It's not that big of a deal. Water quality in Spring and Cypress creeks gets tested, because they feed into the San Jacinto river, which then feeds into Lake Houston, and is a major source for the city's drinking water.
There's a sad amount of trash in the creeks, but it's mostly stuff that blows out of trucks and trashcans and gets washed into ditches and into the creeks over time. By then its just ugly looking but inert.
I know Cypress creek better
For access, any public road that crosses it provides easement access, and there is the Arboretum and Jesse Jones Park on the lower end, and several further up that I am less familiar with. It is shallow and wide for the most part northwest of Jesse Jones, with some channels, holes, etc. south of Jesse Jones, it slows down and widens to permanent low banks, and actually is lined with cypress trees here and there. There's some big overhead powerlines that basically mark this location. From there, it is more or less about 30-40 yards wide, and varies from 3 to 20 feet deep. After 3/4 miles it widens out much more, and merges with Spring Creek with a big deep hole, and then not much further down, it merges with the San Jacinto river, and is almost effectively the upper end of Lake Houston.
Anywhere within a mile or so of this convergence, you can catch white bass year round on occasion, but they are far more concentrated during the spawn. They will come up the creeks past I45 under the right conditions, usually in April or so, if at all. I have caught white bass almost to the Kuykendahl bridge once.