Field measurements of volcanic gases. Vulcano Island (Italy), Kilauea (Hawaii,U.S.A.), Merapi (Java,Indonesia)

A field gas chromatograph, built in 1978, was used in the field to directly analyse volcanic gases before water vapor condensation. Tested in Vulcano (Italy), Kilauea (Hawaii) and Merapi (Indonesia), this field measurement technique provides the actual composition of the volcanic gas mixture. The technique avoids the depletion of sulfur gases and the dissolution of the acid gases in the condensed water during the cooling. Thus the mixture of H₂S and SO₂ in fumarolic and high temperature gases (up to 819°C) in equilibrium at the emission temperature was examined.     

Delivery of upper-basin sediment to the lower neuse river,North Carolina,U.S.A.

Extensive storage of upper-basin Piedmont sediment and apparent low sediment supply to streams in lower-basin Coastal Plain areas generates questions as to the source of alluvium in lower reaches of rivers of the U.S. Atlantic drainage. This was investigated on the Neuse River, North Carolina, using a mineralogical indicator of sediment source areas. The utility of mica flakes for discriminating between Piedmont and non-Piedmont sources of sediment in the lower Coastal Plain reaches of the Neuse was established on the basis of an examination of the U.S. National Soils Database and of 26 soil surveys of the North Carolina Coastal Plain. From the Neuse River estuary to 48 km upstream there are no mica flakes in floodplain soils or in river bank and channel shelf sediments. Mica flakes become more common upstream. This suggests that a very small proportion of the sediment eroded in the Piedmont portion of the watershed is delivered to the river mouth. The small amounts which presumably do reach the lower Coastal Plain are so diluted by Coastal Plain-derived alluvium that no Piedmont origin can be discerned. This demonstrates a dominantly Coastal Plain source and underscores the importance of storage and discontinuous transport in fluvial sediment systems. More importantly, results suggest that upper- and lower-basin sediment dynamics are not only non-linearly related, but may be virtually decoupled.     

Assessing Seasonal Transport and Deposition of Agricultural Emissions in Eastern North Carolina, U.S.A.

— There is an increasing interest regarding the fate of nitrogenous compounds emitted from agricultural activities in the southeastern United States. Varying climate, topography and proximity to the Atlantic Ocean particularly complicates the problem. An increased understanding of the interaction of synoptic scale flow with mesoscale circulations would constitute a significant improvement in the assessment of regional scale transport and deposition potential. This knowledge is necessary to facilitate current and future modeling attempts in the region as well as for planning future monitoring sites to develop a cohesive regional policy for the abatement strategies. The eastern portion of North Carolina is used as a case example due to its high, localized emission of nitrogen compounds from agricultural waste. Three periods: July 2–7, 1998, October 5–11, 1998, and December 12–19, 1998, corresponding to three different seasons were studied. Surface wind and thermodynamic patterns were analyzed using surface observing stations and archived-model analysis results centered over eastern North Carolina. Diurnal and seasonal patterns were identified for dispersion and concentration values obtained using an air pollution transport and dispersion model. This mesoscale information was used to draw qualitative conclusions regarding the possible trends and deviations in the dynamic trajectories as well as the resulting near-surface concentrations and deposition potential in eastern North Carolina. Results show that highly variable seasonal and diurnal atmospheric circulations characterize the study domain. These variations can significantly impact the transport and fate of pollutants released in this region. Generally, summer provides the highest potential for localized deposition, while fall can provide opportunity for long-range transport. The results also suggest that mean climatological or seasonally averaged flow patterns may not be sufficient for analyzing the fate of the agricultural releases in this region. At the very least, mean and variance based analysis is required to capture the climatology of the dispersion and deposition patterns. These patterns in eastern North Carolina appear to be sensitive to the strength and location of air mass boundaries along the coastal plain, indicating diverse scale interactions affecting the variability and uncertainty in the regional pollutant transport.     

Primary fractures within a tuff cone, North Menan Butte, Idaho, U.S.A.

