The Hudson is constantly changing. Tides ebb and flood, organisms move from one place to another. As the seasons pass, the water warms and cools, populations of migratory fish appear and then leave the river, and plants sprout, grow, and die. Over longer periods of time, we note that the physical character of the river and the kinds of plants and animals living in the river are different from those described in early journals, or even recollected by people with long memories.
All of these changes may be normal or even essential parts of the Hudson River ecosystem, or they can lead to irreversible and substantial change in its character. Which of these changes are simply "bends" in the river ecosystem and which are "breaks" - fundamental, long-lasting shifts?
Regular, predictable changes like the tides and the seasons are not only a normal part of the river's ecosystem, but are essential for maintaining its present character (think about how the Hudson River ecosystem would change if we could somehow stop the tides or the seasons!)
Larger and less predictable changes can markedly affect the character of the river's ecosystem. Hurricanes cause high flows that wash out plankton and rooted plants and make the river muddy for many days. Nevertheless, the river's ecosystem recovers from these storms, and returns to its familiar character in a few weeks.
Other examples of impressive but ultimately transitory events include long-term droughts or even sewage pollution before the Clean Water Act. However, some changes, either natural or human-made, change the character of the river forever (or at least for a long time). Human introductions of alien species such as the zebra mussel have caused radical changes, which probably will not be reversed for decades, if ever. Likewise, changes in sea level will alter the salinity of the river, leading to changes in the biological communities of the river, and in the usefulness of the river for drinking water and human recreation.
Understanding the importance and consequences of all types of change in the Hudson is the central theme of the these lessons and an important and current topic of scientific inquiry.