Originating in New York State, the Delaware River flows south into Pennsylvania, gaining strength from many tributaries along the way. This area east of the Appalachian Mountains is know as the Piedmont Plateau and is characterized by rolling, upland hills and farmland. The Delaware River flows across the Piedmont Plateau, forming the border of Pennsylvania and New Jersey, and onto Morrisville, were it meets the Coastal Plain. The Coastal Plain is flatter and made up of much softer, sandier soil without the rock content of the Piedmont. Where two geological areas like this meet, it is known as a fall line. When a river or stream crosses the fall line there is usually a major change in its characteristics. Because of the Piedmont's high rock content, the rivers erosive effects are not as great as in the Coastal Plain where the softer soil is more easily  eroded. Combined with the flatness of the Coastal Plain, it is not uncommon for the Atlantic tides to push their way back up the rivers to the fall line. This is the case with the Delaware River. Because Morrisville is located on the fall line between the Piedmont Plateau and the Coastal Plain, the river at the north end of town always flows down stream and is not effected by the tide. However at the south end of town the river has a large tide change, and in fact, will change directions on a strong incoming tide. This geological phenomena was important to the early settlers for several reasons. First, this was as far up the river as a ship could go. The river is more easily crossed upstream of the falls because the river is shallower and narrower, and bridges can more easily be constructed for the same reasons. Another reason is there is usually a change in elevation making it possible to harness the power of the river to run the mills of colonial America. Not only was Morrisville able to take advantage of its location on the "Falls of the Delaware", but it was also on a straight line between New York and Philadelphia, making it a major trade route.    
The first three pictures were taken at the end of Ferry Road, across from the filtration plant. The first pictures is looking up stream and shows the first line of boulders that form the fall line. The second picture is looking back toward the Calhoun Street Bridge. Note the egret and motor boat on the right. The third picture is the same as the second, but different time of the year. The difference in the water level is due to the amount of water coming off the Piedmont Plateau, not the tide.
From the dike at the end of Maple Avenue, looking back the other way toward the Calhoun Street Bridge. The fall line is more obvious in the next picture, taken from the same spot on a different day, as was the third picture. 
Taken from the base of the dike along Park Avenue, these rocks are mildly effected by the tide, but more so by the amount of flow in the river.
These are the same rocks as the picture above, taken from the "Trenton Makes" bridge
Looking across the falls on a winter day. The Calhouns Street Bridge is just visible in the background left.
On the south side of the "Trenton Makes Bridge" the river is greatly effected by the tide.
Looking under the Route 1 bridge toward the "Trenton Makes" bridge. These boulders are the first line of defense against the Atlantic tide, however they will be submerged when the tide comes in. The last picture is the same picture except zoomed in for a more unique perspective.