The Tide is high. then low. then high again

The River Ouse in Selby can be a confusing beast. One time you see it and it will be bursting out of its banks as if flooding is immediately imminent, the next time you see it the water will be so low that entire beaches will have emerged. One time the water will be flowing exactly as you expect, from the hills and towards the sea. The next time time it will have reversed its direction entirely to go back from whence it came. 

There is no secret or mystery as to why this is happening, it's all very well understood and easily predictable. Every day the North Sea and the Humber Estuary go through their twice daily cycle of high and low tides and that action is powerful enough to force the River Ouse into reverse. The historic name for this tidal river action in this part of the world is Aegir. Although there is some debate as to where that name comes from, the most evocative and therefore most popular explanation is that this is the personification of a Viking Sea God. In the River Trent, which also flows into The Humber, Aegir takes the form of a tidal bore. That is to say, a surfable wave that flows upstream at times during the cycle. In The Ouse, Aegir may be less dramatic but no less important. Nowadays, The Ouse is tidal all the way up to Naburn. That makes sense to anyone who knows the area as the lock and weir there prove impassable even for a Viking God. However, before that particular piece of engineering was built in the 19th Century, The Ouse was tidal all the way past York and up as far as Poppleton. 

There is no definitive record as to exactly why in the year 71 the Roman 9th Legion (Hispania) originally chose the exact location for a new fortress. A fortress they would call Eboracum and which would develop into the administrative and military centre of the North of England for centuries to come. Plenty of potential factors have been suggested and discussed such as the defensive benefit provided by a v shaped confluence of two rivers to the defence from nature provided by a glacial moraine in an otherwise flat as a pancake landscape. However, whatever the reasons, it is clear that having the trade and transport opportunities offered by a major river must have been part of the thinking. The fact that The Ouse was tidal was also a bonus. The classic example used to illustrate this was the transportation of magnesium limestone from Tadcaster to York. That creamy-white stone that proved so resilient to weathering caused by rain, had been brought into York all the way back to Roman times. In fact, the Roman name for Tadcaster was Calcaria which simply means Limestone. However, it was Mediaeval York that really made the most of the stone by building of cathedrals, churches, castles, monasteries and city walls out of the stuff. And that fact that this stone proved so hard wearing means that we can still enjoy the fruits of their labour to this day.

The first step of bringing down the stone was easy, load up barges on the River Wharf and sail downstream with the current until the Wharf meets The Ouse near to Cawood. The next step might have caused a significant obstacle as Cawood is considerably downstream of York and it might have been a difficult fight against the current from that point on. However, all that was required was to wait for the tide to turn and then make use of the skills and knowledge of local pilots who could use Aegir to take the stone all the way to its final destination. The process would then be reversed to get the barges home again ready for the next shipment.