"...To me a swale is an infiltration trench – in the UK the swales that people like Capability Brown built were drainage systems – the permaculture venacular has come to them being ditches with uncompacted embankments on contour – the inspiration for this coming from US Corp of Engineers work across the USA which resulted in the terrace structures pretty well everywhere with a slope east of the Rockies, to very large structures in the arid south west — such as those Bill visits in the Drylands episode of 'Global Gardener'. Some people put a gradient on them, some do not.
However their primary function in the context of this thread is as an infiltration ditch that captures overland runoff. Secondary functions are to direct that runoff to dams.
In my opinion if there is such a thing as a 'Keyline Swale' then it would be a swale placed on the Keyline of a Primary Valley. Of course those who know #KeylineGeography would know that this, and the saddle above it, are the only places one could have an element with that description.
Of course this could be extended out to the adjacent Primary Ridges . Now as Keylines are never on the same elevation that would mean that you'd have a whole lot of ditches ending at the water divide line of the primary ridge – which would be a bit chaotic.
IF you were wanting madly to do such thing, then you would have to place the swale on a Keyline guideline. This is not so simple a task and to do properly you really should have a good contour map to start with so that you can place it effectively. Fig 2.29-2.30 (pp.102-103) of the #RegrariansHandbook outlines the process one could use. In my opinion only then would you have what could constitute a Keyline swale or other swales that would run parellel to this guideline. That being the case then I believe you could apply this moniker.
However if one were using the whole Keyline program, and Keyline is a whole system after all with a whole planning process, then you would find that having a swale per se would probably not give you the water-harvesting performance that a water conservation channel (the Keyline description) or gradient road structure would. If you landscape management is about it all infiltrating then you are going to battle to get much run off to fill your dams and having purposely absorbant ditches as your main vehicle for moving that water is going to be suboptimal. That is why in Keyline the water conservation channels are compacted and in most cases have a gradient..."