In 1066 York was a tidal port. At the time of the battle the seasonal high tides were approaching and would peak five days after the battle. However, the flood levels were already near their peak, perhaps 10-15% below height, which would occur early on Monday. There has to be some estimating and a little guesswork involved in these calculations since the tide no longer reaches York since the construction of the lock at Naburn. Tidal predictions are always subject to unknowable factors, such as recent rainfall, atmospheric pressure and silting in the river.

These figures were very kindly computed by the Hydrographer to the Royal Navy whose help is acknowledged and very much appreciated. Tidal calculation at historic dates in the past can be calculated using their on-line calculator.

Some raw figures were calculated for Goole (53°42'N, 0°52'W), which is the nearest place to the mouth of the Ouse for the Gregorian dates 19 - 30 September 1066. Goole was chosen as the data for this location can be assessed with some confidence. The adjustment for the flow along the river Ouse is discussed below.

"At Goole, the amount of fresh water coming down the rivers has little effect on the heights of high water. The levels for low water are given for a low rate of river flow caused by run-off from the catchment area. High rates of river flow can increase the level of low water by as much as 0.6 m."

The effects of wind, its direction and pressure can alter the observed tide from the predicted values.

"A south-westerly gale can delay high water in the river by as much as half an hour; an onshore gale will increase the height of the tide considerably. Conversely a high level of freshwater in the river would have masked any tidal effects and, by eliminating the flood tide, would have made the City approachable by river, albeit against a strong downstream flow."

Changes in the topography and landscape could also have radical effects on the tide times. A constriction can slow down, and possibly reduce the level of, a tide. The work of Colin Briden suggests that, while significant and rapid changes in the local flow have happened, the general pattern has been stable for centuries.

Colin Briden has written two articles on the tidal Ouse.

"Let us take as our example a tide 2 days after a new moon in September. This is an equinoctial spring tide, one of the lowest and highest tides of the year; after a dry summer the bed of the river has accumulated a substantial thickness of sediment by deposition out of the sluggish current. High water in Hull that day is at 0800 hrs; as we have seen, it is simultaneously low water in Selby. Between the North Sea and Hull the tide is ebbing; between Hull and Selby the water is rising. Between Selby and York the last of the previous tide is ebbing. At 1100 hrs it is low water at York, possibly very low indeed, only inches deep. Almost imperceptibly the ebb slows and stops and the river begins to rise. With little freshwater to prevent it, the river begins to flow backwards, rising all the time. At half-tide, around 1130, the first heavily laden ships begin to appear in the river from Selby and the Humber, taking advantage not only of the favourable current but also of the fact that should they happen to touch on a shoal - the infamous clay hutts - the rising tide coming up behind will float them off. Equally heavily-laden ships are moving off down-river, trading the disadvantage of punching a foul tide against the need for deep water in the upper reaches of the river. At 1245 it is high water York, and the river has risen around 1.5m to 3.40m OD, in 1 3/4 hours. Almost immediately the tide turns, picking up speed to the point where sailing or rowing vessels cannot overcome the current. Any unfortunates who have missed the tide and run aground down river now have little option but to transfer some of their load to lighten ship and hope to make it on the next, lower, tide."

Nevertheless, these estimates provide a good basis for the discussion of its impact on the battle. It is also important to recognise the way the tide rises. It is not a steady rise but a dramatic change. In an hour, the mud is covered to 80% of its eventual depth by muddy, churning water. If the river Ouse just below Naburn is observed today, the change in the minutes after the tidal front arrives inspires one with awe.

The hydrographer’s model of the back-calculated tides, plus the work of Colin Briden and some personal observation, have produced an indication of the tide heights and timings around the time of the battle at Fulford.

This would lead to the following model. The exceptional tide would start to arrive at Fulford about 8:00 on the morning of the battle and peak about 9:00. It would then fall slowly until 5:30 (17:30) in the afternoon, just about the time the sun set, and would peak again shortly after 9:00 (21:00) in the evening.

Tide estimates at Fulford on the day of the battle. These estimated figures have a profound effect on any interpretation of the battle. When the face-off began, there would have been a water-filled moat between the armies. By late afternoon, the beck would have been muddy with almost no flowing water. Any description of the landscape near the beck, must take account of the time of day, since there would have been significant ‘overflow’ flooding from this peak tide.