Predicting Sea Level Rise Impacts on Ag Production

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This project investigated sea level rise impacts on saltwater intrusion and explore methodologies to detect and quantify land subsidence and river channel aggradation changes that may impact agricultural viability now and in future climate conditions; this informed the Snohomish Conservation District's Snohomish Agriculture Resilience Plan and was funded by the The ESRP Learning Program.

The Snohomish Conservation District led a collaborative effort with the Snohomish Sustainable Lands Strategy, UW Climate Impacts Group, Cardno, and NOAA to evaluate how climate change, sea level rise, and vertical land movement are likely to affect agricultural systems in the tidally influenced Lower Snohomish and Lower Stillaguamish river floodplains. This information supports estuary restoration as an appropriate response to protecting regional agricultural viability under sea level rise by describing the value of a strategic retreat from vulnerable low-lying areas, and encouraging a renewed focus on protecting agricultural lands and irrigation water resources at higher elevations.

The project team developed and implemented a land subsidence, channel aggradation, and saltwater intrusion study coupled with community engagement activities to create a community vision for agricultural resilience in Snohomish County. This project complements and leverages several ongoing agricultural resilience and salmon habitat restoration project development initiatives in the Snohomish and Stillaguamish estuaries and watersheds, including: 1) restoration project development in the Snohomish and Stillaguamish floodplains; 2) flood modeling, groundwater modeling, and creation of several decision support tools to inform agricultural sustainability and resilience planning and land use decision-makers. The information created through this project and the complementary ongoing initiatives are expected to inform development of estuary floodplain salmon habitat and agricultural land management strategies by the Sustainable Lands Strategy.

Subsidence and Aggradation[edit]

Subsidence refers to the downward sinking of the ground surface. Subsidence of agricultural lands can occur from the lack of sediment inputs to the floodplain, soil compaction, groundwater withdrawals, and decomposition of soil organics. Aggradation refers to the rising of the ground surface and, in this instance, refers to the accumulation of sediment within the river channel. Aggradation can increase the risk of flooding because it decreases the capacity of the river to carry flood volumes. Subsidence contributes to drainage issues in agricultural fields and can increase the risk of levee failure through settling and shifting. Therefore, aggradation within the river channel and subsidence of adjacent farmland can increase the flood and drainage impacts to some agricultural areas.

In order to study whether land subsidence and aggradation is affecting agriculture in the Snohomish and Stillaguamish River floodplains, the Conservation District contracted Cardno to conduct subsidence and aggradation studies for each watershed. To evaluate subsidence, Cardno re-surveyed elevations in areas that have been surveyed in the past, including monuments and benchmarks, roads, agricultural lands, and levees. The study also involved analysis of the vertical difference between elevations from multiple LiDAR datasets. To evaluate aggradation, Cardno compared recent channel cross-sections to historical surveys, evaluating 48 cross-sections of the Stillaguamish River and its tributaries and 19 cross-sections of the Snohomish River.

The analysis of farmland subsidence for both the Stillaguamish and the Snohomish River floodplains indicated that some areas may be sinking 2-3 inches per decade due to cultivation of organic soils, although error rates in this study are high.

The analysis also showed that the Lower Stillaguamish River channel is aggrading, and this trend is likely to continue into the future. The Lower Snohomish River is not aggrading in general, but upper reaches (from the SR-9 bridge to the Skykomish River) show some aggradation.

Saltwater Intrusion[edit]

Agricultural areas located near marine waters can suffer from saltwater intrusion, which occurs when saline waters move into groundwater aquifers. In the Lower Stillaguamish and Snohomish River floodplains, groundwater with increased salinity due to saltwater intrusion could affect the growing conditions for crops if that salinity reaches root zones. Though salts are crucial plant nutrients, high concentrations of any one salt or many different salts can be toxic to plants. Sea level rise could increase saltwater intrusion into groundwater in these areas as the saltwater interface rises in relation to freshwater aquifers.

Areas closest to the shoreline are at the highest risk of saltwater intrusion. Areas within 5,000 feet of the shoreline are especially vulnerable, and areas within 10,000 feet could also experience increases over time.

Florence Island, in the Stillaguamish River estuary, already experiences saltwater intrusion above crop tolerance thresholds in patches, and those impacts are likely to increase in severity over the next 50 years. Increasing pumping could pull salty water upward in the groundwater table.

In addition to analyzing groundwater levels and ponding, a groundwater study completed by Cardno also assessed the effect of sea level rise on saltwater intrusion into shallow groundwater. Cardno measured salinity levels in the wells drilled for the groundwater level study, as well as analyzed data from partner’s wells. Salinity impacts are measured in millisiemens per centimeter (mS/cm), a metric that measures conductivity values as a surrogate to salinity. Based on the salt tolerance of crops most commonly grown in the Lower Stillaguamish and Snohomish River floodplains (corn, grass, beets, spinach, and cabbage) and the depth of the wells used in the study, it was assumed that 3 mS/cm would best indicate potential impacts of saltwater intrusion on agricultural production. The response of plants to 0-2 mS/cm is mostly negligible, while sensitive plants can experience yield impacts with 2-4 mS/cm. Most plants would be restricted by 4-8 mS/cm, and only tolerant plants can grow under conditions with 8 mS/cm or more.

In the Lower Stillaguamish, existing conductivity measurements at wells within 1,000 feet of Hatt Slough showed a range of 0.1 to 6.7 mS/cm in late August 2016. These readings suggest that crops in the lower estuary may already be stressed by existing salinity conditions. Farmers in this area confirm that this is true in patches, but that most land is still highly productive. Data suggests that rising sea levels of one foot will increase conductivity measurements by approximately 1 mS/cm in the groundwater of farms near the coast.

Climate Impacts to Groundwater Levels[edit]

To better understand the impacts of sea level rise on groundwater, the Conservation District hired Cardno to assess the impact of rising sea levels on groundwater levels in the spring and fall on floodplain agricultural land. The study examined the lower Snohomish and Stillaguamish basin floodplains from the mouth upstream to the extent of tidal influence on groundwater levels for each river system. For the Snohomish River, the study area extended from the mouth of Possession Sound to Thomas’ Eddy at river mile 16.1. The Stillaguamish River study area extended from the mouth of the river at Hatt Slough upstream to the Pioneer Highway Bridge at river mile 7.4.

In order to confirm assumptions about geology and to document groundwater levels across seasons, Cardno installed wells throughout the study areas. They also used data from existing wells operated by Snohomish County, the Stillaguamish Tribe, and the Washington Department of Fish and Wildlife. Cardno used the recently released Projected Sea Level Rise for Washington State – A 2018 Assessment to incorporate projections of relative sea level rise into the analysis. Sea level rise was assumed to affect river channels up to the current extent of tidal influence.

Results indicate that rising sea levels are anticipated to delay the time when farmers access their fields in the spring. While natural variation will continue, sea level rise will generally increase the delay of start times for working fields and this increase will become more and more pronounced with time. For low-lying farmland, delays could reach three weeks by the 2050’s and four to five weeks by the 2080s. Areas closer to the Puget Sound coast (within a few miles) will feel the greatest effects of this change because of their proximity to rising marine waters.

Partners and Roles[edit]

Snohomish Conservation District - Project Lead
Cardno - Technical Study Consultant
UW Climate Impacts Group - Technical Collaborator
UW Crustal Deformation Group - Technical Collaborator