the stream,
the bigger the sediments. The less energetic stream will only move
smaller pieces. The energy of the stream changes from place to place
depending on the slope of its bed and from time to time as the amount of
water in the stream changes. We can see the effects of changes in energy
on the ability of the Wissahickon to move sediments in this bar.
At the upstream end of the bar we see large sediments like those seen in the photo to the right. These sediments and many smaller sediments were being moved by the stream but when the stream approached the area of the bar it began to lose energy. With a loss of energy, the stream was not able to do as much work and so began to drop sediments. The heaviest ones are dropped first, although some smaller stones and sand will be trapped between larger stones and so a mix of different sized particles is found at the upstream end. If you move south along the bar, you find a continuous decrease in size. This means the stream continued to slow and lose more and more sediment at this location. Because the bigger sediments had all been removed at the up stream end of the bar, only smaller sediments are found as you move downstream along the bar. The next two pictures show the middle of the bar and its south end. I think you can see the distinct difference in particle size.
Of course if you are walking on the bar, the creek is not there. How could it deposit these sediments were it
doesn't flow? The creek does cover the bar during periods of high
water after heavy rains. It is when the creek has more water that
it does most of its work. During the next flood, many of the sediments
in the bar will be picked up and moved downstream where they will find a
temporary home in another bar. Sediments from upstream will replace
the lost gravel in this bar. As a result the shape and make-up of the
bar may change over time, but it is a relatively permanent feature. 
