Features appear in each issue of Pennsylvania Heritage showcasing a variety of subjects from various periods and geographic locations in Pennsylvania.

Thousands of years ago, travelers from northeastern Asia­ – ancestors of Native Americans – followed the animals they hunted into what had once been inaccessible regions of Alaska. Over the newly-formed land bridge at the Bering Strait they came, eventually spreading across the North American continent, including the territory that became Pennsylvania.

In the Commonwealth, as elsewhere, much of the archaeological record of the indigenous people has been lost through carelessness, looting, and poor record keeping. However, thanks to the vision of a few individuals and institutions, a great many archaeological findings have been preserved and scrupulously studied. Today, for instance, much of what students and scholars know about the culture of the Susquehannock Indians is owed to the work of both David Herr Landis (1864-1936) of Lancaster and the old Pennsylvania Historical Commission.

Probably few can imagine what the lower Susquehanna River, dividing the counties of York and Lancaster, looked like only a century ago. In the days before hydroelectric darns, speeding power­boats, and campaigns to save the Chesapeake Bay at its terminus, the Susquehanna looked quite different. Natural features, such as the falls at Conowingo and the mammoth rocks at Indian Steps, made it untamed and unspoiled. Through the years, however, largely unchecked agricultural and industrial development along the broad river’s banks began to adversely affect the quality of its water. Economic develop­ment of the Susquehanna River itself, such as construction of the York Haven Dam in 1884, changed the geography of the river. Perhaps spurred by the knowl­edge of these changes and the plans for additional dams and hydroelectric facilities on the river, individuals began actively to collect artifacts related to the native cultures which had many years before inhabited the area.

In 1931 and 1932, the Pennsylvania Historical Commission (now the Pennsylvania Historical and Museum Commission), in conjunction with the Safe Harbor Water Power Corporation, undertook a scientific examination of the area of the river in and around Safe Harbor, about twenty miles south of Harrisburg, in preparation for construc­tion of a huge hydroelectric dam. Archaeological Studies of the Susquehannock Indians of Pennsylvania, a report prepared at the time by Donald A Cadzow, archaeologist for the Pennsylvania Historical Commission, offered a grim picture of what he and fellow archaeolo­gists sadly discovered.

Unfortunately, most of the previous archaeological work in the region had not been conducted along scientific lines. No attempt had been made to record field data and, as a result, there was no accurate information to make comparisons or form preliminary opinions. Unsustained theories advanced by local enthusiasts had to be discounted and a new approach made to the whole problem.

Many of the Indian sites investigated have been known for years and were partially looted – the objects found either destroyed or scattered.

Unlike most of his contemporaries who hunted for artifacts, David Herr Landis assembled one of the largest and best-documented archaeological collec­tions in the Keystone State. His collection, held in public trust by the Hershey Museum since 1936, consists of artifacts, field and research notes, books, articles, correspondence, and pho­tographs. It is an important research tool for those who seek to understand the culture of the Susquehannock Indians who once inhabited the picturesque river valley. The Landis collection is signifi­cant, too, because it allows today’s archaeologists and historians to appreci­ate both the curiosity and perspective of the individuals who, during the late nineteenth and early twentieth centuries, sought to discover, preserve, and interpret the material culture of the earliest inhabitants.

David Herr Landis spent his entire life at Windom Mills in Manor Township, Lancaster County. The substantial complex of buildings which makes up Windom Mills is located midway between Millersville and Washington Boro on the West Branch of the Little Conestoga Creek in an area rich in Native American material culture. Landis, whose family had owned and operated the mill and surrounding tobacco farm for several generations, quickly developed a passion for collect­ing and studying both the history and archaeology of the area.

Widely read in both natural and social history, and armed with a rudi­mentary knowledge of science acquired during brief studies at Millersville State Normal School (the present-day Millersville University of Pennsylvania) in 1888, Landis initiated a scientific study of local Susquehannock Indian sites along the Susquehanna River in Lancaster County beginning in the early 1890s. He eventually assembled a collection of nearly twelve thousand objects, painstakingly documented with field notes, photographs, and drawings. Although an amateur archaeologist, lacking any formal training, Landis used a system of cataloguing and record keeping quite similar to one employed by archaeologists today. He carefully identified each object with a permanent number, recorded all known information about it, and described the site of its discovery. Landis realized that a single site could contain many artifacts from different periods, and interpreted the consequences of those differences in his prolific writings.

