I'm not a Catt expert but some of the following might help.
To me the steelhead part of the river - the lower 35 miles or so below an impassable dam in Springville - can be divided into 4 sections. The first is a short segment from the river mouth to the Rte 90 bridge. There are rock jetties on both sides of the mouth of the river. In this section the river iis wide, slow-moving, and not very scenic. The water is usually well off-color or simply muddy. The north bank is on Seneca Nation land. The south bank is not, from the mouth to the Rtes 5&20 bridge. This water can be very productive for a fly fisherman early in the run when the water is not too off-color.
The next 12 miles or so is one of the best steelhead flyfishing sections in NY, also in the earlier stages of the run & when the water is in decentish shape (which it is some of the time), and when the crowds are down (increasingly less and less of the time). Much of the bottom is gravel and there are a good number of pools, riffles, chutes, etc. This entire stretch is on Seneca Nation land. One can access virtually the entire stretch of SN-bordered river after purchasing a SN fishing license. Last year's price was $35 for an annual license, with discounts for seniors & disabled. There were no shorter-term licenses available lthrough last year.
There are no formal parking lots in this stretch, by the way. A Route 438 roughly parallels the river. Dirt lanes here and there, some of which run down to the river's edge where one parks wherever the land is flat enough. Other dirt lanes turn out to be driveways.... Some require a 4WD, or make you wish you had one, especially when it's rained recently.
The SN land ends just short of the lower bridge in the village of Gowanda. At the upstream end of the village is a railroad bridge. There are a fair number of people who fish just around these bridges. About half a mile above the railroad bridge is a 5-6 mile stretch that runs through a gorge in what is mostly state land, the Zoar Valley Multiple Use Area. This is by far the nicest stretch of river to fish with canyon walls up to 300 feet, some old growth trees, and with very little sign of man's presence. It is a hit-or-miss section catching-wise, tho, since fish tend to blast through it.
Further upstream the river flows through a narrow valley. Much of this section is posted. However, the state has acquired several miles' worth of access rights. At the upstream end of the stretch is the impassable Scoby Dam and a fishermen's park to make it easy to get at the fish that stack up there. There are decent pools and runs scattered throughout this stretch of river. Several of the most productive are now off limits (I've been told).
Some of the Catt's feeder creeks also receive some steelhead.
The NY state web site I've cited below (no, it's *not* commercial) shows the public access sections of the Catt from Gowanda upstream to Scoby Dam (the park is not shown because it's relatively new).