Every kind of reaction in nature, to the best of my knowledge, follows a decaying exponential trajectory
Are you familiar with
Zeroth Order Reaction Rates? They are linear.
Radioactive molecules diminish according to their half-life.
Indeed an exponential form. A First Order Rate equation.
The earth's magnetic field is similarly decaying.
The magnetic field fluctuates and has been known to ebb and flow throughout geologic history.
In other words, when it comes to reactions of any kind in nature, there is no such thing as uniformitarianism.
Any reaction rate that can be
modelled mathematically would seem to be aptly called a uniformitarian model. It simply means that a
rate expression can be forecast and backed out in time.
Uniformitarianism can be generalized to state that processes occuring at a given rate today probably occured at a similar rate in the past. This would seem to account for chemical rates as well. That is how we know that radiometric dating (an exponential decay process) is a good dating technique. The rate is calculable over time.
If a first-order reaction is what you consider to be the only thing of importance then indeed rates may be faster in the past. But that does not mean that streams necessarily ran faster in the past and eroded at dramatically higher rates in the past
just because you have some examples of chemical reaction rates that follow an exponential decay.
If that were the case, then surely we'd see such a change happening in stream erosion patterns all over the earth at this time. It would be pattently obvious.
To my knowledge stream erosion rates are not
universally decaying. They are a function of gravitational potential, weather, rock strength, capacity and competence of the stream.
Indeed a given stream will erode to a level in which it loses capacity and competence
in that locality, but that doesn't affect streams in other unrelated parts of the world.
But further, remember, the way we know how things occured in the past is by the
evidence we see in the rocks today compared to the same type of evidence in soft-sediments under modern conditions.
Now, unless you are going to make equally unfounded claims around some decay of
fundamental laws of physics over time (like did Gravity follow a different formula in the past that it doesn't today?) you will have to explain how modern structures (a function of the physics or chemistry occuring as we watch it) are wholly unrelated to strikingly similar structures we see preserved in rocks.
Just because you can find some exponential decays in nature does not mean that
everything over all of history follows some exponential decay curve.
Indeed, if it were so, then those same decays would be readily apparent today.