Subsided Lands

We used spatial data on elevation, land use, land cover, and restoration activities to measure the extent of subsided lands at a regional scale (Bay and Delta) and to assess the likelihood of continued subsidence driven by types of land use or land cover (Delta). Land uses and covers within subsided areas in the Delta region that are continually or seasonally dry were assumed to be subsiding, including dry agriculture, urban areas, and managed ponds. We assumed subsidence has slowed or halted in perennially or semi-consistently wetted areas in the Delta region, such as rice fields, channels, muted marshes, unnatural ponds, and lakes (minimally subsiding). Only a handful of subsidence reversal projects (in the ground before 2022) were assessed as likely accreting.

In the Bay, we measure the total acreage of subsided lands in order to track changes in the extent of subsidence due to reconnection of restoration projects to tidal action or by additional subsidence. As the total area of the subsided land disconnected from the tides decreases, the assigned score will increase toward Good. We based Bay benchmarks on the 2025 indicator baseline acreage of 69,000 acres (using mapping data from 2020) and the San Francisco Bay Restoration Authority target of 15,000 acres of restoration by 2037. We set the benchmark delineating Poor and Fair at 70,700 acres to capture the baseline acreage plus the potential subsidence reversal due to the 1,700 acres of in-the-ground marsh restoration projects that occurred between the passage of Measure AA in 2016 (when the Restoration Authority began tracking its target) and the 2020 baseline. A score will increase from Fair to Good if the remaining restoration target of 13,300 acres is met in such a way that the amount of subsided lands is reduced to 55,700 acres. The approach to scoring Bay Subsidence Management may change when the Sediment Indicator is completed in future updates.

In the Delta, we measured the total acreage of land that is likely accreting or experiencing minimal change. These two measurements are combined into a single “likely accreting or minimally subsiding” metric. There were only a handful of restoration projects focused directly on subsidence reversal to date, while there are more instances of projects that slow or halt subsidence. If new subsidence mitigation or reversal projects are implemented, we expect the assigned score to increase toward Good as subsiding land acreage decreases. We set benchmarks for the Delta region by summing our current measurement of 25,000 acres of likely accreting or minimally subsiding lands and the target acreage of two Delta Plan Performance Measures for subsidence. The total subsidence management acreage target in the Delta Plan is an additional 36,500 acres, which is the combined acreage for both Delta PM 4.12 and Delta PM 5.2. Thus, we set the benchmark between a Fair and Good status score at 61,500 acres: when the Delta Performance Measures are fully met (36,500 additional acres of new subsidence management projects) + our current measurement of 25,000 acres of likely accreting or minimally subsiding lands. We set the benchmark between Poor and Fair at 43,000 acres: the halfway mark to meeting the Delta Performance Measures target (18,250 additional acres, rounded to 18,000) + our current measurement of 25,000 acres.