When the Biebrza becomes a lake

The chart shows all levels at once. Profile B has a constant slope, so its water surface grows almost proportionally: about 10 m per centimetre near the channel. In profile A two features stand out, which produce a shelf and a step.

• Flat meadow. When the floor rises by a few centimetres per kilometre, one centimetre moves the shoreline by 120–170 m on each side.
• Sill beside the channel (assumption: 14 cm). Up to 14 cm the water does not cross to the west side. At 15 cm it fills the depression behind the sill in a single step: in this model that adds approx. 2.9 km of water surface.
• Mineral bank (from approx. 34 cm). The water surface reaches the rising slopes and the curve flattens.

The sill, the slopes and the 16 km window width are illustrative assumptions, not a measurement of any cross-section of the Biebrza.

In a landscape you do not see a cross-section, only the water surface and whatever sticks out of it. According to the references carried over from V2, Biebrza National Park describes watching the floodplain wetlands from a high bank on the Sambory–Burzyn–Brzostowo stretch (sources).

A photograph from the Brzostowo area (Adikpl, Wikimedia Commons, CC BY 3.0) shows shallow water on an open valley. The page does not load it by itself, so that it works offline: open the photograph on Wikimedia Commons (new tab, requires internet).

How to read such a panorama through the model, qualitatively

• High bank. Dry ground beyond the water, the equivalent of the ends of the cross-section where the curve flattens.
• Shallow water over the meadow. The area where profile A is almost flat: here centimetres decide hundreds of metres.
• Channel under the shared water surface. A flooded meadow does not stop the river; the channel still carries water.
• Far bank. The opposite, higher slope of the valley closes the view.

The photograph is not georeferenced. No part of it corresponds to an elevation or distance from the model, and the breakdown above is an interpretation to be checked, not a description of the photograph.

A season is not a timetable

• Spring. High water levels after the thaw; the largest floodplain wetlands usually occur in early spring.
• Summer and autumn. Low-flow periods and water loss through evaporation and plant transpiration. Exposed meadows do not mean the wetland is disappearing: open water is only one of its states.
• Late autumn. The park’s account describes floodplain wetlands at that time too, together with migratory birds (lapwings, geese, swans).

This is the typical rhythm according to the references carried over from V2, not a guarantee of a view on any particular weekend. The BbPN pages were not re-checked for this version.

The model assumes a level water surface, the same level across the whole section and no passage of time. That is enough to see the geometry, but it leaves out most of what happens in a real valley.

• Longitudinal gradient. Water in a valley flows, so its level in one place is not its level in another. The model shows a single cross-section.
• Evaporation, transpiration, peat retention. These are not calculated, yet according to the references carried over from V2 they affect the water level on the meadows in summer and autumn.
• Side channels and oxbow lakes. In the lower course they take water outside the main flow, which a single cross-section does not show.
• Time. The model does not say how many days it takes for water to reach a given point or how long it will stay.
• Sill and slopes. The values of 14 cm and 34 cm and the slopes are illustrative assumptions. Judging how realistic they are is a task for a hydrologist.

What was removed from version V2 and why

• Trip notebook (localStorage). Dropped: it did not explain the phenomenon. The page stores nothing in the browser.
• Probe with a slider and clicking on the drawing. Replaced by three checkpoints with an explanation of their state.
• A/B profile switch. Replaced by a shared view of both profiles: the difference cannot be seen when one profile is hidden.
• Five metrics and a photograph embedded from the internet. Two numbers remain (width and increment). The photograph is a link, because the page is meant to work offline.
• Three “seasonal states” as the numbers 15/60/85. They had no basis in centimetres, so the seasons are described in text, and the preset levels follow from the geometry (sill, edge of the plain).

Kept: the water level as the main variable, two profiles (flat peat, narrower with slopes), the nonlinearity of the extent and the distinction between the channel and the flooded meadow.