The archaeological sites in western Lancaster County examined by Landis were once home to the Susquehannocks, Eastern Woodlands Indians who flourished between 1575 and 1680. The Susquehannocks came into contact with Dutch, Swedish, and English settlers who offered manufactured goods in return for furs. By the seventeenth century, they had profited by acting as middlemen between tribes who occu­pied western Pennsylvania and the Ohio River valley and the European explorers. The Eastern Woodlands Indians valued trade items and often buried them with their dead. Many of the objects found at Susquehannock village sites by Landis and his contemporaries date from this period of European contact.

As a result of this contact, native cultural traditions were frequently neglected or were altogether displaced by European material culture. The Hershey Museum’s Landis Collection is exceptional because it contains items of both domestic and European origin. Domestic objects include day pots and pot sherds, arrow points, bifaces, stone hoes and axes, banner stones, and shell beads – all of which are accompanied by Landis’ field notes, drawings, and photographs. European-made buttons, spoons, jaw harps, bottles and bottle fragments, earthenware fragments, glass beads, knives, iron axes, brass kettles and kettle clippings, pipe fragments, musket parts, munitions, flints, and various utilitarian and decorative objects removed from Susquehannock village sites reveal the types of European goods that the Susquehannocks used and valued. They also document the rapidity with which they were adapted by the Susquehannocks once contact with European traders had been established.

In addition to the loss of Native American survival skills, material culture, and way of life, contacts between the Native Americans and the Europeans also exposed the Susque­hannocks to diseases, including measles and smallpox, against which they had no immunity. In 1673, weakened by disease and the constant struggle to protect their trading rights, the Susquehannocks were attacked and virtually destroyed by the Iroquois. The few survivors resettled with other Native Americans, most notably the Seneca, where they became known as the Conestogas, at a site known as Conestoga Indian Town. In 1763, the tribe’s remaining members were massacred by a group of white settlers known as the Paxton Boys.

For the following century and a half, various amateur archaeologists, farmers, and “relic collectors” found objects in plowed fields, or purposely opened Native American grave sites in the lower Susquehanna River valley, without giving any thought to the people or culture that had made and used them. In fact, most objects in the Landis Collection were excavated from grave sites either by Landis himself or by the individual upon whose land the discovery was made. Landis actively added to his burgeoning collection by purchasing single pieces and parts of collections from others or through auctions and public sales. To his credit, he did much more than simply acquire and display his collection of “relics.” Instead, he attempted to understand the culture of those who had made or used these objects and to share that knowledge with others.

A prolific writer, lecturer, and active member of the Lancaster County Historical Society, Landis shared his findings with scholars, professional colleagues (including Donald A. Cadzow and Pennsylvania Historical Commission staff members), and interested individu­als. As a result of an article written for the historical society’s journal in 1910, Landis was recognized as “probably the first person to actually identify in print a number of archeological sites as being Susquehannock.” In collaboration with the Lancaster County Historical Society and the Pennsylvania Historical Commis­sion, he was instrumental in securing the placement of several historical markers commemorating early Native village sites in western Lancaster County.

David Herr Landis’ unpublished field notes, entitled Catalogue of My Collection of Indian Curios, dated August 23, 1920, are safeguarded by the Hershey Museum. This fascinating manuscript offers contextual details about the discovery of sites in addition to basic information about the objects unearthed. In a section of the loose-leaf notebook innocuously entitled “The Indian Site At Indian Town,” he vividly records the circumstances surrounding the uncover­ing of a Native American grave site at Conestoga Indian Town:

During the summer of 1914, Will Keener while plowing on the Dr. Hiestand (Brenneman) farm, uncovered the remains of an Indian which was on the top of a hillock on the West side of the run, a few hundred yards West of the grave uncovered by Witmers … Nothing accompanied the remains except a string of white glass beads which were around the neck. At my request, Will Henry and his son Chester carefully disinterred the skeleton during the fall of 1915 for me. The skeleton is almost complete; the teeth were in perfect condition, but some of them were taken by John Witmer and others when first uncovered. This was also the case with some of the heads though I secured a few.

Because most of the Susquehannock archaeological sites are located near the Susquehanna River, flooding often played a role in the discovery of native grave sites and artifacts. A section of Landis’ notebook, “The Indian Site at Washington Boro,” recounts the aid fortuitously lent by nature to further his quest.

