That's essentially correct. Step-pools are the slow, typically deep, pools formed between faster reaches of low-order streams. "Order" refers to how many branches a stream has - primary streams have no branches feeding into them. Secondaries result from joining two primaries, etc. The lowest order streams occur furthest upslope, and often the fastest, steepest, and coldest. Step-pools can occur in any order, but are less likely in high and lowest order streams. A primary stream isn't likely to have enough water content to form such pools, while higher order streams may be so flat and slow that pools also don't form. Pachytriton, Rhyacotriton, and some Desmognathus are likely to occur in tiny streams where the substrate is rocky, the water cold and fast. Paramesotriton, Laotriton, Dicamptodon, Neurergus, some Desmognathus [I'm ignoring many genera] occur where the water is still cold, but the volume is higher and large deep pools form because of rock or log barriers or differing erosion rate of the underlying bedrock. Basically, step-pool species occur in mountain streams, but at locations where water is slightly warmer, deeper, and slower, often between rapids and falls.