North Menan Butte is a tuff cone near Idaho Falls, Idaho. It is a result of the eruption of basaltic magma through shallow water-saturated river alluvium of the Snake River. The cone is characterized by primary fractures that can be classified into four groups on the basis of their physical properties and their orientations relative to the symmetry elements of the cone. Type I fractures are short, closely spaced and usually confined to individual beds. They strike approximately at right angles to cone radii and always dip toward the rim of the tuff cone. Bed segments separated by these fractures have undergone rotation, resulting in normal displacements. Type II fractures have similar attitudes but are more continuous, less frequent, and show no shear displacement. Type III fractures also strike at right angles to cone radii, but they dip away from the cone rim. They cut across several beds and reveal inconsistent senses of shear displacement. Type IV fractures are radial, steeply dipping and tend to be the most continuous of all fracture types. Type I fractures were the earliest to develop; age relationships otherwise are uncertain. Examples of all four types of fractures are exposed on the inner and outer eroded slopes of the cone.Evidence from the cone indicates that the fractures developed in an unconsolidated aggregate of tuff with low cohesion; therefore, analysis of fracture genesis should be constrained by principles of soil mechanics. Type I fractures originated as tension fractures related to early downslope mass movement. Later movement on Type I fractures accompanied the development of Type III shear fractures and possible bedding plane displacements, all caused by overloading the crest of the cone by late-stage eruptive products. The origin of Type II fractures is unknown; shrinkage due to desiccation or large-scale creep are possible explanations. The radial Type IV fractures may be a consequence of desiccation shrinkage or possibly of subcone processes such as magma doming or radial hydraulic fracturing.     

The geology of the Vulsinian area,Lazio, Italy

The Vulsinian volcanic area in central Italy is made up of several volcanic complexes: the older east part of the area, with the large volcano-tectonic depression — in our opinion not simply a caldera — of the « Lago di Bolsena », and the younger Latera volcano are the major features. With the aid of field studies, paleomagnetic measurements, absolute age determinations and petrologic data, a scheme of the volcanic evolution has been drawn up, dividing the volcanic activity into six stages. Volcanicity started about 0.92 M.y. ago and lasted until subrecent. Three rock series are distinguished: a trachytic-phonolitic magma, mainly found as ash flows, a potassic mafic magma with its salic derivatives. mainly as lavas, scoria and pyroclastics, and a trachybasaltic rockseries, mainly found as lavas and scoria.     

Volcanology,history and myths of the Lake Albano maar (Colli Albani volcano,Italy)

The polygenetic Albano maar is the most recent centre of the Colli Albani volcano, located just few kilometres to the south-east of Roma. Presently the maar hosts a 167.5 m deep crater lake, the deepest in Europe. The maar is to be considered quiescent, as phreatic activity is documented throughout the Holocene. This paper illustrates the close relationships between the activity of the maar and the history of settlement in the Roman region as recorded in the geology, archaeology, history and legends of the area. Severe fluctuations of the groundwater table and catastrophic overflows of the Lake Albano from the maar rim had occurred prior to and after the early prehistoric settlements dated in the maar area at the Eneolithic times (ca. III millennium B.C.). Repeated lahars occurred along the northwestern slope of the maar filling in the paleodrainage network and forming a vast plain. Paleohydraulic analyses on fluvial and lahar deposits originated from the Holocene phreatic activity of the Albano maar indicate sediment–water flows in excess of hundreds of cubic metres per second. Absolute age determinations of the paleosoil underlying one of the most recent deposits of the lahar succession at 5800 ± 100 yr B.P. (¹⁴C CAL) are in perfect agreement with the age of the overlying Eneolithic age settlements. The last catastrophic overflow is described in the Roman literature as a consequence of the anger of Poseidon against the Romans in 398 B.C. for their war against the Etruscans. In 394 B.C. the Romans decided to prevent the repetition of such events by the excavation through the maar crater wall of a 1.5 km long drain tunnel, which is still operational, keeping the lake 70 m below the lowest point of the maar rim. This tunnel drain may be regarded as the first prevention device for volcanic hazard in history and shows an unprecedented development of the engineering technology under the pressure of hazard perception. The surprising and still largely unknown results of this study are very important to redefine the hazard of the Roman region.     