During the Floods of May 1884 and June 1889, many of the Islands of the Susquehanna were gutted out or almost swept away. In this manner, “Big Island”, which is about the middle of the River opposite Washington Boro, had about a foot or more of the top surface swept away. Soon after the flood of 1884, John F. Shenk, who farmed tobacco there, found what was doubtless an Indian grave, although no remains were found. However, the following Indian articles were discovered, which are in my collection: an earthen bowl, 170 blue cylindrical glassbeads (#840), and what appeared like a small cushion with a number of small “pearl” beads (#811) unlike any 1 have seen elsewhere, some brass bells, buttons, and bullets. I purchased all these from John Shenk. Another earthen bowl was there embedded in the mud and unfortunate­ly someone came along and broke it all while trying to pick it up when it was soft.

Although most artifacts discovered by Landis and his peers were found in grave sites, care was often taken­ – especially by Landis – to make sure the human remains were reinterred. Following the discovery of three graves on October 31, 1933, while digging the foundation for a cannery at Washington Boro, Landis noted that “the skull of the remains of one grave was covered over with a copper kettle, and the skull was in perfect condition. It was reinterred with the other remains.”

When an accidental discovery of Indian artifacts was made, some landowners simply refused to allow anyone to hunt for additional objects on their property. In one section of his notebook, “The Indian Site at Rowenna,” Landis noted that, “On Sept. 26, 1924, Joe Wertz told me that on the lot S. of and adjoining the Station at Rowenna (Schock’s Mills), a great many Indian relics are found, but the owner of it prohibits any hunting for them .. ” A second entry, “Indian Site South West of Letart,” is even more revealing.

Amos C. Nissley told me there is an Indian graveyard on what had been his father’s (Jonas Nissley) farm, now the Ben Miller farm … Its location is on a knoll about 20 feet east of the western line fence of the farm and about midway between the two roads which are the northern and southern boundary of the farm. Amos stated they would find a fiat stone there occasionally. These stones were said to cover a grave and trees were uprooted there a number of years ago which uncovered several such flat stones, but his father would not allow them to disturb them, because if they did and would find Indian remains there, they would be disturbing or robbing the grave, which he would not permit them to do.

Today’s reader may be struck by the ease at which earlier archaeologists were able to simply “happen upon” objects in fields and meadows. Because of his knowledge of the local terrain and his general experience Landis was quite adept at doing just that. This good fortune, coupled with his researching, writing, and teaching abilities, made him a valuable consultant for professionals, including Donald A. Cadzow and Albert Cook Myers of the Pennsylvania Historical Commission. A section of his field notes dated August 20, 1921, “The Indian Sites at Wrightsville and Long Level in York County,” recounts Landis’ experience in showing Myers, secretary of the Commission, some of the local archaeological sites.

I accompanied Albert Cook Myers and Miss Lewis, in Mr. John Frederick Lewis’ car, with the chauffeur, Russel, and directed them to the following sites, viz; – James Patterson Trading Post, Martin Chartiers, Susquehannock Fort in York Co., Cresaps’ Fort (now Byrd Leibharts’ home), the Blue Rock, Fort Demolished (H. G. Witmers and Chas. Stricklers farm at Old Anchor Tavern), John Cartleges (Geo. Baker farm), Pequehan (Dr. Hiestands’ Binkley farm) and Postlethwaits.

While at the Susquehannock Fort site on the slope W. of John Haines’ buildings, Mr. Myers, Miss Lewis and I found quite a number of pieces of clay pipe sterns (Indian), pottery, and bone fragments (probably human). I found a fragment of a bowl having a human face. All of our finds we gave to Mr. Myers to convince him of the Indian occupation there.

While at the Cresaps’ Fort site, Mr. Myers purchased a lot of Indian and Trader articles found by Burd Lephart’s son, which he gathered in the fields about 100 yds. W. of the buildings, consisting of Indian and Trader clay pipe stems, similar to those found at the H. G. Witmer site, one good trader pipe bowl, a small tomahawk, some lead bullets, arrow points, a few cylindrical red glass beads, and a bear tooth, an image of brass (a European with gun and dog and a whistle, exactly similar and from the same mould as my own, found at the H. G. Witmer site).

