Channel Morphology and In-stream Habitat
Characterizing and classifying river reaches according to geomorphologic features helps to identify areas that have similar physical and habitat conditions, and may respond to disturbance, protection or restoration in the same way. Straightened channels, armored banks, riprap and rubble, disturbed hydrology, and dams complicate the application of traditional channel classification systems in the Bronx River. Nevertheless, classifying the Bronx River reaches primarily by channel slope and substrate (Montgomery and Buffington, 1993) and secondarily by channel width and depth and flood-prone width (Rosgen, 1996) reveals three general freshwater types of channel, and a fourth estuary channel type.
The longest free-flowing freshwater stretch of river in the Bronx is a low-gradient, sand-bedded channel (similar to “dune-ripple” or Rosgen B5 channel types) in the Parkland Section. These flat reaches have very low slopes, ranging from 0.08 to 0.1 percent, and are predominantly sand-bedded with occasional short sections of cobble (50 to 200 feet long), where riprap has been placed. For more information on streambed classification and pebble counts click here. This low-gradient, sand-bedded channel is typically 50 to 60 feet wide and is highly uniform, with few pools or in-channel bars. The natural form of these flat reaches is highly meandering with a wide floodplain, as in the Bronx River Forest, where the floodplain is up to 200 feet wide. However, most of the tight meanders have been straightened, and the remaining bends have been anchored by grouted riprap and concrete. Habitat in the low-gradient, sand-bedded channel reaches is greatly influenced directly and indirectly by bank vegetation. For example, tree roots stabilize banks and create overhanging banks. Fallen limbs and trunks become large woody debris (LWD) that provide in-stream habitat and food for microorganisms and macro invertebrates. Scoured areas around LWD create pools that serve as refuge for fish and other aquatic organisms. Many of the habitat features expected in a low-gradient, sand-bedded channel are uncommon in the Bronx. Due in part to the lack of LWD in the channel, past channel straightening and high sediment loads, there are only about one-quarter the number of pools as would be expected in a similar, undisturbed channel. Scarcity of pools, sparse in-stream and woody lower bank cover and frequent disturbance of the sandy channel bed create a high disturbance environment with little refuge. To combat this condition, reach-level management and projects that increase refuge and cover, and watershed-wide efforts to manage sediment loads and storm water runoff need to be pursued.
The pond-like areas of slow-moving water backed up behind dams constitute another reach type found in fresh- and tidal-water known as impoundments. In the Bronx, there are three impoundments, all located within the Botanical Garden/Zoo Section. The impoundments vary in width from 50 to 60 feet at the Snuff Mill Dam in the NYBG to 250 feet at the dam in the Bronx Zoo. Depth in the impoundments varies from one to six feet and substrate composition varies from unconsolidated, fine organic debris to sand. At impoundments, the decreased water velocity causes sediment carried in the water column to collect on the bed. Typically, coarser sediments settle first, at a lower minimum velocity, and finer sediments are carried further downstream. Impoundments provide slower moving, slightly deeper habitat than the free-flowing, low-gradient, sandbedded reaches of the river. However, the shores of impoundments tend to be shallower, and slower moving water there provides refuge from periods of high flow. Since the impoundments in the Bronx are relatively well vegetated, lightly developed landscapes, LWD and overhanging vegetation are abundant along the shores. Despite relatively shallow depths, little aquatic emergent vegetation is found in any of the impoundments in the Bronx River. In the largest downstream impoundment at 182nd St., however, abundant submerged aquatic vegetation is present, and its influence on sedimentation, organic matter deposition and water quality needs to be investigated. During low-flow periods, the impact of flow stagnation and warm temperatures on habitat quality is also a potential problem. Sediment deposition has reduced deep water habitat area and caused bars and islands to grow which are becoming dominated by exotic invasive plants such as purple loosestrife (Lythrum salicaria) and Japanese knotweed (Polygonum cuspidatum). Habitat diversity will continue to decrease unless sediment loads are managed. The least common channel type is the steeper, gravel-bedded channel (“plane-bed” or Rosgen B3) found in the NYBG and West Farms. The slope is about 0.5 percent, the width is 50 to 60 feet, and the substrate consists predominantly of gravel, cobbles and boulders. These reaches are relatively straight, either because of a natural valley constriction (as in the NYBG ravine) or artificial straightening (as in West Farms). Steeper, gravel-bedded channels have high sediment transport capacity and a relatively narrow floodplains. These reaches are therefore less sensitive to changes in sediment load, local floodplain development or hydrologic disturbance than the other reach types. The stream bed is characterized by cobble- and boulder-sized rock, since the smaller gravel has been flushed downstream, and upstream dams trap gravel and prevent downstream replenishment. The pools in these steeper reaches, though infrequent, are relatively permanent, since the high-energy flow prevents sediment deposition. Steeper, gravel-bedded channels also provide fast-flowing habitat where water can be cooled and aerated. These areas also often serve as a passageway between slow-flow environments. The large gravel and cobble found here provides hiding and clinging surfaces for aquatic organisms. In West Farms, this reach type has been heavily modified by straightening, confinement and dumping of concrete rubble. These alterations have increased slope and flow energy and have contributed to scouring of the channel bottom. As a result, some aquatic invertebrates may be unable to burrow into the channel bed to find refuge or lay eggs. Due to the high-flow-energy environment of the steeper reaches, riparian vegetation on the bank influences the in-stream physical habitat structure less than in other areas. Even in pristine streams, LWD has to be large and frequent enough to provide cover or refuge from high flow in such channels. Consequently, where the channel has been artificially widened or confined, in-stream habitat will not usually benefit from enhancement of bank vegetation alone. In these areas, in-stream habitat is most effectively influenced by adding in-stream structures such as appropriately sized boulders that obstruct flow in strategic locations. The fourth reach type, estuarine, refers to the area from approximately the upstream tidal limit (where the channel is about 70 feet wide) to the mouth of the river, (where the channel is about 800 feet wide). From Drew Gardens to the 174th St. weir, the banks of the river are filled and hardened with grouted boulders. At the weir and downstream, a historic bend is anchored in place by bedrock and artificial banks. The tight angle of the bed produces a backwater where suspended sediment settles, creating a shallow flat of fine, unconsolidated sediment downstream of the weir. From the weir to Westchester Ave., average channel width is about 100 feet and the banks consist mostly of concrete walls, piling, riprap, and bedrock. Downstream of Westchester Ave., the channel widens to about 200 feet at Concrete Plant Park. A protruding bulkhead several hundred feet downstream creates a backwater allowing sediment deposition and creating a littoral shelf. Downstream of Concrete Plant Park, both banks of the river are lined with riprap, steel bulkhead and fill. At intermittent locations in this part of the river, short sections of narrow littoral shelf are exposed at low tide. In the past, the Army Corps of Engineers has maintained a minimum river depth of six feet from the mouth to the weir (NOAA, 2000). The last dredging was in 1991 and there are no known plans to dredge in the near future.