Stratigraphy, chronology, and sedimentology of ignimbrites from the white trachytic tuff, Roccamonfina Volcano, Italy

We describe the stratigraphy, chronology, and grain size characteristics of the white trachytic tuff (WTT) of Roccamonfina Volcano (Italy). The pyroclastic rock was emplaced between 317 and 230 Ma BP during seven major eruptive events (units A to G) and three minor events (units BC, CD, and DE). These units are separated by paleosol layers and compositionally well-differentiated pyroclastic successions. Stratigraphic control is favored by the occurrence at the base of major units of marker layers. Four WTT units (1 to 4) occur within the central caldera. These are not positively correlated with specific extracaldera units.The source of most of the WTT units was the central caldera. Units B and C were controlled by the western wall of the caldera, whereas units D and E were able to overcome this barrier, spreading symmetrically along the flanks of MC. The maximum pumice size (MP) of units increases with distance from the caldera, whereas the maximum lithic size (ML) decreases. MP and ML of the marker layer of unit D (MKDa–MKDp) do not show any systematic variations with respect to the central caldera. In contrast, the thickness of surge MKDa decreases with distance from the source, and MKDp accumulates to the north of MC probably controlled, respectively, by mobility-transport power and by wind blowing northwards.The grain size characteristics of the WTT deposits are used for classifying the units. There is no systematic variation of the grain size as a function of stratigraphic height either among units or within single units. Large variation of components in subunit E1, with repetitive alternation of pyroclastic flow to surge through fallout vs. surge deposits, suggests that the process of eruption took place in a complex or piecemeal fashion.Pumice concentration zones (PCZ) occur at all WTT levels on the volcano, but they are much thicker and pumice clasts are much larger within the central caldera. These were probably originated by the disruption of lava (flow or dome) to pumice fragments and fine ash due to sudden depressurization and interaction with lake waters of the molten lava. Local basal PCZ are, in some cases, similar to the lapilli-rich “layer 1P” that has been described elsewhere, and may have been deposited from currents transitional between pyroclastic surge and flow. Other basal PCZ formed in response to small undulations in the substrate, or can be originated by fallout. Lenticular PCZ within ignimbrite interiors and tops are interpreted to record marginal pumice levees and pumice rafts, some of which were buried by subsequant pyroclastic flows.Lithic concentration zones (LCZ) also occur at various stratigraphic height within the extracaldera ignimbrites, whereas intracaldera LCZ are absent, probably due to the fact that ignimbrite currents are strongly energetic and erosive near vent. LCZ at the top of basal inversely graded layers are formed by mechanical sieving or dispersive pressure in response to variable velocity gradients and particle concentration gradients (a segregation process). Coarse LCZ and coarse lithic breccias (LB), that reside in the interior or tops of pyroclastic flows and that occur in medial to distal areas, are interpreted to be the result of slugs of lithic-rich debris introduced by vent collapse or rockslides into the moving pyroclastic flows along their flow paths. These LCZ become mixed to varying degrees due to differential densities and velocities relative to the pyroclastic flows (desegregation processes).     

The emplacement of intermediate volume ignimbrites: A case study from Roccamonfina Volcano,Southern Italy

A model is presented for the emplacement of intermediate volume ignimbrites based on a study of two 6 km³ volume ignimbrites on Roccamonfina Volcano, Italy. The model considers that the flows were slow moving, and quickly deflated from turbulent to non-turbulent conditions. Yield strength and density increased whereas fluidisation decreased with time and runout of the pyroclastic flows. In proximal locations, on the caldera rim, heterogeneous exposures including discontinuous lithic breccias, stratified and cross-stratified units interbedded with massive ignimbrite suggest deposition from turbulent flows. In medial locations thick, massive ignimbrite occurs associated with three types of co-ignimbrite lithic breccia which we interpret as being emplaced by non-turbulent flows. Multiple grading of different breccia/lithic concentration types within single flow units indicates that internal shear occurred producing overriding or overlapping of the rear of the flow onto the slower-moving front part. This overriding of different parts of non-turbulent pyroclastic flows could be caused by at least two different mechanisms: (1) changes in flow regime, such as hydraulic jumps that may occur at breaks in slope; and (2) periods of increased discharge rate, possibly associated with caldera collapse, producing fresh pulses of lithic-rich material that sheared onto the slower-moving part of the flow in front.We propose that ground surge deposits enriched in pumice compared with their associated ignimbrite probably formed by a flow separation mechanism from the top and front of the pyroclastic flow. These turbulent clouds moved ahead of the non-turbulent lower part of the flow to form stratified pumice-rich deposits. In distal regions well-developed coarse, often clast-supported, pumice concentrations zones and coarse intra-flow-unit lithic concentrations occur within the massive ignimbrite. We suggest that the flows were non-turbulent, possessed a relatively high yield strength and may have moved by plug flow prior to emplacement.     

