When you visit Otter Point Creek on different occasions you may happen to notice that the water level varies quite dramatically from day to day. During a 5 months period in 1993 I made a preliminary study of the water levels in different OPC habitats. Water levels were recorded automatically every 10 minutes and stored in a data logger. The record for the northern distributary channel of OPC's tidal freshwater wetland shows the dramatic variability characteristic of a tidal freshwater wetland system. One compact way to visualize this variability is to look at the fourier spectrum of the data. In this case the fourier spectrum allows us to identify some of the controlling forces that drive habitat evolution.



Looking at the fourier spectrum you can see all of the dominant astronomical tidal signals as well as 'subtidal' signals indicating the role of winds and storms. The M2 tide is the standard 12 hr signal that defines high and low tide as we see it at OPC. Even though it has the tallest spike, the area under that spike is not very great. Meanwhile, the area under the spikes for the 'subtidal' consituents is quite large. Since wind forces are more or less unpredictable, actual water levels at OPC cannot be predicted with any degree of accuracy; some days OPC sees no variations in water levels, while other days the astronomical tides arrive like clock work.

It turns out that OPC does not experience a strong 'spring' or 'neap' cycle that is common in many other areas. This peculiarity may be a result of the shape of the Bush River basin, which allows 'free modes' to exist and interact with the forced modes that we commonly talk about. An example of a free mode at OPC is illustrated by the small peak at ~4.5e-5 Hz (6 hr period) in the Fourier Spectrum.

Because water is the life-blood of a tidal freshwater wetland, hydrological records from the site have received close scrutiny. Our research is presently in scientific peer review, but the key finding is that Otter Point Creek experiences predictable cycles of tidal forcing and highly coherent cycles of wind and storms; all of these drive water level changes, but over different time scales, and thus have different effects on sedimentation and marsh evolution.