It is thus difficult to make generalizations about the overall importance of nutrient export by estuaries. Strong geographic variation exists in the extent of nutrient export due to variation in estuary size and morphology, variation in river input, regional variation in groundwater and precipitation inputs, and seasonal variation in nutrient cycling. The seasonal cycle of primary productivity depends upon temperature, light, and nutrient levels which are in turn affected by various interacting factors, e.g., climatic regimes, basin size and morphology, river input, etc. These factors will determine estuarine variability in both space and time. The Hudson River Estuary is a drowned river valley rising only 1.5 miles along 150 miles between New York City and Troy.
Wetlands are in greatest abundance in the upper third of the estuary.
Strong winds from the south and north can push water into or out of the estuary, obscuring the true tidal regime. The location of these zones varies seasonally as well as daily depending on tidal and fresh water inputs. During average fresh water flow, salt water intrusion reaches West Point, about 50 miles from the Battery. The estuary is generally well mixed during low fresh water flow conditions. A sharp salinity gradient occurs 25 miles north of the Battery indicating distinct fresh and saline layers, but this gradient disappears approximately 41 miles upriver.