Some observations on the ignimbrites,lava domes and lava flows of M. Cimino (Central Italy)

Cimino Volcano, in Northern Latium, is one of the most characteristic centers of the early Quaternary acid post-orogenetic volcanism in Central Italy. Three stages in the volcanic activity are distinguished: extrusion of several lava domes; ignimbrite cruptions; effusion of more or less differentiated lava flows. Ignimbrites, that form a shield of over 300 sq. km, show characters of welding variable with the distance from the source area and, in a single section, with the level in the sheet. Most part of the Cimino volcanics (domes and ignimbrites) have a quartzlatite composition. The olivine-trachyte (selagite) inclusions in the domes and ignimbrites, and the dark quartz-latite to olivine-trachyte lava flows, could be the products of a pneumatolitic differentiation in the various stages of the magmatic cycle.     

The emplacement of acid magma in the epizone,and the relationship with ignimbrites,in North Queensland,Australia

There is an aggregate outcrop of 12,000 square miles of Permian-Triassic acid igneous rocks inland from Cairns and Townsville, North Queensland. The rocks consist of ignimbrite and rhyolite, which are structurally and magmatically related to three high-level intrusions the Herbert River, Esmeralda, and Elizabeth Creek Granites. Most of the igneous rocks intrude the Precambrian Georgetown Inlier, but some of them intrude along a fractured zone on the junction of the Inlier and the shelf zone of the adjacent Palaeozoic Tasman Geosyncline. The Upper Palaeozoic — Triassic igneous period consists of two main epochs, both consisting of granite, ignimbrite, and rhyolite. In both epochs granite intrudes the comagmatic and coeval ignimbrite and rhyolite. Rapid horizontal movement of granitic magma through the epizone and major fracturing of the crust are postulated to explain the widespread intrusion of the granite. The granitic magma was probably initially generated 5 miles below the surface of the crust by partial melting of the sediments at the base of the Tasman Geosyncline. Epeirogenic movement in the Precambrian Inlier area formed sheet-like fractures, which provided channels for rapid horizontal movement of the granitic magma. This magma was emplaced along the fractured marginal zone of the Inlier to form a thick sill-like body of granite — the Herbert River Granite — in the first epoch. Magma for the second epoch was derived from melting of the lower part of the granite of the first epoch. Renewed fracturing of the Inlier area formed cauldron subsidence areas and rift, which were quicly filled with rhyolite and ignimbrite. In these collapsed areas the granitic magma crystallized as the Elizabeth Creek and Esmeralda Granites under an insulating cover of about 1,000 feet of rhyolite and ignimbrite.     

Volcanology of trachytic and associated basaltic pyroclastic deposits at Roccamonfina volcano, Roman Region, Italy

The 274 ka “Basalt-Trachytic Tuff of Tuoripunzoli” (TBTT) from Roccamonfina volcano (Roman Region, Italy) consists of a basaltic scoria lapilli fall (Unit A) overlain by a trachytic sequence formed by a surge (Unit B), repetitive pumice lapilli and ash-rich layers both of fallout origin (Unit C) and a pyroclastic flow deposit (Unit D). The TBTT is widespread (40 km²) in the northern sector of the volcano, but limited to a small area on the southern slopes of the main cone. Interpolation between the northern deposits and the latter one yields a minimum depositional area of 123 km², and an approximate bulk volume of 0.2-0.3 km³. Isopach and isopleth maps are consistent with a source vent within the main caldera of Roccamonfina.Unit A shows a fairly good sorting and a moderate grain size; glass fragments are cuspate and vesicular. Unit B is fine grained and poorly sorted; shards are blocky and nonvesicular. Pumice lapilli of Unit C are moderately sorted and moderately coarse grained. Glass shards are equant and vesicular. Lithic clasts are strongly comminuted to submillimetric sizes. By contrast, the ash-rich internal divisions are very fine grained and poorly sorted. They consist of a mixture of equant shards which are prevailingly blocky and poorly vesicular. Unit D is a massive, poorly sorted, moderately coarse-grained deposit. Glass fragments are nearly equant and slightly or nonvesicular.The TBTT is interpreted as due to eruption of a basaltic magma followed in rapid succession by one trachyte magma. Unit A formed by Subplinian fallout of a moderate, purely magmatic column. Interaction between a trachyte magma and water resulted in eruption of surge Unit B. A high-standing eruption column erupted alternating fallout pumice lapilli and fallout ashes. Pumice lapilli originated prevailingly from the inner part of the eruption column, whereas magma-water interaction on the external parts of the column resulted in ash fallout. The uppermost pyroclastic flow Unit D is interpreted as due to final collapse of the eruption column.     

