The development and specific characteristics of rivers and streams are influenced by surrounding landscapes [1
]. Our current understanding of rivers’ dynamics incorporates a conceptual framework of spatial nested controlling factors in which climate, geology, and topography at large scales influence geomorphic processes that shape channels at intermediate scales [3
]. However, direct human impact on the environment cannot be neglected at a local scale, especially in the last century. In particular, land use/land cover (LULC) changes have a significant impact on basin water cycles and soil erosion dynamics. Human factors also include water abstraction for irrigation, flow regulation, the construction of reservoirs, and mining. An extensive bibliography analyzing the effects of dams and reservoirs on the geomorphic responses of rivers has been produced [4
]. However, the influence of other drivers, such as climate and LULC changes has been much less documented over time, although recent studies highlight their importance in inducing river changes [8
It can be said that agriculture and land abandonment are two complementary aspects of human impacts on the landscape. Land abandonment can affect net soil losses [12
], while recolonization of natural vegetation can lead to a reduction in soil loss and a progressive improvement of soil characteristics [13
]. Moreover, land abandonment and agriculture can also lead to changes of river stream morphologies, in particular narrowing and incision [13
]. Liebault and Piegay [14
] observed on the Roubion River (France), that colonization of unstable gravel bars tends to channel minor floods which, in turn, form a new and narrower channel in the existing river bed. A number of studies [3
] have documented statistical links between LULC and stream conditions, using multisite comparisons and empirical models.
Hydrological alteration is one of the principal environmental factors by which LUCL influences stream ecosystems [3
]. It alters the runoff-evapotranspiration balance, can cause an increase or decrease in flow rate’s magnitude and frequency, and often lowers river’s base flow. In addition, hydrological alteration contributes to a change of channel dynamics, including increased erosion of the channel and its surroundings, and a less frequent overbank flooding [3
]. Zhang and Schilling [16
] noted that increasing streamflow in the Mississippi River was mainly due to an increase in base flow, which in turn was a consequence of LULC (conversion of perennial vegetation to seasonal row crops). Many researchers have studied the effects of LULC changes on river flow and most of them have indicated that intensified afforestation will reduce both runoff peak and total runoff volume [17
]. On the other hand, even a modest riparian deforestation in highly forested catchments can result in the degradation of a stream habitat, owing to sedimentary input. A comparison of different catchments showed that an increased forest area results in lower concentrations of suspended sediments, inferior turbidity at base flow, lower bed-load transport, and less embeddedness [3
Numerous studies have demonstrated that LULC, the abandonment of rural activities, and consequently, a decrease of human pressure on mountain areas, has contributed to increase of vegetation cover [18
]. In the case of Reno River mountain basin, Pavanelli et al. [21
] documented that recolonization of natural vegetation and a consequent increase of actual evapotranspiration, was the key hydrological variable that caused the decrease of the river flow rate. However, not many studies have addressed the effect of the redevelopment of natural vegetation at the river-basin scale on river flow, sediment yield, and riverbed morphology. Picco et al. [22
] noted a consistent increase of riparian vegetation within the corridor of the Piave River (Northern Italy) during the last five decades, concluding that it depended on human activities, both in the main channel and at basin scale. LULC and local climate change (e.g., precipitation, temperature and evapotranspiration), may induce notable alterations of watershed hydrology [13
]. Several studies showed that precipitation increase alone is insufficient to explain increasing flow rate trends in agricultural watersheds [24
], since changes in agricultural land use can also result in increased flow rate.
Collectively, these studies provide strong evidence for the importance of the surrounding landscape, and human activities for the hydrological and morphological characteristics of rivers [3
]. However, it is often difficult to separate human from naturally driven activities [26
]. In addition, most of these articles were mainly conducted on small spatial scales, within a few hundred meters of a stream, without considering larger spatial units. Finally, only few studies have addressed the effect of redevelopment of natural vegetation at the river basin scale on river discharge, sediment yield, and bed river morphology.
Therefore, the aim of this paper was to explore relationships between local climate change that occurred in the last century and agricultural land abandonment (and consequent LULC changes), which culminated in the 1950s, on the one hand; and modifications in morphology and hydrology of the Reno River (Northern Italy), on the other hand.
This study has examined relationships between two major geomorphological changes (channel narrowing and formation of wide vegetated banks) that took place in the Reno River mountain basin in the last century, and the hydrological, climatic, and basin re-naturalization factors that have contributed to these changes. The two phenomena are strongly and positively covariant, indicative of cause and effect, and in fact, wide vegetated banks are formed at the expense of the riverbed. While riparian buffer strips were mostly absent in 1954, currently they are well developed along the entire stream. In addition, the shape of the river channel changed from braided to a single one and the width of the river bed was reduced by around 80%. The riverbed in the past, mostly consisted of gravel bars and sand deposits, and currently clay and silt prevail at the basin outlet.
Based on this study, the steering factors for those changes and significant aspects were:
LUCL changes between 1954 and 2003: A reduction in the agricultural land use (from 37 to 5%), an increase of forest cover (from 40% to 57%), and development of riparian vegetation.
Considerable reduction in SSY (−38%) and flow rate (−36%) during the last 90 years, and a consequent change of runoff coefficient (reduction from about 0.6 to 0.4), was an important parameter for hydraulic watershed management.
The effect of agricultural land abandonment that occurred in the 1950s can be recognized after 1960, confirming the initial hypothesis that this year can be taken as a starting point for the basin change. After that date a decrease in the Reno flow rate was observed and dispersion of data is significantly reduced. All statistical analyses confirm that the hydrological flow data measured after 1960 (period 1960 to 2013) are significantly different from those measured when the basin was still heavily agricultural (period 1921 to 1958). However, although this study has identified the human factor as one of the main causes of the above-mentioned changes, it can often be challenging to separate human from naturally driven activities, and future research is needed in order to do it. The geomorphological evolution of the Reno River shows how these changes are mainly related to the hydrological dynamics and catchment re-naturalization. Although climate changed in the period studied (precipitation reduction of 10.67% and 4–5 °C increase of Tmin) it had little bearing on the observed environmental changes.
This study, applied to a typical North Apennine river, illustrates the effectiveness of combining historical data (hydrological and climate data, so as aerial and satellite images) on the one hand, and the use of modern technology (geographic information systems) and direct surveys on the other. Combining these techniques can certainly contribute to a sustainable management of river systems. Although further research is needed, this study gives an insight to the past and present factors that regulate water course, its hydrology, and morphology. An in-depth knowledge of this factors can certainly make it possible to predict the evolution and dynamics of the Reno river flow and its morphology.