Time and the Nile

It is a new year. We measure time in years and months. It was not always this way. In the ancient Egypt, life revolved around floods. Nile floods. They gave life - or they gave famine. It was thus very important to predict the extent of the annual flood, and to do so, they measure the river depth by Nilemeters. These were connected with the river.  Here is a picture of a sophisticated Nilemeter, which was connected with the river: 

Nilemeters also made it into my book, Spear of Seth. Here is the relevant passage (and adventure):

....Heather was just starting to object when a tremendous thump drowned out whatever she wanted to say. The room sank instantly into darkness as lights went out and the air filled with dust. I stuck my hand into my pocket and pulled out the flashlight. In its light, I saw a rock wall where the exit had lain a moment before. The rockslide had cut us off.
“Jonathan!” yelled Heather. “Jonathan!” she screamed again, in a voiced tinged with fear. “I think he is under the rocks.” She turned to me. “We must pull him out.”
I looked down at the floor. “Fat chance,” I said. “That bastard got out.”
“What do you mean?” demanded Heather.
“Look at his footprints,” I said. “He was moving away, out of the room. And here is his mask. He tossed it before he got out.”
“You just don't like him,” she retorted angrily. “He may be hurt. And if he really made it out, he’ll bring help,” she added, her voice filled with desperate hope.
“Yeah, right,” I said. “After he made sure that nobody else was at the temple, and after he asked us not to tell anybody where we were going. He planned it very well,” I added bitterly.
“I don't believe that,” Heather snapped. “I'm sure he'll bring help,” she repeated stubbornly.
“Well, in case he doesn't, I'm going to try to clear out the next doorway and check to see if there is a way out,” I said and turned away.
I began removing the upper rocks blocking the inner doorway. Heather sat down and observed me sulkily, but at least she held the flashlight shining on the doorway. The air was getting heavier and heavier. If we don’t get out soon, we're going to suffocate down here, and worked harder. Finally I cleared away enough rocks to open a little space at the top of the doorway. I climbed in. Once I got halfway into the next room, I felt an open space beneath me. I tried to back out, but I was too far in.
I lost my balance, fell forward and suddenly slid on a sandy slope toward the center of the room. I heard little rocks and sand moving, rolling and finally falling and hitting water. I realized I must be sliding toward a big hole. I dug my fingers and feet into the sand until I stopped. A light appeared behind me and I heard Heather’s anguished voice. “Alex, are you all right?”
I didn't respond. Two feet in front of me, I saw the mouth of a huge well. It was at least ten feet across. I saw a spiral staircase, hugging the wall of the well, descending into darkness. I shuddered.
Heather came into the room (a bit more gracefully) and stood next to me. “Look,” she said, excitement in her voice. “A Nilemeter.”
“A what?” I growled, trying to back away from the well. Going uphill on my belly turned out not to be the best way to locomote.
“A Nilemeter,” she repeated. “It was used to measure the river’s level. This way, the Egyptians could estimate the strength of the annual flood,” she continued.
I finally succeeded in backing far enough from the well to feel I could stand up safely. “You mean this well? How did it work?” I asked mainly to keep the lecture going while I tried to get my shaking legs under control.
“The Nilemeter? It was connected by a tunnel to the river.”
I walked—carefully— back to the well and leaned forward. I caught a glimpse of light deep in the water, or so it seemed to me. I gathered my courage and descended the staircase. Eight feet down, I reached the water level. I took a deep breath and dived, swimming toward the light. I descended about six feet before I touched the wall and found an edge of a tunnel. I swam back and climbed out of the water. “I think we may have a way out.”
“Are you crazy?” she blurted out. “You want us to dive into that black water and swim into some underwater tunnel? Besides, sometimes they built these tunnels into a complete mazework and we could get trapped.” She spun around and resolutely began to climb back into the pharmacy room. “I’m going to wait until somebody comes,” she shouted over her shoulder. To me, she sounded like a petulant child.
“Look, Heather,” I began and started after her. When I made it into the pharmacy, I tripped over something. The breaking sound told me that I had just finished off the twin jar. “I am sorry, I…” I tried to apologize.
She raised her hand and stopped me. I followed her gaze and looked down at the jar. A green, fluorescent fog rose from the broken neck. It formed a strange shape, until it resembled a man-sized, blind frog. “Quick,” said Heather, dragging me away.
“What is it?” I gasped.
“I don’t know and don’t want to find out,” snapped Heather. We retreated into the Nilemeter room and without hesitation Heather jumped.
I followed her into the water and finally into the tunnel. Fortunately, the tunnel itself was only about fifteen feet long. I looked up, saw light, and after a short ascent broke the surface of the Nile River. Heather surfaced to my left, spitting water and coughing. She looked around and smiled when she saw me.
“We made it,” she said jubilantly, then concentrated on swimming till we reached the landing.