My goodness! It never occurred to me that rainfall and litter would be correlated, but you are right.

The 4th hit was the one below, which gives me the idea that lots of rain during the growing season results in lots of dead leaves and other litter in the fall and winter. That fits with what I've heard in Santa Barbara, where some people have lots of allergies after a spring with heavy rain, because lots of plants grow, with lots of flowers and pollen that can cause allergies.

From the Lecture Notes for Allochthonous Inputs
(site to explore)
Pollen heads of conifers cause spring peaks in pine forest streams. Litter fall was positively related to annual precipitation and decreased with increasing latitude

I think your question about the strength of the correlation has many parts. The correlation between rainfall and litter depends on things such as:

1. How much litter falls directly into the stream, and how much gets into the stream due to wind, or water running down stream banks, or other things.

2. How much undergrowth is there by the stream banks? Does the litter mostly just fall down and settle among the undergrowth, or does it get blown or washed into the stream?

3. What is the altitude? The google hit above says there's less litter at higher altitudes.

4. How wide is the stream? Wide streams have less litter for a specific area of stream [such as a square yard]. I learned that from the website in this email.

The website in this email has lecture notes as a pdf. The lecture notes have a lot of information, and I recommend that you look at them. After you read the first sentence, or maybe the whole Overview, you will want to look at the glossary at the end, because it tells you what the scientific words and abbreviations mean! But the lecture notes are not easy to understand.