Lake-ice collapse trenches in Wisconsin,U.S.A.

A series of trenches about a metre deep, 20 to 30 m wide, and as much as 2 km in length occurs in central Wisconsin, along the east shore of proglacial Lake Wisconsin. They are interpreted to be collapse trenches formed when shore ice melted after being buried beneath an expanding outwash plain.     

The geology of the Yellow Trachytic Tuff, Roccamonfina volcano, Italy

The 227 ka Yellow Trachytic Tuff (YTT) of the Roccamonfina volcano is a multiunit ash-, pumice-, scoria- and lithic-ignimbrite with a proximal sandwave surge deposit. The YTT has an estimated volume of 0.42 km³. It erupted in the northern, subsided sector of the volcano from Gli Stagli caldera, and was channelled down ravines northward between the limestone range of M.Cesima and M. Camino that bounds the depression. Up to 5 YTT units occur close to the outer part of the northern rim of Gli Stagli. The basal four units are separated by lithic-rich marker layers which are inferred to result from gravity segregation followed by shearing. The first three units are consolidated by chabazite cementation, the fourth one is not consolidated. The uppermost unit is altered. One or two units characterize the YTT deposits in medial to distal zones. Here, the unconsolidated unit underlies the consolidated one. Absence of markers precludes correlation with proximal stratigraphy. The YTT is poorly sorted and, except the surge deposit and the altered faciés which are very fine-grained, has moderate median diameter typical of pyroclastic flows. Matrix, pumice, and scoria clasts are poorly vesicular. Matrix shards are equant, blocky-shaped, hydrated, and range from non-vesicular to vesicular. These features suggest that magma-water interaction played a role in the YTT eruption process, with some magmatic fragmentation.The complex near-Gli Stagli-rim YTT sequence could record the arrival of successive flows from the source vent, or also form by interaction of one or two flows with the caldera rim. In both cases, the absence of basal Plinian deposits in YTT units suggests that the eruptions were low pyroclastic fountains. The YTT distribution was controlled by interaction with the northern rim of Gli Stagli caldera and with the limestone range that bounds the northern depression. The near-rim stratigraphy shows the complete record of the eruption, whereas the medial to distal sequences provide only the initial pyroclastic flow possibly with the final flow spilling over the caldera rim. The proximal surge episode probably resulted from higher velocity of a later pyroclastic flow due to steeper slope of the volcano in that locality.     

Rare earth and strontium isotope evidence concerning the petrogenesis of North Chilean ignimbrites

New rare earth element (REE) data, Rb and Sr analyses and Sr isotope measurements are presented for pumice clasts collected from some North Chilean ignimbrites of dacite and rhyolite composition. The samples are light-REE enriched with chondrite-normalised Ce (Ce_N) of 17–98, Yb_N of 4–14 and Ce_N/Yb_N of 2.6–15. While some samples have no Eu anomalies, others do have anomalies with inferred Eu/Eu^* values of down to ca. 0.4. Eleven samples have present-day⁸⁷Sr/⁸⁶Sr ratios between 0.7053 and 0.7100, and noting that some samples are up to 12 Ma old, initial⁸⁷Sr/⁸⁶Sr ratios are below ca. 0.709. These trace element and Sr isotope characteristics resemble those of contemporaneous andesite and dacite lavas, suggesting a common origin for all these rock types. The higher Rb/Sr ratios and larger Eu anomalies in most of the dacitic and rhyolitic ignimbrites are consistent with an origin by plagioclase-dominated fractional crystallization of mantle-derived andesite magma.     

A detailed gravity study of the Chattolanee Baltimore Gneiss Dome,Maryland, U.S.A.

