce that’ – but the reality is the unimpaired flows never happened under natural conditions,” he said. “What they are is a calculated number. These flows have not existed and do not exist in the real world. The cal- culation assumes that all the existing infrastructure is in place. The levees are there, the bypasses are there, the expansive flood plain in the Central Valley with its tules and marsh habitat is not there, the Delta is not a marsh but the existing set of farmed islands. Under natural conditions, today’s channelization and physical works that confine historic flows were not in place. Under natural conditions, this valley flooded.”
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Some people favor increased wetland habitat and an expanded floodplain in the Yolo Bypass as part of an effort to restore the Delta ecosystem.