Marshall, Pilot Station, St. Marys (which is accessible from the Yukon at Pitkas Point), and Mountain Village. After Mountain Village, the main Yukon channel frays into many channels, sprawling across the delta. There are a number of communities after the "head of passes," as the channel division is called locally: Nunum Iqua, Alakanuk, Emmonak, and Kotlik. Of those delta communities, Emmonak is the largest with roughly 760 people in the 2000 census. Emmonak's gravel airstrip is the regional hub for flights. Hazards Navigational obstacles on the Yukon River are the Five Finger Rapids and Rink Rapids downstream from Carmacks. Bridges Despite its length, there are only four vehicle-carrying bridges across the river, listed from upstream to downstream: The Lewes Bridge, north of Marsh Lake, Yukon, on the Alaska Highway; The Robert Campbell Bridge, which connects the Whitehorse suburb of Riverdale to the downtown area; The Yukon River Bridge in Carmacks, Yukon, on the Klondike Highway; and The Yukon River Bridge, north of Fairbanks, Alaska on the Dalton Highway. A car ferry crosses the river at Dawson City in the summer; it is replaced by an ice bridge over the frozen river during the winter. Plans to build a permanent bridge were announced in March 2004, although they are currently on hold because bids came in much higher than budgeted. There are also two pedestrian-only bridges in Whitehorse, as well as a dam across the river and a hydroelectric generating station. The construction of the dam flooded the White Horse Rapids, which gave the city its name, and created Schwatka Lake. Transportation is also performed along the river in summer by barge, enabling heavy goods, oil, and vehicles to be transported to communities along the Yukon, Tanana, Innoko, and Koyukuk rivers. This service is performed by Ruby Marine and reaches Tanana on the Yukon River and Nenana on the Tanana River. Geography and ecology Some of the upper slopes of this watershed (e.g. Nulato Hills) are forested by Black Spruce. This locale near the Seward Peninsula represents the near westernmost limit of the Black Spruce, Picea mariana, one of the most widespread conifers in northern North America. The river flows into several parklands and refuges including: Innoko National Wildlife Refuge Nowitna National Wildlife Refuge Yukon-Charley Rivers National Preserve Yukon Delta National Wildlife Refuge Yukon Flats National Wildlife Refuge Discharge Yukon at Pilot Station (121 miles upstream of mouth) minimum, average and