A detailed gravity survey over the Chattolanee Baltimore Gneiss Dome in the Maryland Piedmont suggests that the dome is an arched recumbent fold. The Baltimore Gneiss, which cores the dome, has a negative density contrast with the surrounding Cambro-Ordovician marbles and schists and is coincident with a large minimum in the simple Bouguer gravity. Three north-south profiles, which cut across the east-west-trending surface exposure of the dome were modeled two-dimensionally. The models suggest that the Baltimore Gneiss is thickest and tightly folded in an inverted V shape to the east and thinner and broadly arched to the west. It is also possible to fit the gravity data with a mushroom-shaped body at the easternmost profile, which could suggest a diapiric origin for the dome, but this interpretation is not favored based on geological arguments. The Baltimore Mafic Complex, located to the south of the Chattolanee Dome, can be modeled as an approximately 1 km thick slab with a subhorizontal base, suggesting that it is a thrust sheet. By analogy with the Phoenix Baltimore Gneiss Dome, mapped by Crowley [2], the Cambro-Ordovician sediments surrounding the Chattolanee Dome may also be involved in the recumbent folding which would suggest that the dome was formed during the Ordovician Taconic orogeny.     

Hydrogeochemical processes involved in salt-dissolution zones,texas panhandle,U.S.A.

Permian evaporite deposits have been extensively dissolved beneath the perimeter of the Southern High Plains in the Texas Panhandle. Hydrologic and geochemical data were collected from six test wells to determine hydrogeochemical processes involved and the source and flow paths of ground water moving in salt-dissolution zones. Geochemical similarities and hydraulic-head relationships indicate that ground water dissolving halite and anhydrite moves downward from aquifers in post-Permian formations and follows flow paths influenced by topography. Holocene salt-dissolution rates probably are lower than Tertiary and Pleistocene rates owing to regional changes in physiography and climate that probably decreased the amount of recharge to salt-dissolution zones. Present as well as palaeohydrologic ground-water velocities and salt-dissolution rates are probably less beneath the Southern High Plains than in adjacent, peripheral salt-dissolution zones because of lower hydraulic conductivities and lower hydraulic-head gradients. Salinities in peripheral salt-dissolution zones are low (67 000 to 95 000 mg L^?1) despite high solubility of halite, reflecting relatively open circulation of ground water. In interior salt-dissolution zones beneath the Southern High Plains, ground-water circulation is low and water composition tends to reach halite saturation.     

Temporal cycles of karst denudation in northwest Georgia,U.S.A.

Time patterns of karst denudation in northwest Georgia (U.S.A.) were investigated at three spring sites for 12 months and at five stream sites for 10 years. Rainfall was evenly distributed and showed no significant seasonality. At the springs, as well as the streams, water hardness was largely controlled by discharge. At the springs, soil pCO₂ and water pH were strongly correlated (r + -0·69 to -0·83). Solute transport in spring waters was highly seasonal, with two conduit flow springs removing more limestone in the winter, and the diffuse flow spring removing more during the growing season. At the stream sites, most denudation occurred during the winter and spring seasons, and least during the summer. Fourier analysis showed that variations in denudation occur on deterministic (long-wave) as well as stochastic (shortwave) time scales. As contributing variables, discharge varied in short-wave and long-wave cycles, whereas soil pCO₂ showed only a long-wave cycle. The 12 month deterministic cycles were the most important, with changes in discharge taking precedence over soil pCO₂. Time series regression explains up to 69 per cent of changes in denudation through rain and soil pCO₂. Time cycles in available water are the key controlling factor of denudation, and amounts of available soil CO₂ may not be as important in the temporal patterns of karst downwearing as has been believed previously.     

Erosional effects of cattle on streambanks in Tennessee,U.S.A.

The geomorphological effects of cattle on streambanks in a humid region, which have consequent potential effects on water quality, are examined. Field observations suggest that cattle are important agents in causing streambanks to erode, but so many variables are involved that it is difficult to isolate the role of cattle. Instead, an empirical approach based on long-term controlled experiment was adopted along a small perennial stream in the Central Basin of Tennessee. The results showed that uncontrolled grazing caused about six times as much gross bank erosion as occurred on the protected control stretch. However, most of this difference was due to breakdown of banks by trampling and consequent erosion, rather than by bank scour caused by removal of bank vegetation by grazing. That is, bank vegetation alone did not appear to be a primary control. A relatively inexpensive grade-control structure reduced the gross bank erosion by about 50 per cent. The rapid destruction of streambanks observed in this study suggests that reduction of geomorphic resistance by uncontrolled stock access to streambanks has been an important factor in the stream widening that has taken place during historical time in the eastern United States.