Of an individual site, Landis devoted the most coverage in his catalogue to “Byrd Leibhart’s Indian Site” at Long Level. Although he writes that he had known of the site much earlier, he specifically mentions that it was only in October 1933 that a major find was accidentally discovered by Leibhart while plowing his fields. Eight graves and a number of refuse pits were eventually uncovered in the three-acre village site. Landis, impressed by the quality of objects recovered, included more than twenty pages of notations and drawings in his catalogue.

Throughout his file, Landis undertook a number of highly important field projects, often working on his own, sometimes relying on the help of his son Nelson or other interested parties, and almost always working at his own expense. In addition to his extensive archaeological work in and around Washington Boro, Landis undertook two systematic photographic studies of the petroglyphs (or rock carvings) of the lower Susquehanna River. The rocks were threatened with obliteration by water impounded by the Safe Harbor, Holtwood, and Conowingo hydroelectric dams. He located and cleared many of these rocks in Pennsylvania and Maryland, covered their faces with lampblack, and filled the incisions with white pigment to photograph them clearly.

David Herr Landis knew, as early as 1905, that the Susquehanna Power Company planned to construct a large dam at the village of Conowingo in Maryland. Although financial problems put the project on hold until 1926, the Pennsylvania Water and Power Company (today known as Pennsylvania Power and Light) did in the meantime construct a mammoth hydroelectric facility at Holtwood, about ten miles north of the Maryland border. Prompted by his knowledge of these plans, Landis began in the late 1890s to compile the fast of his two detailed photographic records of river petroglyphs. Entitled simply Photographs of Inscriptions Made By Our Aborigines On Rocks in the Susquehanna River in Lancaster County and released as a limited edition work in 1907 – of which only five copies were actually made – it consisted of fifty photographs of rocks and rock groups, including Cresswell Rock, the Neff’s Island Rocks, Big and Little Indian Rocks, and the Indian Steps on the York County side of the river. In his introduction to the
work, Landis explained his mission.

Bent on contributing a mite to our collection of local history, and more perma­nently preserve the rapidly disappearing traces of our aborigines, it was not work but my pleasure and recreation, to gather, from time to time, during the past nine years these photographs … Although a number of these rock inscriptions have been described in various ways heretofore, it is astonishing how little effort has been made to give their exact location, and how very few persons have any knowledge of them, although living within a few miles of them all their lives. For this reason 1 have made a special effort with map, kodak, and pen, to describe their location, and that they may be found in the future by anyone.

On March 8, 1926, the Philadelphia Electric Company commenced construction of the Conowingo Hydroelectric Dam on the Susquehanna River in Maryland, about eleven miles north of Havre de Grace. The mile wide concrete dam was designed to generate the largest electrical output of any power plant in the world! Because the lake formed by the dam would extend more than fourteen miles, petroglyphs at Bald Friars in and around Miles Island were threat­ened with submersion. Faced with this threat, many petroglyphs were simply cut and removed from the river. They were blasted, chiseled, numbered, and taken to Maryland by the Maryland Academy of Sciences, and to Pennsyl­vania by the North Museum in Lancaster. Several were believed to have been delivered to the Pennsylvania Historical Commission in Harrisburg as well. Plaster casts of carvings on the larger stones were also made.

Responding to the threat posed by the construction of the Conowingo Dam, Landis visited Bald Friars on Wednesday, July 8, and again the following Wednesday in 1925, to photograph petroglyphs on Miles Island. His unpublished album of photographs contains forty photographs of various views of the inscriptions on Miles Island. Each photograph is accompanied by a typed inscription. Because of his familiarity with the rocks, Landis again visited the rocks with Francis Nicholas of the Maryland Academy of Sciences on Sunday, October 16, 1927, where he assisted in choosing sections which were to be quarried and preserved. Equally as important as the photographs made by Landis are his descriptions of the immediate area contained in. the unpub­lished album.

The petroglyphs or Indian inscriptions on rocks in the Susquehanna River at Bald Friars, Maryland, are found on a small rocky island, known as Miles Island and several other rocks and rocky islets within a few hundred yards of Miles Island. The island is composed of four rocks or masses of rocks on which there are inscriptions … When the Susquehanna Water Power Company of Penna. has constructed the dam at Conowingo, all of these rocks and islands will be at least thirty feet or more under water.

