This paper discusses the concept of stability in a concrete natural situation: the gallery forests in the Chaco region. On the basis of Pickett's definition of minimal structure, stability is considered as the response which enables a system to keep its (minimal and configurational) structure once an alteration has taken place (Pickett et al. 1989). Resilience is, instead, another kind of response which enables the system to keep its minimal structure intact regardless of what might happen to its configurational structure. The system to be analysed is that of the gallery forests in the Lower Bermejo Basin, in Argentina, which is subject to intense river bed migrations and floods caused by this allochthonous watercourse. The effect of this alteration is analysed on different space-time scales. Locally floods behave as a completely unpredictable phenomena, starting a complex floristic changes mechanism that would eventually enable the system to recover its structure once the alteration is over. These forests appear to be resilient but little adaptable, with low resistence and low elasticity. The latter, when combined with the alternation's return interval, which is under the ecosystem's generation time, upset the theoretical stability conditions, thus making the system seem ‘apparently unstable’. Regionally, instead, migrations act as regular and permanent phenomena (highly predictable) always affecting some part of the area. The autochthonous system river side forests have become adapted through evolution to this new situation, so much so that this phenomenon has become a part of the system's internal dynamics. Chaco physiognomy, were it not due to the allochthonous phenomena affecting it today, would certainly be very different from the present one.