The topography suggests that, between them, Boggy Mead Spring and Butterwick Brook, both of which are still viable streams, could have been referred to in the name Smallford.
The ford was across the lower end of Colney Heath Lane, which, today, has been culverted, and in any case, a lower water table results in it carrying less water. The stream would have provided the main water supply for residents of the early settlement.
The photograph below is Boggymead Spring at Hatfield Road, half a mile north of the original Smallford location.
The picture below was not taken locally but shows the possible size of the ford, the track representing the lower end of Colney Heath Lane.