I have a [name_u]Beck[/name_u] myself, so I appreciate a good river name! [name_u]Calder[/name_u] and [name_u]Brooks[/name_u] are lovely, I really like them together. [name_u]Calder[/name_u] [name_u]James[/name_u] & [name_u]Brooks[/name_u] [name_m]Jacob[/name_m]? They sound amazing.
It’s a nice theme. I wonder about the potential for Calder to be mispronounced or pronounced in different ways where you live? If you’re interested in other nature-themed and/or Celtic names that go well with Declan (a terrific name), suggest:
Callum and Finn
Kieran and Rowan
Lachlan and Ross
Rory and Finn
James and Jacob are excellent middle names (like John), though they are variations of the same biblical name with James being the English version.