A river that moves both ways
Far upstream, the Delaware flows in one direction, from the hills toward the sea. At Camden the picture is more complicated. The river here is tidal, so its level and flow respond to the ocean as well as to rainfall upstream. Twice a day, the water rises and falls, and at times the current runs upstream.
Tides at this location come from the ocean at the mouth of Delaware Bay, which transmits the tidal wave upriver. The effect travels as far as the fall line near Trenton, where the tide ends. The range is typically a matter of several feet at Camden and Philadelphia, though exact figures differ by location and are published in official tide tables.
Reading the tide
A few basic terms help:
- Flood tide. The incoming tide, when the water level rises and the current generally flows upstream.
- Ebb tide. The outgoing tide, when the level falls and the current flows downstream.
- Slack water. The short period around high or low tide when the current is weak.
- Spring and neap tides. Larger tides around the new and full moon, and smaller tides in between.
The tide does not follow a simple clock. Timing shifts by roughly an hour each day, and the moment of high water differs from place to place along the river, so predictions for a specific location are needed.
What else shapes the water
Tides are only part of what moves the water.
River flow. After heavy rain or snowmelt upstream, more fresh water arrives, which can strengthen the ebb and weaken or delay the flood.
Wind and weather. Strong winds, particularly from certain directions, and storm systems can raise or lower water levels beyond the predicted tide. Low-lying shorelines are especially exposed during storms, a topic covered in the article on the tidal floodplain.
Salinity. Salt water from the bay pushes upriver, more so in dry periods when less fresh water flows down. This affects ecology and also water intake for industry and municipalities.
Ice. In cold winters, ice can form and affect navigation.
Channels and charts
Large ships need deep water, and natural depths in a river vary. A navigation channel is therefore marked and maintained at a depth that is adequate for the ships the ports expect, with periodic dredging to remove accumulated sediment. The channel has been deepened over time to accommodate larger vessels.
Mariners rely on:
- Nautical charts, which show depths, hazards and channels.
- Buoys and markers, which indicate the edges of channels and hazards.
- Tide and current predictions, published by national agencies.
- Notices to mariners, which announce changes.
Larger commercial vessels are guided by pilots with specialized local knowledge, as described in the article on ports and shipping. Tugboats assist with turning and docking.
Why timing matters for ships
For deep-draft ships, a few feet of water can matter. Ship operators may time movements to take advantage of higher water or favorable current, and they plan docking around slack water when possible. Currents of a couple of knots, which are common in parts of the river, can make maneuvering in confined areas more difficult. Such details are why professional pilots and tug crews play a central role.
Recreational boating
Smaller boats share the river with large commercial vessels, and the differences in size and maneuverability are substantial. A large ship cannot stop or turn quickly, and its wake and suction can affect small craft. Anyone who boats on the tidal Delaware should consult current charts, local regulations and weather and tide forecasts, stay clear of shipping lanes and avoid assuming that other vessels can see or avoid them. Formal boating safety courses are offered by official and volunteer organizations.
The same caution applies to the shoreline: currents near piers and pilings can be strong, and cold water is dangerous in most of the year. This article is general background, not safety instruction, and boaters should rely on official guidance.
The river and the city
Tidal behavior has shaped how Camden and Philadelphia grew. Ferries of the colonial period had to deal with currents and ice, which is part of the story of river ferries, and later bridges had to clear a navigation channel, so their heights and spans reflect shipping needs. Even today, planners of waterfront parks and buildings consider tides and flood levels, as discussed in the material on green infrastructure.