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Hydrology

The existence and function of wetlands in a watershed influences and is dependent on the watershed hydrology, that is, on the distribution and effects of water as it moves from the atmosphere through and across the landscape. The hydrology of the entire Bronx River watershed has been significantly altered by development, which has increased hard surfaces, reduced natural water storage in soil and wetlands on the landscape, and increased piping of stormwater to downstream water bodies. In an urban landscape, such as the Bronx, most of the land is developed and covered with roads, buildings and parking lots (impervious surfaces) which prevent rainwater from naturally being intercepted by vegetation and infiltrating into the soil. Instead, stormwater flows across these impervious surfaces into storm drains and gutters, which convey the water to stormwater and sewer pipes. These pipes transport the water to treatment plants or discharge stormwater directly into downstream rivers or bays.

The hydrology of the Bronx River watershed north of the Bronx primarily affects the riverine wetlands in the Bronx. Under a natural, undeveloped pattern of flows, the floodplains of the Bronx River would be inundated by flood waters once or several times a year for several days. This overflow would help maintain the water table to support wetlands vegetation, particularly in depressions on the floodplain. Under the urbanized pattern of flows, flows are more frequently higher, more erosive, and carry more sediment, but do not last as long. The channel incision, or erosion, accompanying more frequent erosive flows may have also contributed to the drying of the floodplain wetland, by also lowered the water table and disconnecting the floodplain from the channel. Thus these urbanized flow patterns are capable of more disturbance, but less able to sustain wetland vegetation.

Stream flow gauges that continuously measure the volume of flow in a channel over time provide information about how hydrology upstream in a drainage basin has changed over time. The U.S.G.S. stream flow gage located in Bronxville, three miles upstream of the Bronx, which operated from 1945 to 1989, revealed that the highest, or peak, flows each year increased as the watershed became more developed. (Also see Interfluve 2001, Preliminary Design Report). This increase in peak annual flows is typical of developing watersheds, where the stormwater is more rapidly collected and conveyed via pipes to the channel. A new U.S.G.S. stream gauge was installed on the Bronx River in the Bronx, at the New York Botanical Garden, in 2006. Over time, this gage will help us learn whether the hydrology in the watershed is continuing to change. Changes may be the result of changes in the landscape as well as in the climate.

In the Bronx, about 70% of the Bronx River watershed (3200 of 4500 acres) is drained by pipes that capture storm water and transport it to the Hunts Point treatment plant, located in the next watershed to the west, on the East River. Thus, there is a complete disconnection between the watershed and the river. Only during storms when the capacity of the system is exceeded is some of the storm water (combined with sewage) released to the river at CSOs (combined sewer overflows) in the tidal reaches of the river. As a result of this modified hydrology, we commonly refer to the sewersheds rather than watersheds of the Bronx. In these sewersheds, there is no significant natural infiltration, recharge to the groundwater or collection of water in the soil that could support a wetland. The wetlands existing in the Bronx today all lie in the remaining un-sewered area of the Bronx River watershed, in the Bronx River floodplain, the Bronx Zoo and Botanical Garden, and the estuary.

During the period of time when there was no U.S.G.S. surface water flow station operating along the river, groups and agencies occasionally took flow measurements for various reasons along the Bronx River in the Bronx. Measurements of velocity, depth, and average flow on a give day at a give location have helped provide resource managers and educators a general understanding of the conditions in the Bronx River that affect aquatic organisms and vegetation on the banks of the river.

There is no published hydrologic data for any of the non-riverine freshwater wetlands in the Bronx, most of which are located within the New York Botanical Garden and the Bronx Zoo. Knowing the depth, duration, and frequency of inundation in a given wetland is an important for understanding the vegetation it can support, and how it may be protected, managed, restored, or utilized for functions such as storm capture or water quality enhancement.

Hydrology Monitoring Work

Section Title Source Year
All RiverKeeper Streamflow Measurements RiverKeeper 1990-1993
All Opportunistic Point Velocity Measurements 2003
Parkland Historic Mean and Peak Flow at Burke Avenue Bridge USGS 1944-1989
Parkland Depth and Discharge at Burke Avenue Bridge NRG 2002-2005
Botanical Garden/Zoo Daily Peak Flow USGS 2006-present
Botanical Garden/Zoo Stream Velocity at New York Botanical Garden NRG 2004
West Farms Stream Velocity at West Farms NRG 2004
West Farms Stream Velocity at Drew Gardens NRG 2002

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