Consistency is a very important factor in a river. Major changes in the volume of water that flows down it can have far-reaching effects, leading to turbulence, flooding or even drought. Fortunately the Seine is a very predictable river, for a number of reasons.
Firstly, we have to consider the materials that the river flows through. Much of the Seine Basin is constructed of permeable rocks which can absorb huge quantities of water, like gigantic sponges; this help to regulate the flow by taking it in during periods of heavy inundation, or allowing it out when water levels fall.
Secondly, we have to consider the prevailing weather conditions. Regions which feature monsoon conditions, or periods of sustained drought, can result in major changes in the levels of water in rivers. Throughout most of it's length, however, the Seine runs through regions with moderate amounts of rainfall, spread fairly evenly throughout the year. This, of course, can vary from year to year so the Seine does have occasional extreme conditions, although far less so than most other European rivers. A particular variable factor is snow; for example the Yonne River, a tributary of the Seine, originates in mountains of mainly impervious rocks, so a thaw of a heavy snowfall can cause water volumes to grow rapidly.
Drought conditions can cause the water levels to fall but extensive storage facilities which have been built along the river can normally be used to maintain them at an acceptable level.
As a result of the above, floods which cause serious problems are extremely rare, and the river water levels do not suffer from problematic fluctuations very much.