To the left of Miles Island stand two huge rocks of milky quartz, each one stand about ten feet above the average level of the River and each is about 10′ X 12′ in length and width … The early Marylanders compared these bald rocks with the shaven heads of the early friars and in that manner the name “Bald Friars” was derived.

Officials of the Pennsylvania Water and Power Company had long consid­ered harnessing the power of the river a second time. In conjunction with the Baltimore Gas and Electric Company, they created the Safe Harbor Water Power Corporation to build a hydroelec­tric dam at Safe Harbor, eight miles up river from the Holtwood facility. In 1930, as work began on the dam, the Pennsyl­vania Historical Commission mounted an ambitious project to rescue selected petroglyphs photographed by Landis whose existence was now jeopardized. Commission staff members were also intent on exploring archaeological sites adjacent to the river. Working under Cadzow’s direction, specimens were copied in plaster, chiseled into small pieces, and taken to shore. Timing was critical; the Safe Harbor hydroelectric dam was to begin operation in late 1931.

David Herr Landis became associated with Donald A. Cadzow when the Pennsylvania Historical Commission was given the opportunity to conduct archaeological fieldwork in the Safe Harbor area before the construction of the new hydroelectric facility. Many of the petroglyphs originally identified by Landis in his 1907 publication, as well as several archaeological sites he identified in his series of articles written for the Lancaster County Historical Society, became the focus of Cadzow’s work. “During the summer of 1930,” Landis wrote, “the Pennsylvania Historical Commission at Harrisburg with the assistance of the Safe Harbor Water Power Corporation, not only took plaster casts and drilled out the rock inscrip­tions on rocks at Neff’s Island (Star Rock Station) and the rocks south of Cresswell Station, but also excavated Indian graves in this section.”

In his Catalogue of My Collection of Indian Curios, Landis devoted many pages to detailed drawings and descrip­tions of excavated articles and bones, and plaster casts of petroglyphs he examined at the Safe Harbor “store rooms ” between late May and October 1930. In addition to observing the work of the Commission staff, Landis spoke with Cadzow on several occasions, including an exchange he recorded in the Catalogue: “They are now beginning to explore the Hiestand Frey (former John Stehman) property, and Dr. Cadzow told me the Safe Harbor Water Power Corporation has purchased the Oscar A. Leibhart (former John Hains) property, at Long Level, the (my) site of Susquehannock Fort.”

The archaeological and photographic fieldwork carried out by Landis before 1930 proved to be an invaluable resource for Cadzow and the Pennsylvania Historical Commission. Interestingly enough, a photograph of Cadzow, dated April 15, 1930, discovered in the Hershey Museum’s Landis Collection, vindi­cates – after many years – the efforts made, and at least some of the methods used, by Landis. Cadzow was pho­tographed with a series of inscriptions on a Neff’s Island petroglyph first brought to public attention by Landis in preparation for his 1907 publication. This particular view of the inscription and the method used to highlight it are nearly identical to those made by Landis for his publication. The importance of Landis and his contributions to the work of the Pennsylvania Historical Commission were recognized by Frances Dorrance, vice chairman, in her introduction to Cadzow’s completed field report, Archaeological Studies of the Susquehannock Indians of Pennsylvania, published in 1936.

… in response to a call for information on existing artifact collections, manuscript and printed accounts, knowledge of sites, trails, and, in fact, anything relating to the life of the Red Man in our Commonwealth” (made in 1925) … a book of photographs of the Indian rocks in the Susquehanna River near Safe Harbor, Lancaster County (was received) from David H. Landis, who stated that the drawings on the rocks were threat­ened by the projected erection of a power dam at that point on the Susquehanna.

Today, only Big Indian Rock and Little Indian Rock, located in relatively shallow water, just below the Safe Harbor dam, have escaped inundation. Little Indian Rock is visible only when water levels are unusually low, generally during summer. The top of Big Indian remains visible in all seasons, although the traces of its carvings have been obliterated by vandal.ism and erosion. Despite numerous theories based on the observations of Landis, Cadzow, and others, the meaning and origin of the inscriptions on these petroglyphs of the lower Susquehanna River remain a mystery. Both Big Indian Rock and Little Indian Rock are crucial remnants of the material culture of the pre-historic inhabitants of the area and were entered in the National Register of Historic Places in 1978.

