The predominant feature of the Eureka area is Humboldt Bay, which is separated from the ocean by spits. Humboldt Bay, the northern end of which is known as Arcata Bay, extends 12 miles parallel to the coastline and is 0.5 to 4 miles wide. The land on the inland side of the bay is flat to gently rolling. The shoreline of the bay has a well-developed beach from which dunes extend inland a short distance over the alluvial plain.
The major streams that drain the area are the Eel River, which flows into the Pacific Ocean south of Humboldt Bay, and the Mad River, which flows into the Pacific Ocean north of the bay. Several small streams also flow into Humboldt Bay. All the streams are tidally influenced and have brackish-water marshes and mud flats along their banks for as much as 1 to 2 miles inland.
Coastal northern California has a Temperate Oceanic climate, which is characterized by moderate temperature and precipitation. Dense fog is frequent and tends to attenuate temperature fluctuations. The average annual precipitation at Eureka is approximately 40 inches per year, most of which falls during the autumn and winter months. Precipitation increases with altitude, and amounts are greater inland in the foothills and mountains.