Key Findings in Napa

Hydrology changes

Agricultural ditches are now commonplace and have resulted in a much higher degree of hydrological connectivity between the hillsides, fans, valley floor, and the mainstem of the river. Whereas under pre-settlement conditions multiple channels (distributaries) were associated with wetlands, today, agricultural, flood protection and water supply needs promote maintenance of single-thread channels. The total length of the drainage network in the valley has increased by about 25% as a result of these changes - not including storm drains or sub-surface agricultural drains. The total length of tributary channels that are directly connected to the main stem has increased by about 50%. There has been a slight increase in the length of the main stem due to its artificial channelization through historically braided reaches where no single-thread main stem channel had previously existed. The total length of secondary or overflow channels along the valley floor (historically called “sloughs”) has dramatically decreased.

The historical hydrograph is remarkably different than the modern hydrograph. The historical hydrograph is much broader and flatter. This is probably due to historical changes in land cover (loss of wetlands, loss of forest, increases in urbanization and agriculture), storage of runoff in wetlands and as groundwater, reduced surface connectivity between some tributaries and the mainstem, and the distribution of high flows across the valley floor. Approximately 50% of current urban and agricultural lands were converted from grassland/savanna, 25% from forest and 25% from wetlands. Runoff occurs much more rapidly under the modern conditions described above, and in comparison to the historic hydrograph, the overall discharge volume is higher.