#game-container {
    position: relative;
    width: 800px;
    height: 600px;
    border: 1px solid #000;
    margin: 0 auto;
    background-color: #00aaff;
}

#river {
    width: 100%;
    height: 100%;
}

#frog {
    position: absolute;
    bottom: 0;
    left: 50%;
    transform: translateX(-50%);
    width: 50px;
    height: 50px;
    background-color: #00ff00;
    border-radius: 50%;
}