On November 13, 1936, the executors of David Herr Landis’ estate sold his entire collection, .including a vast array of artifacts, books, research notes, photographs, and correspondence, to Hershey Estates, which operated the Hershey American Indian Museum, for seventy-five hundred dollars. The museum, established in 1933 by choco­late company founder Milton S. Hershey, and housed in the former residence and surgery of Dr. Martin L. Hershey, contained two floors of artifacts repre­sentative of various North American Indian tribes and regional native cultures (see “An American Indian Museum for Mr. Hershey’s Model Town” by Lois Miklas Hartmann and James D. McMahon Jr. in the summer 1993 edition of Pennsylvania Heritage). Possibly encouraged by the success of his Indian Museum, Hershey purchased in 1935 an extensive collection of Pennsylvania German objects from the estate of George H. Danner, a resident and merchant of Manheim, Lancaster County (see “George H. Danner: The Retailer and His Relic Rooms” by Jonathan P. Cox in the summer 1987 issue of Pennsylvania Heritage). This acquisition required the relocation of the Indian Museum to a much larger building, the former Convention Center and Ice Palace, which was remodeled and opened as the Hershey Museum in 1938.

With the purchase of the Danner Collection in 1935 and the Landis Collection the following year, the new Hershey Museum acquired two impor­tant collections of regional significance that greatly increased both the size and scope of the institution. Although it is not known if the administrators of Landis’ estate approached Hershey officials or vice-versa, it is certain that the acquisition was facilitated by the professional friendship between Donald Cadzow and Richard Light, curator of the Hershey Museum from its inception in 1933, who became acquainted during their work with the Society for Pennsylvania Archaeology.

Today, the Hershey Museum’s Landis Collection remains one of the preeminent archaeology collections in the Commonwealth. It is highly regarded by professional archaeologists for its content, precision, and attention to detail. Because of the vast number of objects in this collection, it is maintained as a research collection by the museum, although pieces are exhibited in a permanent interpretive exhibit on the Eastern Woodlands Indians.

With the passage of the Native American Grave Protection and Repatriation Act of 1990, human remains and sacred and funerary objects which once belonged to tribal groups, rather than individuals, are eligible to be reclaimed by native groups through designated tribal representatives. As early as 1986, in response to a growing awareness and sensitivity to native cultures and beliefs, the Hershey Museum made arrangements with the Seneca Iroquois National Museum to rebury the remains of an unidentified Susquehannock removed by Landis in 1915 from the Conestoga Indian Town site. Because no descendants of the Susquehannocks live today, the museum reinterred the remains in the Horseshoe Community Cemetery on the Allegany Indian Reservation in New York.

The Hershey Museum continues to reevaluate all aspects of its Native American holdings, which include objects from the continental United States and Alaska. As required by federal law, the museum has notified all designated tribal representatives of the contents of its collections. In the special case of the Landis Collection, the Hershey Museum has notified the National Park Service of the funerary content of this collection, welcoming future dialogue on the repatriation issue. With the continuation of respectfulness and sensitivity toward both the Native American culture and its relics, the spirit of David Herr Landis lives on.


For Further Reading

Cadzow, Donald A. “Archaeological Studies of the Susquehannock Indians of Pennsylvania.” Safe Harbor Report Number 2. Harrisburg: Pennsylvania Historical Commission, 1936.

Kent, Barry C. Susquehanna’s Indians. Harrisburg: Pennsylvania Historical and Museum Commission, 1984.

Stranahan, Susan Q. Susquehanna, River of Dreams. Baltimore: The Johns Hopkins University Press, 1993.


The editor wishes to thank Steven G. Warfel, senior curator of archaeology for The State Museum of Pennsylvania, for his review of this manuscript.


James D. McMahon Jr., curator of collections and exhibitions for the Hershey Museum, recently reorganized the institution’s David Herr Landis Collection. He received his bachelor of arts degree from Franklin and Marshall College in Lancaster and his master’s degree from Penn State Capital College in Middletown. He has contributed to Pennsylvania Folklife, The World Book Encyclopedia, and Pennsylvania Heritage. A native of Lancaster County, he is a user of the Lake Clarke Recreation Area and the Lake Aldred Recreation Area, both of which were created by water impounded by the hydroelec­tric dams constructed on the Susquehanna River at Safe Harbor and Holtwood.